Hydration-dependent dynamic crossover phenomenon in protein hydration water

The characteristic relaxation time $\tau$ of protein hydration water exhibits a strong hydration level $h$ dependence. The dynamic crossover is observed when $h$ is higher than the monolayer hydration level $h_c=0.2$–0.25 and becomes more visible as $h$ increases. When $h$ is lower than $h_c$, $\tau$ only exhibits Arrhenius behavior in the measured temperature range. The activation energy of the Arrhenius behavior is insensitive to $h$, indicating a local-like motion. Moreover, the $h$ dependence of the crossover temperature shows that the protein dynamic transition is not directly or solely induced by the dynamic crossover in the hydration water.

Figure 1

Quasielastic neutron scattering spectra of the hydration water at (a) $T=295$ K and (b) 235 K for the sample with $h=0.30$ (red open squares) and 0.45 (green open circles) at $Q=0.5$ ${\AA }^{-1}$. The fitted curves are denoted by solid lines. At 295 K, the protein hydration water at $h=0.30$ is seen to relax more slowly than the one at $h=0.45$. However, this ordering is reversed at 235 K.

Figure 2

Arrhenius plot of the mean characteristic relaxation time $\langle \tau \rangle$ for the samples with $h=0.30$ and 0.45 at (a) $Q=0.5$ (blue squares) and 0.9 ${\AA }^{-1}$ (orange circles) for the sample with $h=0.30$ and (b) $Q=0.5$ (green up triangles) and 0.9 ${\AA }^{-1}$ (magenta down triangles) for the samples with $h=0.45$. The dynamic crossover takes place at about 220 K in the former case and at about 235 K in the latter case. The result of Doster et al. [17] for the CPC with $h=0.3$ and at $Q=1$ ${\AA }^{-1}$ is also plotted in (a) and is denoted by + markers. (c) shows the $\langle \tau \rangle$ at $Q=0.5$ ${\AA }^{-1}$ for both cases of $h=0.30$ and 0.45. It is seen that the hydration water at $h$ = 0.45 exhibits a more visible dynamic crossover than that at $h=0.30$.

Figure 3

Quasielastic neutron scattering spectra of the hydration water along with the corresponding fitted curves (denoted by solid curves) at $h=0.18$ and $Q=0.9$ ${\AA }^{-1}$. The energy resolution function is denoted by a dashed line.

Figure 4

Arrhenius plot of the mean characteristic relaxation time $\langle \tau \rangle$ at $Q=0.5$ (black open squares) and 0.9 ${\AA }^{-1}$ (red open circles) for the sample with $h=0.18$. In this case, the $T$ dependence of $\langle \tau \rangle$ can be described by the Arrhenius relation in the measured temperature range.