Conformation of sequential and random lysine–phenylalanine copolypeptides

The sequential copolypeptides (Lys-Phe-Lys)_n and (Lys₂-Phe-Lys)_n and a series of related random copolypeptides were investigated with respect of their ability to adopt the α-helix or β-conformation. Conformational transitions were induced by increasing the pH or by addition of NaClO₄ or methanol and were observed by recording the CD spectra. In contrast to the respective alternating copolypeptide (Phe-Lys)_n with its strong tendency for the β-structure reported previously, (Lys-Phe-Lys)_n can adopt either secondary structure, whereas (Lys₂-Phe-Lys)_n strongly favors the α-helix. Together with the random copolypeptides, whose composition varied from 20 to 50 mol % phenylalanine and whose average molecular weights ranged from 10,000 to 90,000, the influence of the phenylalanine content and of the chain length on conformational stability and the rotatory strength of the respective secondary structures were elaborated.