Fingerprints of Amorphous Icelike Behavior in the Vibrational Density of States of Protein Hydration Water

The low-frequency modes of protein hydration water are investigated by inelastic neutron scattering. Experiments on both protonated and fully deuterated maltose binding protein samples allow us to unambiguously single out the contribution from water. The low-energy vibrational density of states of hydration water at 100 K is similar to the density of states of high- and low-density amorphous ice, and quite different from that of simple forms of crystalline ice. This result can be related to the picture of hydration water mass density depending on the protein surface curvature, which supports its glassy behavior.

Figure 1

Normalized spectra of hydrated powders. All curves are average of measured spectra for $2{\theta }_m=65°$. All manipulations of data (sums and subtractions) have been done before taking the averages. The line is the sum of $\mathrm{MBP}(\mathrm{D})\mathrm{\text{−}}{\mathrm{H}}_2\mathrm{O}$ and $\mathrm{MBP}(\mathrm{H})\mathrm{\text{−}}{\mathrm{D}}_2\mathrm{O}$.

Figure 2

Spectra of $\mathrm{MBP}(\mathrm{D})\mathrm{\text{−}}{\mathrm{H}}_2\mathrm{O}$, $\mathrm{MBP}(\mathrm{H})\mathrm{\text{−}}{\mathrm{D}}_2\mathrm{O}$ rescaled by 0.26 to estimate the contribution from exchangeable protons, and the resulting difference representing the contribution from protein hydration water.

Figure 3

(a) DOS of the protein hydration water at 100 K, ice $\mathrm{I}h$ at 37 K [33], ice $\mathrm{I}c$ at 160 K [33]. (b) DOS of the protein hydration water at 100 K, HDA ice at 75 K [34], and LDA ice at 75 K [34]. The line is the weighed sum of LDA and HDA contributions (see the text). In Refs. 33, 34 the DOS of $\mathrm{I}h$, $\mathrm{I}c$, LDA, and HDA ice were estimated from deuterated (${\mathrm{D}}_2\mathrm{O}$) samples under the incoherent approximation [40].