Homopolymeric Polypeptides

Spin-lattice relaxation times and 13C chemical shifts were used to study conformational changes of poly-L-lysine, which undergoes a coil-helix transition in a pH range from 9 to 11. In order to adopt a stable helical structure, a minimum number of residues for the formation of hydrogen bonds between the C = 0 and NH backbone groups is necessary therefore for the polypeptide dodecalysine no helix formation was observed. Comparison of the pH-dependences of the 13C chemical shifts of the carbons of poly-L-lysine and (L-Lys)12 shows very similar values for both compounds therefore downfield shifts of the a, / and peptide carbonyl carbons can only be correlated with caution with helix formation and are mainly due to deprotonation effects. On the other hand, a sharp decrease of the 7] values of the carbonyl and some of the side chain carbons is indicative for helix formation [854]. 

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Alternate Helical Structures. Since Pauling and Corey s description of the a helix [76], many other helical structures for polypeptides have been proposed and described. However, most of them are rare or found only in homopolymeric systems. These include the helix, the it helix, and the an helix. 

Quite recently,the anionic homopolymerization of a few substituted p-lactams and the copolymerization of some of the above pairs have been smdied in order to prepare polyamide 3-derived polypeptides displaying biological properties. The solution polymerization or copolymerization, initiated by li amide disubstituted with trrmethylsilyl groups and activated with 4-tert-butylbenzoyl chloride, does not have living character. From that study, some insights emerged into the reactivity of the above p-lactams in terms of their acidities, as well as electrophUidty of the imide end groups. 

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