Helical poly(L‐alanine) in trifluoroacetic acid

In 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.