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Unusual conformational transitions of leucine‐containing basic polytripeptides

The conformational transitions of synthetic basic polytripeptides (Lys‐Leu‐Gly)n, (A2bu‐Leu‐Gly)n, (Lys‐Leu‐Ala)n, and (A2bu‐Leu‐Ala)n induced by high salt concentrations and elevated pH were investigated by CD, ir, and 1H‐nmr spectroscopy, sedimentation analysis, viscometry, and light scattering. Sheet aggregates of chains in a conformation similar to the polyglycine II (polyproline II) helix, bound together by hydrogen bonds, are the most probable form of (Lys‐Leu‐Gly)n and also, partly, of (A2bu‐Leu‐Gly)n in a high‐pH or high‐salt solutions. The conformation (Lys‐Leu‐Ala)n, in a low‐salt concentration, is an α‐helix. Since (A2bu‐Leu‐Ala)n is disordered under similar conditions, it appears that this α‐helix is stabilized by hydrophobic interactions between Lys and Leu side chains. In a high concentration of water structure‐making ions, CD data for (Lys‐Leu‐Ala)n indicate distortion of the α‐helix, with a parallel increase in the average molecular weight corresponding to trimer formation. Hydrodynamic data are consistent with a model of bundles of three closely touching spherocylinders. (A2bu‐Leu‐Ala)n shows a limited tendency to α‐helix formation.