Proton magnetic resonance and optical spectroscopic studies of watr-soluble polypeptides: poly-L-lysine HBr, poly(L-glutamic acid), and copoly(L-glutamic acid42, L-lysine HBr28, L-alanine)

The helix-coil transition has been studied by high-resolution NMR for three water-soluble polypeptides. Such systems are better models for protein behavior than those in TFA-CDCl₃ solvent. An upfield shift of ～7 cps is observed for the α-CH peak of poly(L -glutamic acid) and poly-L -lysine as the helix content increases over the transition. No such shift is found for copoly(L -glutamic acid⁴², L -lysine²⁸, L -alanine³⁰). The width of the α-CH peak for poly L-lysine increases rapidly as helix content rises but for poly L -glutamic acid and the copolymer, the width of this peak remains unchanged up to 60% helicity. This demonstrates a rapid rate of interconversion between helical and random conformations in partly helical polymer for the latter two polypeptides. All three polymers however, show no apparent α-CH peak at 100% helicity. Side-chain resonance lines also broaden as helix content increases and, to a greater extent, the closer the proton is to the main chain.