13C Nuclear magnetic resonance study of salt induced conformational change of basic polypeptides: Poly (L‐lysine), poly (L‐arginine), and poly (L‐ornithine)

A 13C-nmr study of the salt-induced helix–coil transition of the basic polypeptides poly(L-lysine) [(Lys)n], poly(L-arginine) [(Arg)n], and poly (L-ornithine) [(Orn)n] was performed to serve as a reference of the helical portion of histones and other proteins. As is the case with pH-induced helix–coil transition, the downfield displacement of the Cα and carbonyl carbon signals are observed in the helical state. The upfield shift of the Cβ signals, on the other hand, is noted in the salt-induced transition. Regardless of the differences in the side chains and also the salts used, very similar helix-induced chemical shifts are obtained for (Lys)n and (Arg)n. However, the displacement of the Cα, Cβ, and carbonyl carbons of (Orn)n in the presence of 4M NaClO4 is found to be almost 50% of that of (Lys)n and (Arg)n. This is explained by the fact that the maximum helical content is about 50%, consistent with the ORD result. Further, the motion of the backbone and side chains of the helical from was estimated by measuring the spin-lattice relaxation time (T1), nuclear Overhauser enhancement (NOE), and line width. In the case of (Lys)n, the motion of the side chains is charged very little in comparison with that of the random coil. Indicating that the aggregation of the salt-induced helix is small in contrast to that of the pH-induced helix. For (Arg)n, however, the precipitate of the helical polymers is mainly due to aggregation.