Proton magnetic resonance and optical spectroscopic studies of water‐soluble polypeptides: Poly‐L‐lysine HBr, poly(L‐glutamic acid), and copoly(L‐glutamic acid42, L‐lysine HBr28, L‐alanine30)

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Abstract
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.