This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.
About 13.7Â billion years ago (Ga), theÂ Big BangÂ occurred which led to the formation of theÂ universe. The universe evolved by self-organization of matter towards increasing complexity. The accretion of the material in the solar nebula results in the formation of micron-size dust grains (Petit and Morbidelli, 2006). These grains collide with each other, growing larger objects named â€žplanetesimals" that eventually reach the size of tens of kilometers. The larger planetesimals grows much faster than the smaller ones which lead to the increase in realtive mass difference. This phase of runawat growth ends with the formation ofthe cores of the gaint planets of outer solar system like Jupiter, Saturn, Uranus and Neptune and the formation of asteriods and small planets of the inner solar system. The core further accrets large amount of gases and forms the gaint planets. In the Late Heavy Bomdardment (LHB) has been explained by a rapid migration of the giant planets that caused gravitational perturbations in the inner solar system ("Nice model"; GOMES et al. 2005). Due to this the smaller planets collide with each other and forms the inner terrestrial planets like Mercury, Venus, Earth and Mars around 4.6 billion years ago.
Geological history of Earth
The Proto-Earth grew byÂ accretion, until the inner part was hot enough to melt the heavyÂ metals. Due to the high temperature metals were in liquid state. The higherÂ density metals like iron began to sink to the center of the Earth. ThisÂ resulted in the separation of aÂ primitive mantleÂ from the metallicÂ coreÂ after around 10 million years after the formation of Earth. A cloud of gaseousÂ silicaÂ which surrounded the Earth during the accretion of material later condenses as solidÂ rocksÂ on the surface. The surrounding or an early atmosphere the planet which consists of three lightest chemical elements from the solar nebula, like hydrogen, helium and lithium existed. But, heat of the Earth and the solar winds would have removed this atmosphere (ref). A few hundred million years later the first stars were born and began to produce heavier elements like carbon, nitrogen and oxygen.
Initially it was assumed that the primordial atmosphere contained high concentrations of hydrogen, methane and ammonia, and thus was similar to the present-day atmospheres of the giant planets Jupiter and Saturn. However, today a different model of the atmosphere was put forward where the main atmospheric constituents were probably nitrogen, carbon dioxide and water vapour and was practically absent near the surface before the origin of life (Kasting and Catling 2003; Kasting and Howard 2006; Catling and Kasting 2007; Shaw 2008). As a consequence, a UV-absorbing ozone (O3) layer was also missing that absorbs the UV radiations. Therefore short wavelength UV radiation may have reached the planet's surface and could have damaged the organic molecules formed. However, carbon dioxide, volcanic sulfur dioxide, a thin haze of organics produced by photoreactions of methane, and dust from volcanic eruptions and episodical meteorite impacts could have protected the surface (Westall, et al. 2006). The concentrations of hydrogen and ammonia were also low because the former rapidly escaped into space and the latter was destroyed by UV radiation. The solar luminosity of the Sun was 20-30 % less of the present day which steadily increases with time. Therefore, without a sufficiently strong greenhouse effect, the mean surface temperature would have been below the freezing point of water. This is referred as "faint young Sun problem" (Kasting and Catling 2003). Haqq-Misra et al. (2008) have found that 0.03 bar of CO2 and 3000 ppm of CH4 could be sufficient to sustain the mean surface temperature at ~17 Â°C. In total the atmosphere was probably mildly reducing and volcanic exhalations could have locally created more strongly reducing conditions. The frequency of meteoroid, asteroid and comet impacts were higher in the Hadean eon. Some of the impactors that hit the Earth might have had diameters of a few hundred kilo metres (Sleep et al. 1989).
Origin of the Oceans
Oceans played an important role in the evolution of life. The first organic molecules on Earth have been likely synthesized in or nearby oceans (Stetter, 1998; Holms and Andersson, 1998; Nisbet and Sleep, 2001). Oparin andÂ HaldaneÂ independently developed theories suggesting that the simple organic chemicals that were synthesized on a primitiveÂ reducing atmosphere were accumulated in the ocean and called it as a "primordial soup". From this "primordial soup", elementary life has emerged in time. Oceans protected organic molecules formed from UV radiations (Cleaves and Miller, 1998) and cometary and meteoritic impacts/bombardment (Sleep, 2001; Nisbet and Sleep, 2001) because during the late heavy bombardment (LHB) around 4.1-3.9 Gyr ago, Earth was repeatedly strike by asteroids and meteorites.
According to the oldest known terrestrial minerals, the Jack Hills zircons (hydrothermal zircons) are 4.3-4.4 Ga old found from Australia and 4.2 Ga old zircons xenocryst (magmetic zircons) from northwestern Canada shows the presence of oceans and continental crust (Wilde et al., 2001, Mojzsis et al., 2001, Watson and Harrison, 2005, Harrison et al., 2005 and Lijuka et al., 2006). However, there presence is still debatable (Hoskin, 2005 and Nemchin et al., 2006) as the Earth was too hot to sustain "permanent" liquid water oceans durind that time. 3.82 Ga old rocks were found from West Greenland which shows the presence of solid crust and liquid water (Moorbath, 2005). Therefore, it could be concluded that oceans were present by 4.2 Ga ago (Valley 2006).
