5. Poly-L-glutamic acid adopts a random coil structure at physiological pH. Under what pH conditions might poly-L-glutamic acid adopt an α-helical structure?

Poly-L-glutamic acid adopts an α-helical structure at conditions of pH 2.0 - 6.0

It becomes prominent & starts appearing properly at pH 5.0

Explanation:

The helix formation dynamics of poly-L-glutamic acids (PGAs) happen as thus.

The helix formation of PGA from random coil usually starts by initiating a pH jump from pH 8.0 to pH 4.9.

The structure of the γ-PGA solution is highly related to the deprotonated degree of the COOH groups in the γ-PGA side chains.

Generally the side chain or R group of glutamate contains carboxylic acid group with pKa of 4.25. At pH 5, side chain of glutamate is largely ionosed with a 1-charge. Above pH 5, this carboxylic acid is not as ionized and doesn't force the structure to adopt an α-helical structure.

The random coil structure comes back because the intramolecular hydrogen bonds in the γ-PGA-H structure decrease and intramolecular electrostatic repulsion increases as pH increases. Hence, at high pH degrees, the structure takes on a random coil structure.