Broadened far-infrared absorption spectra for hydrated and dehydrated myoglobin

The far-infrared absorption spectra of myoglobin powder at hydration levels between 3.6 and 42 wt% were measured with terahertz time-domain spectroscopy and Fourier transform infrared spectroscopy. Absorption is dominated by the water content, but even the driest specimens 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 powders suggests that the absorption mechanisms are similar to those in liquid water, and hinders the spectroscopic identification of biomolecules in this frequency range.