Question

Water, hydrogen sulfide and selenium hydride are three similar molecules, but only one is the solvent of life. With reference to their chemical structure and the associated intermolecular forces, describe the molecular basis for some of the analous properties of liquid water and explain the significance of this with reference to H bonding, Coulumbic and Hydrophobic interaction IN BIOMOLECULES??

Answer 1

Water is considered as solvent of life (H₂O). They form hydrogen bondings with each other and strongly polar compounds. The hydrogen bonding cause its many unique property i.e having a solid form less dense than the its liquid form, a relatively high boiling point of 100^o C and its high heat capacity.

The non covalent interactions depend entirely on the properties of water.:

1) Coulomb's law (Ionic interactions/charge-charge interaction) : According to Coulomb's law

F = k Q₁ Q₂ / r²

F = strength of interaction between charged particles, is dielectric constant, r is distance between charged group,

water is highly polar, high dielectric constant . They include attractive and repulsion forces.

Small ions in water influence the interaction between biomolecules by shielding the charges of biomolecules.

The strength of small ions can be calcukated by using ionic strength I = 0.5 (sum of M_i Z_I)

M_i is molarity Z_I is charge of i th ion

2) Hydrogen bonding : An interaction between a) hydrogen atom covalently bonded to electronegative group, electron donating group. (OH, NH)

b) lone pair of non bonded electrons on another electronegative group, acceptor group (CO, NH)

In biological system only oxygen and nitrogen can acts as hydrogen bond acceptor. As most of the biomolecules form hydrogen bonding due to its small energy requirement, they make stability of biomolecules.

Directionality of hydrogen bonding can be explained in the below image: attraction between partial negative charges is great when 3 atoms are involved and which lie in a straight line.

3) Hydrophobic interaction : Hydrophobic means fear of hydro (water). This effect involves in the association of non polar groups with each other in aqueous solution due to unfavourable interaction of non polar groups./molecules with water. For example protein folding which require free energy

Non polar molecules aggreagate in water because of entropy of water is increased through release of water molecules. This is hydrophobic effect which helps in promoting many biochemical process.