About 4 billion years ago, the salt content of the oceans were probably two times higher than the modern value (Knauth, 1998) and the sea water pH from weakly acidic in Early Hadean to nearly neutral for Late Hadean Ocean (Morse and Mackenzie, 1998). The salinity of the ocean was due to the weathering of the continental crust and oceanic hydrothermalism. Oceanic hydrothermalism was the main source of salinity during the Archean and Hadean periods as the continets were smaller and the major inputs of the dissolved species were predominant. The main anions were chloride and sodium, potassium, magnesium and calcium were the main cations.
SIPF or "salt-induced peptide formation" deals with the possible role of salts in the prebiotic chemical evolution (Suwannachot and Rode, 1999 and Rode et al., 1999). SIPF reactions used the Cu2+ ÂÂ-catalyzed condensation of Î±-amino acids in concentrated NaCl solutions. The presence of copper minerals in 'green zones' in Precambrian rock formations gives evidence that copper was available as Cu(II) (Cloud, 1973 and Rode 1999). The oxygen content of the primitive earth atmosphere was in the range of 10-15 to 0.1 of the present atmospheric level (Levine et al., 1982) which could keep the copper ions in its divalent state. Also the partial pressure of 10-35 atmosphere is sufficient to oxidize Cu(I) to Cu(II) (Ochiai, 1978).
Formation of Clay minerals their structure and properties
Clay minerals drew special attention in chemical evolution due to their wide range in variation of properties. Clays were probably common on earth before prebiotic chemistry (Nagy, 1975). They have evolved through sequence of steps and follow three primary mechanisms: (1) gradual separation and concentration of the elements from the pre-solar nebula; (2) an increase in range of physical variables like pressure, temperature, and H2O, CO2, and O2 activities; and (3) formation of far-from-equilibrium conditions by living systems (Hazen et al., 2008). Clays are formed as weathering products of volcanic ash as an alteration poduct of silicate minerals and also during diagenesis of sediments after the formation of an early hydrosphere (Ponnamperuma et al., 1982). It is believed that there were high levels of volcanic activity during that era (Fripiat and Cruz-Cumplido, 1974, Reynolds 1988, Ferris, 2006, Chamley, 1989 and Jackson, 1959). Alkaline conditions favor smectites under normal temperatures and pressures on Earth in the presence of Ca and favor mica in the presence of K, whereas acidic conditions support formation of kaolinite and hydrated silica (Chamley, 1989 and Jackson, 1959).
A model for the origin of life based on clay was proposed Cairns-Smith in 1985. The complex organic molecules arose gradually on a pre-existing, non-organic replication platform-silicate crystals in solution. Complexity in the molecules is increased due to the selection pressures on types of clay crystal. "Mother" crystals with imperfections were cleaved and used as seeds to grow "daughter" crystals from solution. The daughter crystals had many additional imperfections. According to the "clay-locale hypothesis of origin of life" life emerged from the clay as it provides environment that suits origin of life through 'self-organization' of monomers into the complex systems. Clay minerals are among the most competitors of the solid surfaces as they are widely distributed in geological time and space (Ponnamperuma et al., 1982).
Clays were thought to act as information-carrying crystals. During the evolution of the first crystalline primitive gene which is an inorganic mineral is genetically taken over by organic macromolecules (Cairns-Smith, 1965). It is interesting to note that clays may have played a key role in prebiotic chemistry, for example as catalysts (Brack, 2006). According to Bernal's hypothesis clay minerals play important role during the stages of chemical evolution on the primitive Earth (Ponnamperuma, 1982):
Clay minerals catalysed the synthesis of monomer from gaseous constituents of the primordial atmosphere (Yoshino et al., 1971).
Clay minerals adsorbed monomers on their surfaces, and provided highly concentrated local area in which the monomers had a specific orientation.
Clay surfaces activate monomers to form oligomers by providing matrix/templates for reacting monomers on their surfaces.
Lahav and white (1980) have pointed out that heterogeneous surface may provide greater selectivity than aqueous medium.
They could have played protected the biomolecules formed from the energy source (phytolysis) and hydrolysis from which they have being created. According to Weiss (1969) proteins adsorbed on clay is not attacked either by micro-organisms and hence protected from being decomposed. Cohn et al., (2001) also suggested the protective effect of clay minerals.
They may have some role in the origin of homochirality of amino acids (Degens et al., 1970, Jackson, 1971a, b and Julg and Ozias, 1988).
Many experiments have already been done involving the catalytic properties of clays, like (1) they have a high specific surface area, (2) have a specific distribution of surface charge, (3) are associated with exchangeable metal cations, (4) they have plasticity.
Clay minerals have two kinds of layer structure. They consist of 1:1 layers and 2:1 layers. Kaolin is 1:1 clay with tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra for e.g. kaolinite. Smectite is a three layered or 2:1 clay where two tetrahedral silica layers sandwiching a central octahedral layer for e.g. monontronite and nontronite. The individual layers of montmorillonite is negatively charged and separeated by vander waals force with interlayer containing ions like Mg2+, Ca2+, Na+ which balances the negative charge. This negative charge is neutralized by the presence of positive counterions Na +, K +, Ca 2+, Mg 2+, polynucleic A1 or Fe basic cations (Fripiat and Cruz-Cumplido, 1974 and Anderson and Banin, 1975).
