Dynamics at the protein-water interface from 17O spin relaxation in deeply supercooled solutions

Abstract

Most of the decisive molecular events in biology take place at the protein-water interface. The dynamical properties of the hydration layer are therefore of fundamental importance. To characterize the dynamical heterogeneity and rotational activation energy in the hydration layer, we measured the (17)O spin relaxation rate in dilute solutions of three proteins in a wide temperature range extending down to 238 K. We find that the rotational correlation time can be described by a power-law distribution with exponent 2.1-2.3. Except for a small fraction of secluded hydration sites, the dynamic perturbation in the hydration layer is the same for all proteins and does not differ in any essential way from the hydration shell of small organic solutes. In both cases, the dynamic perturbation factor is <2 at room temperature and exhibits a maximum near 262 K. This maximum implies that, at low temperatures, the rate of water molecule rotation has a weaker temperature dependence in the hydration layer than in bulk water. We attribute this difference to the temperature-independent constraints that the protein surface imposes on the water H-bond network. The free hydration layer studied here differs qualitatively from confined water in solid protein powder samples.

FIGURE 1

Temperature dependence of different contributions to the water ¹⁷O relaxation rate R₁ at 81.3 MHz for a 5.1 mM ubiquitin solution at pH 5.0, plotted on linear (left) and logarithmic (right) scales. The depicted R₁ contributions correspond to the three terms in Eq. 1, representing 10,000 bulk water molecules (○), 443 water molecules in the hydration layer (•), and a single internal water molecule (▪).

FIGURE 2

Temperature dependence of the ADPF ξ(ω₀,T) at 81.3 MHz for BPTI, ubiquitin, and BLG. The contributions from hydration water (light shading) and internal water (dark shading) are indicated. The curves were obtained by fitting the two model parameters ν and (Table 1) to the data (solid circles).

FIGURE 3

Temperature dependence of the hydration-layer DPF ξ_H(T,p) for BPTI (solid curves), ubiquitin (dash), and BLG (dash-dot). The three panels show the DPF for all N_H hydration waters (left), the 90% most mobile waters (middle), and the 50% most mobile waters (right).

FIGURE 4

Arrhenius plot showing the mean correlation time 〈τ〉 for the 50% (narrow shaded band) and 90% (wide shaded band) most mobile hydration waters and the correlation time τ₀ in bulk water (solid curve). The shaded bands each contain three nearly linear curves for BPTI, ubiquitin, and BLG.