GLASS-TRANSITION OF ZN-SUBSTITUTED MYOGLOBIN PROBED BY ABSORPTION AND SITE-SELECTIVE FLUORESCENCE SPECTROSCOPIES

The temperature dependence of absorption and laser-induced fluorescence spectra including the resonance line has been measured for Zn-substituted myoglobin (ZnMb) in the temperature range between 4 and 300 K. The spectral shape observed below 180 K has been found to be well explained by assuming that the site-energy distribution function is temperature independent and is equal to that determined at 4 K. This may indicate that ZnMb is glass-like below 180 K. The spectral shape of the absorption spectrum at temperatures higher than 180 K has been explained by using a configuration-coordinate model, assuming the conformational substates are distributed thermally. This means that ZnMb is liquid-like in this temperature region. As a result, it is concluded that ZnMb undergoes a liquid-glass transition around 180 K.