Heme protein dynamics studied by phosphorescence of an external phosphorescent probe molecule

The rate constant for quenching, kq, of the phosphorescence of 6-bromo-2-naphthyl sulfate (BNS) by cytochromes c, cytochrome c peroxidase, catalase, and myoglobin has been measured as a function of temperature and solvent viscosity. In aqueous solution at pH 7.0 for cytochromes c and myoglobin the value of kq is nearly equal to the rate constant for diffusional intermolecular contact, which is estimated from the value of kq for microperoxidase-11. For cytochrome c peroxidase and catalase kq is at least 350 times smaller than the rate of diffusional quenching, which shows that quenching of BNS phosphorescence occurs predominantly over the short distance between donor and acceptor. The mechanism for cytochrome c and myoglobin is found not to involve static quenching, deep penetration of BNS into the globin, or unfolding of the protein to allow contact between heme and BNS. It is concluded that quenching occurs by interaction of BNS with the exposed heme edge and by surface insertion of BNS into the protein to a depth sufficient for quenching by the unexposed heme. The effect of rapid-diffusional enhancement on kq is small. From a comparison of the results for the heme proteins, a model emerges that describes cytochrome c and myoglobin as having dynamic surfaces. Sufficient fluctuations persist to allow penetration of polyatomic probe molecules into the protein matrix, but the dynamics and/or interior microenvironment acts to increase resistance with increasing depth of penetration.