Light-Induced and Thermal Relaxation in a Protein

After photodissociation of carbon monoxide bound to myoglobin, a thermally driven conformational relaxation towards the equilibrium structure of the deligated protein occurs above 160 K. Here we show that a relaxation can already be induced at lower temperatures by absorption of visible light. Both thermally induced and light-induced relaxations occur in discrete steps. The light-induced relaxation is characterized by a broad distribution of quantum yields. Similarities to spectral hole burning of dyes in glassy matrices suggest that the light-induced protein relaxation is initiated by electronic excitation of the heme group.