In the montmorillonite clay the origin of the negative charge at all pH values above 2 or 3 and this is mainly in the octahedral rather than tetrahedral layer (Grim, 1953 and Amphlett, 1958). The charge on the clay lattice originates from one or more of the following reasons: Isomorphic substitution, lattice imperfections, broken bonds at the edges of the particle and disassociation of exposed hydroxyls. Isomorphic substitution involves the replacement of the metal ions of the lattice by cations of lower charge. Isomorphous substitutions in tetrahedral and octahedral sheets affect the properties of smectites. Unequivalent isomorphous substitution (e.g., AI (III) for Si (IV), Mg (II) and/or Fe (II) for AI (III)) causes net negative layer charges, which are compensated by hydrated cations in the interlayer spaces. Smectites are swelling clays and widely distributed on Earth and Mars while kaolinite is non swelling clay. Momorillonite has ability to adsorb water in the interlayer space. This water can interact with the interlayer cations or with the silicate layer surface inside the interlayer space, or with both.
In kaolinite, laminae do not disperse readily in water, while in montmorillonite hydrogen bonding is absent so, the laminae could separate more easily and take up a large quantity of water resulting in swelling of clay particles Good, (1973). Kaolinite has a low cation exchange capacity because of difference in bonding energy of the two different surfaces, the O" ions of the silica tetrahedron on one surface and OH- ions of the alumina octahedra on the other surface (Mitra and Rajgopalan, 1952).
The process of swelling involves the exchange of cations, the siloxane layer of montmorillonite or hydroxylated alumina layer of kaolinite (Swartzen-Allen and Matijevic, 1974).
Table1. Surface characteristics of montmorillonite and kaolinite.
Lattice stacking units (Si tetrahedral layers: Al octahedral layers)
Total specific surface area (m2g-1)
Average surface charge density (esu cm- 2)
3.3-4.3 x 104
2.3-5.9 x 104
Free swelling in water (cm3 g- 1)
* Depending upon exchangeable cation, CEC = Cation Exchange Capacity
Geological Martian History
The geological Martian history is summarized by Bibring et al., (2006) and Carr and Head (2010). Mars Noachian period occur around 4.1 to 3.8 Ga ago. The pre-Noachian period was characterized by a magnetic field and subjected to numerous large basin-forming impacts. The Noachian period ended around 3.7 Ga ago, was characterised by high rates of cratering, erosion and valley formation. Episodically surface conditions were like to cause widespread production of hydrous weathering products such as phyllosilicates. Extensive sulfate deposits accumulated late in the period and into the Hesperian. In late Noachian, activities such as large impacts, volcanic eruptions causes a major climatic change. The impact rates, valley formation, weathering and erosion all decreased whereas volcanism continued at a relatively high average rate in the Hesperian, particularly in the first half. Large water floods formed episodically, particularly in the later parts of the Hesperian, possibly leaving behind large bodies of water in the northern lowlands.
After the end of the Hesperian, around 3 Ga ago the pace of geologic activity slowed further. The average rate of volcanism during the Amazonian was lower than in the Hesperian by a factor of ten lower. Canyon development was largely restricted to formation of large landslides. Erosion and weathering rates remained extremely low. The most distinctive characteristic of the Amazonian is formation of features that have been attributed to the presence, accumulation, and movement of ice. Therefore, three sequential eras on the Mars due to the surface alteration were: (i) a nonacidic aqueous alteration, dominated by phyllosilicates (the ''phyllosian'' era); (ii) an acidic aqueous alteration, traced by sulfates (the ''theiikian'' era); and (iii) an atmosphere with non-aqueous alteration, traced by anhydrous ferric oxides (the ''siderikian'' era). The transition between the first two eras occurred when Mars global climate change from an alkaline, possibly moist, environment to an acidic environment. This change was driven by the extensive out gassing of volatiles, including sulfur, coupled to the volcanic activity that modeled Mars' surface (Bibring et al., 2006). The global change to an acidic environment would have been coupled with a rapid drop in atmospheric pressure. The decrease in the atmospheric pressure could be due to the heavy bombardment of Mars that could have resulted in an escape of its early atmosphere (Chassefière and Leblanc, 2004).
Chevrier and Mathé (2006) have reviewed the mineralogy of the surface of Mars. They have described a possible chemical and mineralogical evolution of the surface materials, resulting from weathering driven by the abundance and activity of water. Surface evolution is strongly affected by volcanism, meteoritic impacts, hydrothermalism and time.
