Optical and IR absorption as probe of dynamics of heme proteins

The spectroscopy of horseradish peroxidase with and without the substrate analogue benzohydroxamic acid (BHA) was monitored in different solvents as a function of the temperature in the interval from 10 to 300 K. Thermal broadening of the Q(0,0) optical absorption band arises mainly from interaction of the electronic pi --> pi transition with the heme vibrations. In contrast, the width of the IR absorption band of CO bound to heme is controlled by the coupling of the CO transition moment to the electric field of the protein matrix. The IR bandwidth of the substrate free enzyme in the glycerol/H2O solvent hardly changes in the glassy matrix and strongly increases upon heating above the glass transition. Heating of the same enzyme in the trehalose/H2O glass considerably broadens the band. The binding of the substrate strongly diminishes the temperature broadening of the CO band. This result is consistent with the view that the BHA strongly reduces the amplitude of vibrations of the heme pocket environment. Unusually strong thermal broadening of the CO band above the glass transition is interpreted to be caused by thermal population of a very flexible excited conformational substate. The thermal broadening of the same band in the trehalose glass is caused by an increase of the protein vibrational amplitude in each of the conformational substates, their population being independent of the temperature in the glassy matrix.