Photosensitized Singlet Oxygen Luminescence from the Protein Matrix of Zn-Substituted Myoglobin

A nanosecond laser near-infrared spectrometer was used to study singlet oxygen (¹O₂) emission in a protein matrix. Myoglobin in which the intact heme is substituted by Zn-protoporphyrin IX (ZnPP) was employed. Every collision of ground state molecular oxygen with ZnPP in the excited triplet state results in ¹O₂ generation within the protein matrix. The quantum yield of ¹O₂ generation was found to be equal to 0.9 ± 0.1. On the average, six from every 10 ¹O₂ molecules succeed in escaping from the protein matrix into the solvent. A kinetic model for ¹O₂ generation within the protein matrix and for a subsequent ¹O₂ deactivation was introduced and discussed. Rate constants for radiative and nonradiative ¹O₂ deactivation within the protein were determined. The first-order radiative rate constant for ¹O₂ deactivation within the protein was found to be 8.1 ± 1.3 times larger than the one in aqueous solutions, indicating the strong influence of the protein matrix on the radiative ¹O₂ deactivation. Collisions of singlet oxygen with each protein amino acid and ZnPP were assumed to contribute independently to the observed radiative as well as nonradiative rate constants