Unusual conformational transitions of leucine‐containing basic polytripeptides

The conformational transitions of synthetic basic polytripeptides (Lys‐Leu‐Gly) n , (A 2 bu‐Leu‐Gly) n , (Lys‐Leu‐Ala) n , and (A 2 bu‐Leu‐Ala) n induced by high salt concentrations and elevated pH were investigated by CD, ir, and 1 H‐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 (A 2 bu‐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 (A 2 bu‐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. (A 2 bu‐Leu‐Ala) n shows a limited tendency to α‐helix formation.