| Absorption measurements were made of the heme protein myoglobin mixed with water from 1.2 to 98 wt% (weight percentage) in the frequency range 0.1-2.0 THz, using THz time-domain spectroscopy. It was found that the absorption is dominated by the water content, but even the driest specimens with hydration level below 4 wt% have a nearly continuous spectrum without identifiable sharp features. Inhomogeneous broadening plus the intrinsically high spectral density of vibrational modes in the region below 2.0 THz apparently combine to obscure the lowest frequency vibrational modes expected for protein molecules of this size. A continuous absorption spectrum for hydrated protein samples suggests that the absorption mechanisms are similar to those in liquid water, and hinders the spectroscopic identification of biomolecules in this frequency range.;The interaction of proteins with an aqueous environment leads to a thin region of "biological water" whose molecules have properties that differ from bulk water, in particular reduced absorption of far-infrared radiation caused by protein-induced perturbation of the water dipole moment. Based on the myoglobin far-infrared absorption measurements, the effect of biological water on myoglobin is carefully studied. Measurements show that absorption per protein molecule is increased by the presence of biological water. Analysis shows greater THz absorption when compared to a non-interacting protein-water model. Including the suppressed absorption of biological water leads to a substantial hydration-dependent increase in absorption per protein molecule over a wide range of concentration and frequencies, meaning that water increases the protein's polarizability. |
