Dynamics of proteins at low temperatures: fibrous vs. globular

Abstract

We have measured quasielastic neutron scattering from H2O-hydrated collagen and haemoglobin at T≤270 K. The data consist of sets of nearly elastic peaks showing (i) Q,T-dependent decreases in window-integrated intensities Sqe(Q;T) proportional to effective Debye–Waller factors and (ii) small line-shape changes due to various types of proton motions with ns>τ>10 ps. Relative to haemoglobin, the 200-K dynamic transition is shifted upward by 20–25 K in collagen, and the T-dependence of m.-sq. displacements derived from Sqe(Q;T) suggests that in triple-helical systems there are three rather than two regimes: one up to around 120 K (probably purely harmonic), an intermediate quasiharmonic region with a linear dependence up to ≈240 K, followed by a steeper nonlinear rise similar to that in globular proteins.