Room Temperature Phosphorescence and the Dynamic Aspects of Protein Structure

While the phosphorescence of aromatic chromophores in solution is normally quenched through diffusion of dissolved oxygen and other solvent-mediated processes, the phosphorescence of some proteins in solution is observed at room temperature. The tryptophan phosphorescence arises from residues which are hindered from interaction with oxygen by the folding of the polypeptide chains. Measurements of the phosphorescence lifetime of horse liver alcohol dehydrogenase (alcohol: NAD(+) oxidoreductase, EC 1.1.1.1) as a function of oxygen concentration indicate that internal tryptophan residues are periodically exposed to oxygen. This permits the calculation of rate constants for conformational oscillations in the enzyme. The present article illustrates the feasibility of employing phosphorescence in the study of proteins in solution in general and the utility of such experiments in probing the dynamic aspects of protein structure.