Question

Consider the following pentapeptide: LTAYS. What is the maximum number of water molecules that can simultaneously Hydrogen-bond with the side chains (just the side chains, not the N- and C- termini)?

Answer 1

Answer-

In a water molecu;e, each hydrogen atom shares an electron pair with oxygen atom. The oxygen nucleus attracts electrons more strongly than hydrogen nucleus; oxygen is more electronegative. The sharing of electrons between H and O is therefore unequal; the electrons are more often in the vicinity of the oxygen atom than of hydrogen. THis unequal electron sharing creates two electric dipoles in water molecules. The oxygen atom bears partial negative charge, and hydrogen atom bears partial positive charge. When polar molecules are introduced into water, they form hydrogen bond with water molecule (one electropositive atom binds to other electronegative atom).

Polypeptide contains different classes of amino acids based on different side chains; hydrophobic amino acids ( gly, ala, val, leu, pro etc), aromatic amino acids (tyr, phe, trp), polar ( Ser, Thr, Gln etc), negatively charged (Asp, Glu), positively charged ( Arg, Lys, His). Hydrophobic amino acids don't form hydrogen bond with water.

Pentapeptide LTAYS stands for (L = Leucine, T = Threonine, A = Alanine, Y = Tyrosine, S = Serine). Among them alanine will not form hydrogen bond with water as it carries (-CH3) in its side chain and also Leucine which carries nonpolar side chains; other 3 molecules will form hydrogen bonds with water. So, 3 water molecules can form hydrogen bonds with pentapeptide LTAYS.

Picture is provided below: