Side‐chain effect on conformation of ionizable polypeptides in aqueous solution

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Abstract
In order to study the effect of side‐chain length on the conformation of polypeptides, conformational changes of various ionic polypeptides with various lengths of side chain, poly‐Nε‐glutaryl‐L‐lysine (PGL), poly‐Nδ‐glutaryl‐L‐ornithine (PGO), poly‐Nε‐succinyl‐L‐lysine (PSL), and poly‐Nδ‐succinyl‐L‐ornithine (PGO), were investigated by ORD, potentiometric titration, and dilatometric measurements in aqueous solution. The results of optical rotation and potentiometric titration measurements indicate strongly that the α‐helix stability increases in the sequence PSO < PSL ∼ PGO < PGL, which corresponds to increased side‐chain length. The volume change associated with the helix–coil transition also increased in the above sequence. This series of polymers seems to be more hydrophobic compared with poly‐L‐glutamic acid or poly‐L‐lysine, as suggested from the values of enthalpy and entropy changes for coil–helix transitions.