Abstract

It is believed that a protein molecule has a large number of conformational substates because of its complexity and flexibility [1]. Proteins in different substates have the same coarse structure and almost the same energy. Microscopically, however, they have slightly different structures and accordingly slightly different energies. At physiological temperatures, a protein molecule fluctuates among many substates, and it is said to be fluid-like. At sufficiently low temperatures, on the other hand, each protein molecule freezes in some substates, and it becomes glass-like. Such a liquid-glass transition behavior is expected to appear in the temperature dependence of optical spectra of proteins. From the analysis of the optical absorption spectrum, we have already shown that deoxymyoglobin exhibits glass-like behavior below about 250 K [2]. In this paper, we describe the temperature dependence of the absorption spectra of Zn-substituted myoglobin (ZnMb) in the temperature range between 4 and 300 K. By analyzing the data on the basis of the result of the site-selective fluorescence spectroscopy, we show that ZnMb undergoes a liquid-glass transition around 180 K.

© 1994 Optical Society of America