Helix‐coil transition of poly(L‐glutamic acid) in N‐methylacetamide

Abstract

The folding of randomly coiled poly(L‐glutamic acid) to the helical state has been studied in N‐methylacetamide by titration methods. Since this solvent would be expected to form amide‐peptide group hydrogen bonds with the unfolded form of the polymer, to a first approximation no helix stabilization could come from intrapolymer hydrogen bonds. The titration data, collected from 30 to 70°C yield the following values per residue for the thermodynamic parameters governing the coil‐helix reaction for the uncharged polymer: ΔG30°C°, −1. 9 ± 0.1 kcal; Δ H°, 0 ± 0.1 kcal; ΔS30°C°, 6.3 ± 0.6 eu. In N‐methyl acetamide, the helix is an order of magnitude more stable than in water, and this stabilization appears to be entirely the result of the entropy gained by solvent molecules which are released from the polymer upon folding.

Applicability of broken-rodlike chain model to conformational analysis of polypeptide chain

“…It is confirmed from the potentiometric titration behavior 15 and the previous studies [11][12][13] that these two samples are in helical states.…”

“…This conclusion is compatible with the suggestions made by Holtzer et al 13 and others. 11,12 I am indebted to Professor M. Nagasawa of Toyota Technological Institute for valuable discussions.…”

“…From some studies with optical method, 11 potentiometric titration, 12 and intrinsic viscosity, 13 it is suggested that the latter polypeptide chain may form a much stronger helix than the former one.…”