Weak attractions in aqueous system? Hydrogen bonding in water.
Crucial role of weak attractions in macromolecular structure and function.
- Muhammad Sanain Javed
Water is also known as universal solvent because it is the only solvent which mostly use in almost every reaction. There are many types of weak attractions which are present in aqueous system.
The weak attractions which are present in the aqueous system are: Hydrogen Bonding Hydrophobic forces of attractions van der Waals forces of attractions Electrostatic forces of attractions
First of all we will discuss about hydrogen bonding which is present in the aqueous system. Oxygen is more electronegative element as compared to the hydrogen atom, so in this way we can say that the water is a polar molecule. So that there will be dipole-dipole attractions present between partial positively charged hydrogen atom and partial negatively charged oxygen atom. So that we can say that the hydrogen bonding is something more than that of simple dipole-dipole forces. The oxygen atom has two lone pairs. While on the other hand hydrogen has sufficient partial positive charge. In this way both the hydrogen atoms present in the water molecules produce strong electric field due to its small sizes. So that the other oxygen atoms, may be of other molecules, will get link to the hydrogen atoms by means of coordinate covalent bond, using one of its lone pair of electrons.
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Thus the loose bond is formed, in the result, which is definitely stronger than that of simple dipole -dipole attractions. Because of the small size of hydrogen atom it can take part in this type of bonding. This bonding acts as a bridge between two electronegative oxygen atoms. So that we can say that the hydrogen bonding is the electrostatic force of attraction between a highly electronegative atom and partial positively charged hydrogen atom.
Hydrogen bonding plays very important role in the water. We can say that it is the blessing of Allah. As the molecule of water have tetrahedral structure. Two lone pairs of electrons on oxygen atoms occupy two corners of the tetrahedron. In case of liquid state the molecules of water are extensively bonded with each other but these associations are break, just minute to minute because the mobility of molecules of water. When the temperature of the water decreases up to a limited extent then the ice is formed and the and the molecules of water become more and more regular and the regularity of water molecules extends through-out the whole structure of water. So in this way the empty space are created between in the whole structure. When the water freezes it occupies 9% more space as compared to the original water. So in this way its density decreases and ice floats on the surface of the water. The structure is just like to the structure of diamond because each atom of carbon in diamond is at the center of tetrahedron just like the atoms of oxygen of water molecule present in the ice. The lower density of ice at the temperature of 0 degree helps the water to float on the surface of water from upward to downward direction. When the water attains the temperature of 4C by the fall of temperature in the surrounding it starts to freezes. As the temperature of the surrounding more falls, the water at its surface become less dense. In this way the water on the surface will freezes and animals underlying he surface of the ice will survive.
Another important attraction present in the aqueous system is the hydrophobic attraction which is in fact the self association attraction present in the non polar molecules sometimes the hydrophobic attractions are incorrectly referred as hydrophobic bonds. Hydrophobic forces of attractions can easily be described as the interaction between the water and the hydrophobes. This can be explained as low water soluble molecules so in this way we can say that these are the non polar molecules having long chain carbon atoms and some alkyl groups which do not react with the water molecules.
Electrostatic forces of attractions:
Electrostatic forces of attractions are also present in the aqueous system. This force of attraction is present between the atoms or molecules having opposite charges. These types of forces are only exist in the aqueous solutions so sometimes these are also known as salt bridge. Salt bridge is referred as the electrostatic forces of attractions which are present between oppositely charged groups present within or between the bio-molecules. The strength of the salt bridge can be compared with the strength of the hydrogen bonding but act over larger distance. In this way they also facilitate the attractions of charged particles with the nucleic acids in aqueous system.
Van der Waals forces:
Van der Waals forces of attractions are also present in the aqueous system. This type of weak forces arises due to attraction between transient dipoles which are produce by the rapid movement of free electrons present in the atom or a molecule. These forces are much weaker than that of hydrogen bonding but on the other hand they are numerous, van der Waals force of attraction decreases as the sixth power of the distance separating atoms. So that due to this reason and due to weak force the van der Waals forces act over very short distance may be up to 2 to 4 aungstrom. Van der Waals forces contain two components one of them is the attractive forces which are commonly known as London dispersion forces. These attractive forces depend on the polarizability of the molecule or an atom. While on the other hand repulsive forces are also present which are commonly known as steric repulsion. This force of repulsion depends on the size of the atom. The force of attractions dominates almost at the longer distance. It is almost about 0.4 nm to 0.7 nm while on the other hand the repulsion dominates at very shorter distance. Van der Waals forces are very weak force and it can easily be broke. We can also say that this is a universal type of attractive force
because it can be generated easily between two molecules which are near to each other due to the rapid movement of their electrons.
Crucial role of weak attractions in macromolecules:
Weak attractions are crucial to macromolecular structure and functions because all the non covalent interactions such as hydrophobic attractions, ionic attractions and van-der Waals attractions are much weaker than that of the covalent attractions. Hydrophobic attractions are also much weaker than that of covalent attractions although they are highly strengthened by a highly polar solvent while on the other hand the hydrogen bonding may be compare able with the covalent bonding but the hydrogen bonding is always less stronger than that of the covalent bonding.
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All these four types of bonding such as hydrophobic attractions, ionic attractions, hydrogen bonding and van-der Waals forces of attractions are present in the aqueous system. The combined effect of all these four attractions play very important role in the formation of structure of the macromolecule. For example if we look upon the structure of the proteins, DNA and RNA then we can easily see that these bonding specially hydrogen bonding plays very important role in the formation of structure of the macromolecules. More these weak attractions present in the macromolecule hence the more will be the stability of the macromolecule. This shows the combine effect of the weak attractions in the structure of macromolecule. In case of proteins or even in case of nucleotides all the base pairs are attached with the help of these weak attractions such as hydrogen binding. If we see the attachment of adenine and thymine then we can easily observed that these are combined with each other with the help of two hydrogen bonds while on the other hand if we observe the attachment of guanine with cytosine then easily observe that these are also combined with the help of three hydrogen bonds. While in case of proteins two polypeptides are also combined with each other by the help of hydrogen bonding but only at specific
sites. In case of antigen antibody complex formation hydrogen bonding also plays very important role because antigen binds with the antibody with the help of hydrogen bond formation. Other weak attractions are also present in the macromolecules. In case of the enzyme catalytic reactions, when the substrate binds with the catalytic site of enzyme then the energy produced in this reaction is used the catalysis of the bio molecule. The substrate binds with the active sites of enzyme is also due to the formation of weak forces of attractions in the aqueous system. The binding of hormone or a neurotransmitter with the cellular receptor is also due to these weak forces of attractions.
When we see the structure of the protein such as hemoglobin then we can easily see that there are many water molecules which are tightly bounded and they look as the part of that protein. Same is true for the DNA or RNA so that in this way we can see that water it self and there related weak attractions play very important role in the stability of the macromolecule.
In this way we can say that the weak forces of attractions plays very important role in the stability of the macromolecules. So we can say that without these weak attractions in aqueous system the stability of the macromolecule must be impossible.
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