Probing the solvation of the α-helix with extended amide III bands in Raman optical activity

Experimental and theoretical Raman optical activity (ROA) study of alpha-helical peptides and proteins has suggested that the relative intensity of two extended amide III ROA bands at similar to 1340 cm(-1) (I band) and similar to 1300 cm(-1) (II band) can be used to monitor the permittivity of the surrounding medium of the alpha-helix. So far, the ROA intensity ratio, I-I/I-II, has been interpreted from two different viewpoints. The first one is in terms of a direct effect of permittivity around the alpha-helix. The second one is based on a structural equilibrium of two types of alpha-helical structures, "hydrated" and "unhydrated" ones. In the present study, temperature- and solvent-dependences of I-I/I-II are measured for highly-alpha-helical peptides and compared to the theoretical spectra while varying the permittivity or the type of alpha-helical structure. A fragment method with partial optimization in the normal modes is adopted in density functional theory calculations. The main features of the experimental spectra and a trend of the observed I-I/I-II are well reproduced by the simulations, which leads us to a conclusion that the I-I/I-II is dominantly governed by a direct influence of the permittivity of the environment and just accessorily by the equilibrium of the two types of alpha-helices. The simulations also opposed the conventional assignments of the I and II bands to "hydrated" and "unhydrated" alpha-helical structures, respectively. In the case of alpha-helical proteins, solvent exposure of the alpha-helix may be monitored by the ROA ratio.