Abstract
β-Helical poly(L-glutamic acid) in a gel state was found to be easily converted to the antiparallel β form by heating. Two β forms were obtained, depending on the temperature of heating. Temperatures between 40° and 85°C produced a β form with a spacing between pleated sheets (d001) of 9.03 Å, termed β1. If the heating was carried out at temperatures higher than 85°C, the β1 form underwent another conformational transition reducing the d001 value from 9.03 to 7.83 Å (termed β2) without any prominent change in the fiber repeat distance (i.e., the polypeptide backbone conformation). The time course of these two transitions was followed by measuring the infrared spectra of the samples, and it was concluded that the α → β1 transition in its initial stage obeys a pseudo-first order rate process with activation enthalpy and entropy of 54 kcal/mol and 92 eu, respectively. On the other hand, the typical sigmoidal conversion curves observed for the transition between the two types of β forms (β1 → β2) indicate that this transition proceeds via a socalled “nucleation and growth” process. The kinetic theory of phase transitions developed by Avrami can be applied with success to explain this transition. The infrared spectra, in the region from 1800 to 200 cm−1, were measured for these two β forms and the results showed that the conformation of the side chains and the mode of the hydrogen bonding between the side-chain carboxyl groups undergo appreciable change during the transition.
The heat-induced conformational transition of poly(L-Glu78 L-Val22) was also studied. The copolymer was transformed from the α-helical conformation directly to the β2 form. The reason for this was thought to be due to the fact that the L-valine residues and the L-glutamyl residues near the L-valine residues have a strong tendency to take the more compact β2 form.