Far-Infrared Emission by Boson Peak Vibrations in a Globular Protein

The Raman- and neutron-scattering spectra of proteins and supercooled liquids display a common feature at low frequency, the boson peak. We elucidate its microscopic nature in relation to biological activity and the glass transition. Our experiments show that optical pumping of a heme protein leads to nonthermal emission in the far-infrared related to boson peak vibrations. The vibronic relaxation of the heme group channels energy into far-infrared modes which are damped with temperature and hydration. The results are consistent with a viscoelastic model of boson peak excitations.