Alanine has pK2 value of nearly 9.2 while polyalanine has pK2 value at 7. Why?

Answer 1

the pka value denotes the dissociation of an acid in numerically manageable terms. Alanine is a branched chain amino acid so the only thing that is ionising is the COOH of the backbone. On the other hand, polyalanine is a chain of alanines - a peptide chain - which will have many insoluble CH3s on the outside of the helix it will form, and again only ionise at the COOH of the backbone. This form is mole for mole less favourable to ionisation than single aminos.

Answer 2

The long carbon chain of the Polyalanine is an electron donor so it reduces the ability of the molecule to ionise.