Presence of phyllosilicates on Mars
Phyllosilicate-bearing deposits have been discovered on Mars using near infrared reflectance data from the Observatoire pour la Minéralogie, l'Eau, les Glaces, et l'Activité (OMEGA) and the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) (Poulet et al., 2005; Bibring et al., 2006; Mustard et al., 2008). Nontronite, Mg-rich smectites saponite and Fe-rich chlorites were detected throughout the most ancient Noachian terrains particularly in the Nili Fossae region which is located west of the large Isidis Basin and near the outflow channel of Mawrth Vallis (Poulet et al., 2005; Mustard et al., 2008; Mangold et al., 2007; Michalski et al. 2010 and Bishop et al., 2008). In Mawrth Vallis a common phyllosilicate observed is nontronite at the bottom, covered by a ferrous phase, montmorillonite and kaolinite (Bibring et al., 2005; Poulet et al., 2005; Bishop et al., 2008). Montmorillonite and kaolinite may have formed through leaching of Fe and Mg by change in aqueous chemistry. It could be concluded that the moist and probably warmer martian environment sustained during the first few hundred million years of its formation.
Presence of Carbonates on Mars
Earth-based extended telescopic measurements from 3.76 to 3.95 Âµm indicated the presence of carbonate on Mars (Pollack, 1990; Calvin, 1994, Bell et al., 1994 and Lellouch, 2000 Cloutis et al., (2010)). Carbonates are present in Martian meteorites and as a minor phase with <5% abundance in the Martian dust (Bridges et al., 2001 and Bandfield 2003). They were detected specifically as magnesium carbonates; in a regional- rock layer in the Nili Fossae region (Ehlmann et al., 2008) and their wide spread occurrences in both light and dark regions (Palomba et al., 2009).
Carbonates are weathering product of water and basalt in an atmosphere with CO2 (Gooding, 1978 and Catling, 1999). Presence of only small amount of carbonate among the other identified minerals suggested that a warm, wet early Mars with greenhouse gases other than CO2 was present or the formation of carbonate deposits were inhibited or destroyed by acidic aqueous activity (Fairén, 2004 and Bullock, 2007 and Fernández-Remolar et al., 2010).
Presence of Sulfates on Mars
Presence of sulfates were acknowledged in the Martian regolith at the landing sites of the Viking 1 and 2 (Clark et al., 1981 and Clark et al., 1982), of Mars Pathfinder (Reider et al., 1997; Foley et al., 2003), of MERA and MERB (Gellert et al., 2006; Wang et al., 2006). The OMEGA/ Mars-Express experiments identified sulfates-rich regions in the equatorial Valles Marineris canyon system (Bibring et al., 2005; Gendrin et al., 2005) and in the Northern polar area like in Juventae Chasma and the Iani ChaosLangevin et al., 2005; Fishbaugh et al., 2007, Meridiani and Gusev, 2008 and Kuzmin et al., 2009).
Three main types of sulfate minerals detected are kieserite, gypsum and polyhydrated magnesium sulfates. Gypsum's spectra were detected only within the Juventae Chasma and the Iani Chaos (Bibring et al., 2005; Gendrin et al., 2005). Wray et al., (2010) reported the detection of the Ca-sulfate bassanite (CaSO4Î‡0.5 H2O) on the outflow channel floor of Mawrth Vallis region. Polyhydrated sulfates were identified in the association with crystalline hematite in the chaotic terrain east of Valles Marineris. Erosion of the sulfate/hematite-bearing outcrops leaves the hematite behind and breaks the sulfates down to wind-transportable sizes (Noe Dobrea et al., 2008). These observations in correlation with hematite and polyhydrated sulfates also suggest an aqueous genesis on Mars. (Squyres et al., 2004 and Bibring et al., 2006). Sulphate evaporite beds in the Tithonium Chasma were located in the Valles Marineris region of Mars Wezel and Baioni, (2010).
Presence of Chlorides on Mars
Chlorides commonly precipitate during the evaporation of surface water or groundwater and during volcanic out gassing. Chloride-bearing materials were identified through 2001 Mars Odyssey Thermal Emission Imaging System on Mars. These chloride deposits are found throughout regions of low albedo in the southern highlands of Mars. The deposits are small but globally widespread, occurring in middle to late Noachian terrains with a few occurrences in early Hesperian terrains. The identification of chlorides in the ancient southern highlands suggests that near-surface water was available and widespread in early Martian history (Osterloo et al., 2008).
Energy Sources on the Prebiotic Earth
Any form of energy source is an important requirement on the primitive Earth by which monomers for the life can be synthesized. Deamer and Weber (2010) summarized various energy sources available on the prebiotic Earth. It includes photochemical energy from sun as ultraviolet light, atmospheric electric discharge, heat involved with volcanic discharge and geological electrochemical energy. They were relatively high energy sources to synthesize monomers. Another is relatively low energy reactions that incorporate condensation processes by which monomers forms polymers. It includes anhydrous heat (thermal energy), mineral-catalysed synthesis, and sugar-driven reactions. In case of thermal energy, potential reactants are trapped in a solid were the process of diffusion is retarded thereby the bond formation is limited between neighboring molecules. Sugars are reactive and contain considerable self transformation energy by which they react with ammonia, yielding many types of molecules. These sugar-driven syntheses require no additional source of chemical energy (Weber, 2000).
Sulfur chemistry is particularly relevant to the origin of life as there was minimum amount of molecular oxygen on the early Earth. Wächtershäuser (1988) proposed that when hydrogen sulfide reacts with iron in solution it produces iron sulfide. In the reaction electrons were generated that can be donated to the bound compounds. DeDuve (1991) proposed that the thioesters could have activated energy-dependant steps in primitive metabolic pathways as the thioester bond has an energy content equivalent to the pyrophosphate bond of ATP.
Sources of Amino Acids on the Prebiotic Earth
Simple molecules such as water, ammonia, methanol and formaldehyde were formed from atomic particles, which originated from nuclear fusion processes in stars of earlier generations. Among other compounds, amino acids were produced from these simple molecules in asteroids and on the early Earth.
The prebiotic Earth had two sources of amino acids: exogenous source and endogenous source.
Exogenous amino acids
The exogenous amino acids were likely synthesized in interstellar medium, on the meteorites, comets and Interstellar dust particles (IDPs) and transported to the Earth through impacts (Cronin and Pizarello, 1983, Botta et al. 2002, Llorca 2004; Llorca 2005; Brack 2007; Elsila et al. 2007). It was thought that the delivery of prebiotic compounds through impacts is less efficient as the impacts are associated with the high heat. But in recent reports it was shown that amino acids such as glycine could be yielded via "shock synthesis" from cometary ice (Goldman et al. 2010).
Exogenous amino acids
The endogenous amino acids could have been synthesized from several sources like: submarine hydrothermal vents, Urey miller type synthesis from volcanoes, lightning and mixture of gases (Ferris and Hagan 1984; Basiuk et al. 1998; Holm and Andersson 2005 and Simoneit, 2004).
Amino acids produced from the hydrothermal vents is a controversial topic as it is considered that in extreme conditions which persists hydrothermal vents like high temperature and pressure could have destroyed more amino acids than their formation (Bada et al., 1995).
First time Stanley Miller (Miller, 1953), demonstrated the synthesis of protein as well as non-protein amino acids formed by electrical discharges in reducing atmospheres (H2O, CH4, NH3, H2 and CO) of simulated primitive Earth conditions. This step is considered as a breakthrough in world of prebiotic chemical evolution. It is now considered that the early atmospheric composition of the Earth was not reducing but was redox neutral and dominated by CO2, N2 (Walker, 1977; Kasting, 1993 and Kasting and Brown, 1998). In contrast to the Urey-Miller finding it was shown that the syntheses of organic compounds with electric discharges under slightly reducing to redox neutral atmospheric conditions are less efficient (Abelson, 1966, Folsome et al., 1981, Schlesinger and Miller, 1983; Plankensteiner et al., 2004 and 2006). Cleaves et al. (2008) have shown that amino acids can be efficiently produced from neutral gas mixture. According to them the low yields were due to the low pH that inhibits the Strecker synthesis and oxidation of amino acids. Buffering with calcium carbonate prevented lowering of the pH by produced nitrite and nitrate in the spark discharge apparatus used and therby increasing the yield dramatically. Furthermore, addition of oxidation inhibitors such as ferrous ions prevented oxidative degradation of amino acids by nitrite and nitrate, increasing yield even further.
Amino acids to be used for simulation experiments
Amino acids are not only interesting as building blocks of proteins, they are also important for the formation of other "biomolecules" such as pyrroles and pyridines under the simulated conditions of the volcanic coast scenario (Yusenko et al., 2008). It has been shown that amino acids like glycine, alanine, Î²-alanine, valine, isovaline, norvaline, aspartic acid, Î²-aspartic acid, glutamic acid, serine, Isoserine, Î±-aminobutyric acid, Î±-aminoisobutyric acid, Î²-aminobutyric acid, Î²- aminoisobutyric acid, Î³-amino-n-butyric acid, phenyl alanine were formed during the electric discharge experiments (Miller, 1953, Miller, 1955, Schlesinger and Miller, 1983 and Johnson et al., 2008). The concentration of some non-protein amino acids like Î±-aminoisobutyric acid, Î³-aminobutyric acid, Î²-alanine, Î±-isovaline, Î²-amino-butyric acid could be greater than or similar to the concentration of glycine concentration (Zaia, 2008).
Simulation experiments were performed using glycine, alanine, Î²-alanine and valine as they were the most commonly formed and found in meteorites and also in Urey-Miller type of synthesis.
Glycine and its modifications
Glycine is the simplest amino acid and was observed as a major product in nearly all simulation experiments (Miller, 1953). Unlike other amino acids, it has no centre of chirality and is optically inactive.
Short heterochiral polypeptides with high glycine content are expected to have played a prominent role in evolution at the earliest stage of life before nucleic acids were available. In the solid state, glycine crystallizes in three forms (Î±, Î² and Î³). The polymorphs of the same compound in solids have different physical and chemical properties. Various polymorphs of glycine have been studied by x-ray (Marsh, 1958 and Iitaka, 1961). The other two polymorphs were introduced as Î´-glycine and Îµ-glycine under high pressure (Dawson et al. 2005).
The Î± form consists of hydrogen bonded double layers of molecules, which are packed by van der Waals forces (Marsh, 1958). It can be formed by spontaneous nucleation from pure aqueous glycine. Garetz and Matic (2002) produced the Î± polymorph from circularly polarized light.
The unstable Î² form, whose single molecular layers possess an internal arrangement same as the Î± form, readily transforms to the Î± form in air. Î²-glycine is formed by the precipitation from ethanol-water mixtures (Iitaka, 1959 and 1960).
