The Dielectric Spectrum of Ubiquitin in Aqueous Solution

We report on frequency-dependent dielectric spectra (1 MHz < ν < 2 GHz) at 298.15 K of aqueous solutions of the globular protein ubiquitin at weight fractions of the protein up to wp = 0.03, corresponding to concentrations up to about 3.4 mmol dm-3. The spectra show a bimodal structure with two dominant dispersion/loss regimes near 20 MHz and 10 GHz, corresponding to the so-called β- and γ-processes found for other proteins. We establish a minor contribution near 100 MHz, which corresponds to the familiar δ-process. The static dielectric constant shows a comparatively large increment. By comparison with results from recent computer simulations, we confirm the usual interpretation of the β- and γ- processes in terms of protein tumbling and reorientation of bulk waters, respectively. The δ-process is attributed to protein−water cross interactions. The results contradict a recently developed microscopic theory in which cross interactions between hydration and bulk water due to exchange processes are responsible for β-relaxation.