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

²H 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 Tₓ₁≈225 K (X₁) and Tₓ₂≈200 K (X₂). We show that neither X₁ nor X₂ are related to a fragile-to-strong transition. Our results rule out an existence of X₁. Also, they indicate that water performs thermally activated and distorted tetrahedral jumps at T<Tₓ₂, implying that X₂ originates in an onset of this motion, which may be related to a universal defect diffusion in materials with defined hydrogen-bond networks.