The Î³ form crystallizes with a trigonal hemihedral symmetry. The Î³ glycine structure consists of helical chains of roughly parallel head-to-tail glycine molecules. These chains are hexagonally packed via lateral hydrogen bonds. It has been reported that growth of Î³-glycine crystals is achieved either by a slowly evaporating solvents (Kunhisha, 1974). The Î³-glycine can be formed by various methods like, in the presence of small amounts of sodium chloride (Bhat and Dharmaprakash, 2002 a and b), from the aqueous acetic acid or ammonia solution (Iitaka, 1961) or by the addition of compounds that inhibit the growth of Î± glycine, such as racemic hexafluoravaline (Weissbuch, 1994). They can also be produced by linearly polarized near-infrared laser pulses (Garetz and Matic, 2002). In order to see the difference in the thermal behaviour of Î± and Î³-glycine experiments were performed with both.
Thermal properties of amino acids
Sublimation of amino acids occur rapidly from theÂ solidÂ phase to theÂ gasÂ phase without passing through an intermediateÂ liquidÂ phase. It is an endothermicÂ phase transitionÂ that occurs at temperatures and pressures below a substance'sÂ triple pointÂ in itsÂ phase diagram.
Decarboxylation of amino acids releasesÂ carbon dioxideÂ from the carboxylic group, removing a carbon atom from a carbon chain.
When two separate amino acid molecules react, the condensation is termedÂ intermolecular condensation. The condensation of twoÂ amino acidsÂ forms theÂ peptide bond by a loss of water molecule commonly occurs on providing heat energy.
In intramolecularÂ condensation the condensation takes place between atoms or groups of the same molecule. For example differenrt small pyrroles were formed from the amino acids when heated at temperature 350 oC or more.
Deamination is the process by whichÂ amino acidsÂ are broken down releasing the amino groups. removed from the amino acid and converted toÂ ammonia.Â
Importance of amino acids and Peptides during the Origin of life
Life does not originated in one step rather in few consecutive steps. The first of which was the development of monomers that are the building blocks of life such as amino acids and nucleotides. Several abiotic ways for their formations were already described above.
Clay mineral catalysis shows the possibility of formation of bio-relevant macromolecules, but still many questions remain unsolved. The first of which refers to the formation of protein-like structures from the simple oligopeptides produced in the prebiotic scenario. To explain this time factor could be one difficulty. It takes only few days to produce the small peptides, whereas it may take much more time to assemble them to stable macromolecules (Rode, 1999).
Sidney Fox studied the spontaneous formation of peptide structures under the prebiotic Eatrh conditions. The amino acids spontaneously forms small peptides which could form closed spherical membranes, called protenoid microspheres, which show many of the basic characteristics of 'life' (Harada and Fox, 1958, Fox et al., 1974, Fox, 1968 and 1965).
It has been reported that small peptides (less than five amino acids) and amino acid can catalyse the asymmetric synthesis yielding sugars. (Cordova et al. 2005 a and b, Cordova et al., 2006 and Dziedzic et al., 2006 and Klussmann et al. 2006).
There is growing evidence that the RNA world (Crick, 1968; Orgel, 1968, Wattis and Coveney, 1999) must have been preceded by a simpler pre-RNA world made up of achiral constituents (Bada, 1995 and Nelsson et al., 2000). An alternative carrier of genetic code is peptide nucleic acids or PNA (Nielsen, 1993). Although PNA are chiral (Tedeschi et al., 2002), there are also achiral forms of PNA (Pooga et al., 2001). The PNA molecules acts primarily as a charge carrier, i.e. a very primitive functionality compared to the genetic code in contemporary cells (Rasmussen et al., 2003).
Homochirality associated with Origin of life
A key attribute of life which is also important for the origin of life is molecular handedness or chirality. The crucial organic molecules associated with life are chiral i.e. they possess non-superimposable 'right-' (D) or 'left-handed' (L) structures. Generally, the amino acid monomers occurring in proteins have only the L- configuration, while the ribose and 2-deoxyribose monomers in RNA and DNA are exclusively of the D-configuration.
There were some exceptions where D-amino acids sometimes to play an important role for example, bacteria releases the D-amino acids to decrease cell wall formation during a non-growth phase (stationary phase). Marine invertebrates (crabs, shrimps) may use D-amino acids for osmoregulation under adverse conditions. In humans also D-amino acids are absorbed by the intestine and travel to tissues where they are metabolized. Enzymes such as amino acid oxidase break down the D-amino acids to ammonia, hydrogen peroxide, and a keto acid, which can then be used by other enzymes to make L-amino acids (D'Aniello et al., 1993). D-amino acids may be important in the brain for neurotransmission and in tumor inhibition (Wolosker et al., 2008). Although D-amino acids may not be as common as their mirror image, but there importance cannot be not dismissed. Racemic mixtures of amino acids are a clear problem for origin of life.
