NONPHOTOCHEMICAL HOLE-BURNING IN ZN-SUBSTITUTED MYOGLOBIN

Persistent holes were burned in the inhomogeneously broadened 595 nm absorption band of Zn-substituted myoglobin (ZnMb) at 2 K. A prominent laser-induced hole-filling effect was observed, which suggests that the burning mechanism is non-photochemical. Zn-substituted protoporphyrin IX and the surrounding polypeptide chain in this material are considered to correspond to a localized center and a glassy host in ordinary hole-burning materials. The phonon sideband hole showed a maximum around 25 cm-1 from the zero-phonon hole, and the shape of the sideband was almost the same for the freeze-dried ZnMb and for ZnMb dissolved in a glycerol-water (3:1) mixture. This result indicates that the phonon sideband represents the low frequency vibrational modes in the protein molecule. The time evolution of the hole depth during the burning process was found to obey a logarithmic law, which can be explained by assuming a large dispersion of the burning rate. The distribution of the conformational barriers between nearly-degenerate ground states was also determined by using a heat cycle method. The distribution obtained obeys a power law which has no characteristic value of energy. All of the above results suggest the glass-like character of protein molecules. © 1990.