Question

Polyglutamate (a peptide consisting of all GLU residues) assumes an alpha helical structure at pH = 1.75 but a random coil arrangement at a pH greater than 5.0. Explain why this peptide assumes different structural conformations at the different pHs.

Answer 1

Glutamic acid consist of 2 acid group and in a peptide chain there will be numberous acid group mainly from sidechain. The pKa of these acid group are 2.19(terminal) and 4.25(side chain) . So at pH 1.75 no carboxylic acid will not get deprotonated and extensive hydrogen bonding is formed. Hence the peptide have a helical structure. At pH 5 all the carboxylic acid gets deprotonated and there will electrostatic repulsion between neighbouring carboxylate group and therefore the tend to be in random coil form.