There are certain theories proposed for the origin of homochirality. There are biotic (selection theory) and abiotic theories. According to biotic theories competing D- and L-organisms arose on the primitive Earth, and chance events eventually eliminated one of the species through the development of a 'killer enzyme' (e.g. a D-peptidase) and surviving only L-organisms (Balasubramanian, 1983, 1985). According to Bada and Miller (1987) the origin of chiral molecules on Earth must have occurred at the time of the origin of life or shortly thereafter. Chiral purity of polynucleotide is necessary for their complementarity in double-stranded helical structures as any L-sugars would distort them in such a way that the H-bonding between their bases would be prevented. Similarly, homochiral oligopeptides were more preferred to form helical structures (Joyce et al., 1984 and Schmidt at al., 1997) and any inclusion of D amino acids within an oligopeptide composed of L amino acids disrupts the latter helical arrangement due to the steric effects by side chains (Krause et al., 2000). Heterochiral peptides are capable of forming helical structures which with further could lead to epimerization of the minority units to the configuration of majority units in repetitive cycles (Green et al., 1999 and Green and Selinger, 1998).
Therefore, biogenic scenario for the origin of homochirality is not viable, since without preexisting chiral purity the self replication characteristic of living matter could not occur. Thus, it can be concluded that an abiotic mechanism was only possible for the primordial origin of homochirality.
According to abiotic theories, the development of molecular chirality and chiral homogeneity preceded the origin of life on Earth. It can be classified into categories, out of which chance mechanisms is the important. Chance mechanism is analogous to the flip of a coin where there is an equal probability of producing an excess of either the D- or L-enantiomer. It includes a model proposed by Frank (1953). According to this model an optical isomer were both a catalyst for its own production and an anti-catalyst for the production of its enantiomer. The efficiency of the reaction is high, because the amount of the catalyst increases during the reaction. Kondepuddi (1990) first time showed the spontaneous resolution on chiral symmetry breaking in Sodium chlorate (NaClO3). These crystals are optically active although the molecules of the compound are achiral. When crystallised from an aqueous solution statistically equal numbers of L and D NaClO3 crystals were found if the solution is not disturbed. When the solution was stirred, however, almost all of the NaClO3 crystals (99.7 percent) in a particular sample had the same chirality, either levo or dextro.
Other aspect of chance mechanism is homochirality produced by clay minerals such as kaolinite and montmorillonite. It was thought that the clay minerals have their enantio-differentiating properties. Degens et al. (1970) first reported that kaolinite catalysed the stereoselective polymerization of aspartic acid, with L-aspartic acid polymerizing over eight times as fast as the D-enantiomer. Jackson (1971a, b) subsequently repeated these claims and reported further that kaolinite preferentially adsorbed L- rather than D-phenylalanine. He suggested that the 'edge faces' of the kaolinite crystals were responsible for these stereoselective effects and that there might be a preponderance of such 'L-fixing' minerals in nature. Also, numerous substances like 1,1-binaphthyl derivatives and chiral sulfoxides have been resolved by the use of clays (Yamagishi, 1987). However, it is still unclear whether clay minerals are chiral or achiral in their crystal structure as the above results were not reproducible.
Abiotic stereoselectivity of amino acids is also shown by the Salt-Induced Peptide Formation reaction (SIPF) (Rode et al., 1992 and Plankensteiner et al., 2005) where it was found that the yields obtained for L-alanine were higher than for the D-enantiomer. Homodipeptides from L- and D-alanine showed an L-L preference over D-D by 10%. The geometry of the active complex is responsible for the stereoselectivity (Plankensteiner et al., 2004). Since the two axial water ligands are at an elongated distance due to Jahn-Teller distortion the equatorial 'plane' consisting of one chelating amino acid, one end-on amino acid or peptide, and one chloride ligand can be easily distorted towards a tetrahedron-like conformation. This leads to central chirality at the Cu(II) ion in addition to its relatively high inherent optical activity povided by parity violation. Chiral ligands like most amino acids or peptides induce an even stronger chirality at the copper centre. The Cu(II) complex, therefore, seems to act as a chemical 'amplifier' for the inherent small chirality due to parity violation. The stronger the distortion of the 'plane' towards a tetrahedron is, the higher is the central chirality at the Cu(II) centre and this can increase parity violating energy differences between the L-amino acid complex and its enantiomer.
Sandar (2003) proposed another model for abiotic homochiral growth of polymer and named it as "Toy model" for polymerization. The assumptions made in this model are that the polymerization of monomers of opposite handedness terminates further growth on the corresponding end of the polymer. This is referred to as enantiomeric cross-inhibition. However, if a polymer has reached an appreciable length, it will have catalytic properties promoting the production of monomers of the same chirality as that of the catalysing polymer. This model was revised by Brandenburg (2005) which incorporates the possibility of polymers to break at an arbitrary location. The polymers otherwise would grow to an infinite length which is unrealistic.
