Solution properties of synthetic polypeptides. V. Helix–coil transition in poly(β‐benzyl L‐aspartate)

Simple approximate expressions have been derived from the theory of Zimm and Bragg for use in the analysis of experimental data on the helix‐coil transition in polypeptide. On the basis of the resulting expressions practical procedures are proposed to determine two basic parameters characterizing a thermally induced transition, i.e., helix initiation parameter σ and enthalpy change for helix formation, Δ H . They have been applied to the data for poly(β‐benzyl L ‐aspartate) (PBLA) with the result: σ = 1.6 × 10 −4 and ΔH = −450 cal/mole for PBLA in m ‐cresol; σ = 0.6 × 10 −4 and Δ H = 260 cal/mole for PBLA in chloroform containing 5.7 vol‐% of dichloroacetic acid. This result gives evidence that σ may change not only from one polypeptide to another but also for a given polypeptide in different solvents. The change in limiting viscosity number [η] accompanying the transition was measured in the same solvents. The curve of [η] versus helical content had a relatively monotonic shape for the chloroformdichloroacetic acid solutions as compared with that for the m ‐cresol solutions, indicating that [η] depended largely on σ. Provided that [η] is a direct measure of the mean‐square radius of gyration, 〈 S 2 〉, the results are consistent with the theoretical predictions of Nagai and of Miller and Flory for 〈 S 2 〉.