Abstract
AbstractIn contrast to previous reports of other authors different 1H NMR chemical shifts were found for poly(L‐alanine) and poly(D,L‐alanine) in pure trifluoroacetic acid. The 13C and 15N NMR signals of poly(L‐alanine) are likewise different from those of poly(D,L‐alanine), and they are also different from the signals of the L‐alanine blocks in random copolypeptides such as (Ala/Gly)n, (Ala/Leu)n, (Ala/Val)n, and (Ala/Phe)n. It is concluded that poly(L‐alanine) is helical in trifluoroacetic acid (TFA); however, the optical purity, and thus, the helix stability is dependent on the reaction conditions used for the polymerization of L‐alanine‐NCA. The base initiated polymerization of L‐alanine‐NCA can cause substantial racemization in contrast to the primary amine‐initiated NCA polymerization. Sulfonic acids were found to be better solvents for polypeptides than trifluoroacetic acid, and their addition to TFA solutions of L‐alanine containing polypeptides exerts a strong influence on the chemical shifts and on the optical rotation. However, no clear experimental evidence was found for the helix‐coil transition of poly(L‐alanine) in TFA/sulfonic acid.
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