Enantiomeric excess from Space
Abiotically formed amino acids are racemic. An enantiomeric excess (ee) could have appeared in space, for example under the influence of circularly polarized light (Nishino et al., 2001 and Bonner and Rubenstein, 1987). Indeed, scientists found enantiomeric excesses on meteorites (Pizzarello and Huang, 2008, and Pizzarello and Cronin, 1999 and 1997). In less altered meteorites (CR meteorites) amino acids with small enantiomeric excess were found (Glavin and J. P. Dworkin, 2009) whereas in more modified meteorites like CM and CI meteorites higher enantiomeric excesses of some amino acids can be observed. The mechanism responsible for the production of large enantiomeric excess could be sublimation (Perry et al., 2007 and Feringa et al., 2007). In space the parent bodies (asteroids) could have a temperature gradient. Because of this, sublimation of amino acids could have occurred. During this sublimation, part of the parent body could get enriched with a higher enantiomeric excess. Feringa et al., (2007) studied the effect of sublimation of a wider range of amino acids. They prepared different enantiomeric excess of leucine by mixing the racemic DL compound with excess pure L enantiomer with different mixing methods like vigorous shaking, extensive grinding with a mortar and pestle or fully dissolving the compound in boiling water followed by removal of the solvent. They found that partially sublimed L-enriched leucine always gives highly enantiomeric enriched sublimates regardless of the mixing procedure. While Perry et al., (2007) found no selectivity in sublimation for alanine and other amino acids except serine (via octamer clusters). They have prepared ee % of amino acids by mixing the appropriate amounts of pure D and pure L enantiomer.
It was observed that (S)-a-(trifluoromethyl)lactic acid with initial enantiomeric excess of 74%, increased to 81% ee after closed and partial sublimation whereas the sublimate was only of 35% ee. Soloshonok et al., (2007) have also obtained reproducible results in open air. Clearly all the above independent research groups have different results. These different aspects have been scrutinized by Blackmond and Klussmann on phase-behavior models. The vapor pressure of enantiomers and of their mixtures may be markedly different and may chiefly depend on the enthalpy of decomposition of the racemic compound and on the temperature at which this process occurs (Farina, 1987 and Jacques et al., 1981). According to Blackmond (2009 and 2010) when the separate solid crystals (D or L) of amino acids are added to the solvent, the kinetics of two physical processes competes: 1) solution-solid (or gas -solid) equilibration; and 2) solid-solid equilibration. Amino acid forms an LD crystal at equilibrium, but before that equilibrium is established, the two separate crystals L and D each begin to dissolve without regard to the other. This results in behaviour more like that of a conglomerate (where each crystal comprises a single enantiomer, and there is no direct molecular interaction between L and D molecules) than that of a racemic compound. Thus during the transient initial regime, a system of separate D and pure L crystals acts as a ''kinetic conglomerate''before solution-solid and solid-solid equilibrium are attained. But, if scalemic samples is prepared from LD amino acid plus extra enantiopure amino acid (as in the Feringa, 2007), then the solution composition at the outset would reflect a value closer to the eutectic enantiomeric excess. Feringa et al., (2007) found 9-10% ee in leucine samples. In these samples solid-solid equilibration had already occurred prior to sublimation. In Perrys' experiment alanine samples were prepared from separate L + D crystals, it is likely that the slight depletion of ee observed for the sublimate occurred because the system was still struggling to emerge from its ''kinetic conglomerate'' state even while the sublimation process began. I have performed sublimation experiments of alanine using different enantiomeric excess at different time intervals to see the behavioural differences in enantiomer and racemate alanine crystals due to their intermolecular. The amino acids found in small but significant enantiomeric excess in meteorites are rare or unknown in our biosphere like isovaline (Pizzarello and Cronin, 1997). It is found around 18 % ee (Pizzarello et al., 2003) in Murchison meteorite and approximately 15 % ee in Orgueil meteorite (Glavin and Dworkin, 2009).
Tab.1: Î±-methyl amino acids from Murchison meteorite.
2-amino-2-methylbutanoic acid (Isovaline)
It has been shown that Î±-methyl amino acids those which cannot undergo racemisation can take part in synthesis of normal L amino acids under prebiotic conditions. The small preferences can be amplified into solutions with very high dominance of the L amino acids due to the difference in solubilities of the pure L form and DL racemic compound crystal (Levine et al., 2008 and Breslow and Cheng, 2009). Isovaline is Î±-methyl amino acid that cannot undergo such kind of racemisation reaction due to the absence of an H atom. This is also a non-proteinogenic amino acid therefore is very rare in terrestrial organisms therefore contamination is very unlikely. Whereas, the common Î±-H amino acids are found in almost racemic mixtures in meteorites as they are easily racemised in water solution through the action of heat or by the catalysis of metal ions. The mechanism involves the keto-enol tautomerism. Experiments of Isovaline were performed to look at the effect of Î±-H or methyl amino acids in context to sublimation.
Similarity between prebiotic Earth and primodial Mars
Liquid water is the essential requirement for life, and the evidence that there must have been significant liquid water habitats on early Mars. This fact is the motivating for considering the possibility of the origin of life on Mars. The suggestion that life may have originated on Mars is based on analogy to the Earth. Life on Earth requires liquid water; Mars had liquid water. Both early Earth and early Mars have sulfurous hydrothermal regions, active volcanism, large stable bodies of water, anoxic conditions, meteorite and cometary impacts, etc with the exception of tidal pools (McKay, 1987). In the above expected similarities, the major uncertainty would have been the duration of such environments on Mars compared with the time required for the origin of life (McKay 1986, McKay and Stoker, 1989). It could be assumed that the processes that may have been important for the formation of organic matter from a primordial atmosphere or the organics that come via comets would have occurred on both planets.