Lysine polypeptide helix-coil transition

These severe requirements are met principally by ionic, water-soluble polypeptides in which there is little solvent change over the helix-coil transition and in which X = Xn. Poly-L-glutamic acid and poly-n-lysine, which... 

In summary, we have therefore seen that poly-L-lysine presents a valuable model for a partially helical polypeptide chain, one which is amenable to conformational analysis by optical rotatory dispersion. The method by which residues in a helical conformation may be discerned and counted against a background of disordered regions has been illustrated with this polypeptide under almost ideal conditions. The adequacy of the method is corroborated by copolymers a step closer to proteins in complexity, but some of the limitations that will be encountered in its application to proteins are already foreshadowed. Before this application is discussed, however, two other phenomena relevant to protein structure that are clearly exhibited in synthetic polypeptides, the helix-coil transition and the /3-conformation, will be considered. 

Another class of polymers equipped with azobenzene moieties comprises a-helical polypeptides, in particular pQly(L-glutamate)s and poIy(L-lysine)s. In solution, these azobenzene-modified polypeptides can undergo photoinduced helix-coil transitions. Polypeptides partially (30 to 50%) substituted with azobenzene moieties are surface active and form stable monolayers. Because of the partial substitution, there is sufficient free volume, and the azobenzene moieties can be isomerized in the monolayer. The photoisomerization changes the area per molecule, and the monolayer shows a photomechanical effect. LBK films of a photosensitive poly(L-lysine) with 31 mol... 

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]. 

In contrast to the spiropyran-modified polypeptides derived from poly(L-glutamic acid), the poly(L-lysine) analogs have been reported to show no photomodulation of the side-chain conformation in pure HFP, although they exhibited similar photochromic behavior (Figure 2).32,33 However, when appropriate amounts of triethylamine were added to the HFP solution, exposure to visible light resulted in the reversible formation of a helix by the poly(L-lysine) chains.35 Thus, addition of triethylamine to the HFP solution induced the coil - helix transition, but the amount of base necessary was different for the dark-adapted sample as compared with the irradiated one. The authors claimed... 

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