Proton magnetic resonance and optical spectroscopic studies of water‐soluble polypeptides: Poly‐ L ‐lysine HBr, poly( L ‐glutamic acid), and copoly( L ‐glutamic acid 42 , L ‐lysine HBr 28 , L ‐alanine 30 )

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‐CDCl 3 solvent. An upfield shift of ∼7 cps is observed for the α‐C H 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 acid 42 , L ‐lysine 28 , L ‐alanine 30 ). The width of the α‐C H 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‐C H 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.