Electric and hydrodynamic properties of polypeptides in solution. IV. A new conformational change of poly( L ‐glutamic acid) in dimethylsulfoxide–methanol mixtures

The electric birefringence of poly(L‐glutamic acid) (PLGA) in dimethylsulfoxide (DMSO)–methanol mixtures has been measured by use of the rectangular pulse technique. The length distribution curve, the mean molecular length, and the mean apparent permanent dipole moment of PLGA in solution have been obtained from the decaycurve and field strength dependence of the steady‐state birefringence according to the method developed for analyzing the electric birefringence of a polydisperse system. The length distribution curve exhibits one or two peaks. The length corresponding to a high peak and the mean length of PLGA undergo an abrupt change in the vicinity of 50 to 60 vol % DMSO at 30°C. Moreover, a sharp change of the Moffitt b0 parameter with the solvent composition is observed. These results provide evidence for the existence of a solvent‐induced transition from a helical conformation (presumably α‐helix) to another helical conformation with shorter length per amino acid residue. Further, the temperature dependence of the length distribution of PLGA in 50 vol % DMSO suggests the existence of a temperature‐induced helix ⇌ helix transition.