Origins of Apparent Fragile-to-Strong Transitions of Protein Hydration Waters

${}^2\mathrm{H}$ NMR is used to study the mechanisms for the reorientation of protein hydration water. In the past, crossovers in temperature-dependent correlation times were reported at $T_{x1}\approx 225\mathrm{K}$ ($X_1$) and $T_{x2}\approx 200\mathrm{K}$ ($X_2$). We show that neither $X_1$ nor $X_2$ are related to a fragile-to-strong transition. Our results rule out an existence of $X_1$. Also, they indicate that water performs thermally activated and distorted tetrahedral jumps at $T<T_{x2}$, implying that $X_2$ originates in an onset of this motion, which may be related to a universal defect diffusion in materials with defined hydrogen-bond networks.