Effect of pH on the Conformation of α-Helical Secondary Structures The unfolding of the α helix of a poly-peptide to a randomly coiled conformation is accompanied by a large decrease in a property called specific rotation, a meas-ure of a solution’s capacity to rotate circularly polarized light. Polyglutamate, a polypeptide made up of only L-Glu residues, has the α-helix conformation at pH 3. When the pH is raised to 7, there is a large decrease in the specific rotation of the solu-tion. Similarly, polylysine (L-Lys residues) is an α helix at pH 10, but when the pH is lowered to 7 the specific

All the proteins or polypeptides are formed of polymers of amino acids. These amino acids comprise of an alpha carbon, where a hydrogen, a carboxyl group, an amino group and a variable group R is attached. All these four groups have their specific stereochemistry which gives the polypeptide a particular shape in their coiled form.

Here in case of polyglutamate, the polymer is formed of chains of glutamic acid. This glutamic acid has carboxyl group in the R group which remains free even in the polymerized state. In acidic pH, the carboxyl group has its normal structure - COOH, but as the pH increase to 7,the hydrogen ion dissociates making it - COO⁻. So the stereochemistry changes and the specific rotation also changes.

Similarly in poly lysine, there's amino group in the R group which remains stable in alkaline pH of 10,but in case of neutral or acidic pH, the structure becomes - NH3⁺. So, the specific rotation changes.