Quasielastic neutron scattering studies on couplings of protein and water dynamics in hydrated elastin

Performing quasielastic neutron scattering measurements and analyzing both elastic and quasielasic contributions, we study protein and water dynamics of hydrated elastin. At low temperatures, hydration-independent methyl group rotation dominates the findings. It is characterized by a Gaussian distribution of activation energies centered at about E_m = 0.17 eV. At ∼195 K, coupled protein-water motion sets in. The hydration water shows diffusive motion, which is described by a Gaussian distribution of activation energies with E_m = 0.57 eV. This Arrhenius behavior of water diffusion is consistent with previous results for water reorientation, but at variance with a fragile-to-strong crossover at ∼225 K. The hydration-related elastin backbone motion is localized and can be attributed to the cage rattling motion. We speculate that its onset at ∼195 K is related to a secondary glass transition, which occurs when a β relaxation of the protein has a correlation time of τ_β ∼ 100 s. Moreover, we show that its temperature-dependent amplitude has a crossover at the regular glass transition T_g = 320 K of hydrated elastin, where the α relaxation of the protein obeys τ_α ∼ 100 s. By contrast, we do not observe a protein dynamical transition when water dynamics enters the experimental time window at ∼240 K.