Photosensitized
Singlet Oxygen Luminescence from the
Protein Matrix of Zn-Substituted Myoglobin
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Abstract
A nanosecond laser near-infrared
spectrometer was used to study
singlet oxygen (<sup>1</sup>O<sub>2</sub>) 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 <sup>1</sup>O<sub>2</sub> generation within the protein matrix. The quantum yield
of <sup>1</sup>O<sub>2</sub> generation was found to be equal to 0.9
± 0.1. On the average, six from every 10 <sup>1</sup>O<sub>2</sub> molecules succeed in escaping from the protein matrix into the solvent.
A kinetic model for <sup>1</sup>O<sub>2</sub> generation within the
protein matrix and for a subsequent <sup>1</sup>O<sub>2</sub> deactivation
was introduced and discussed. Rate constants for radiative and nonradiative <sup>1</sup>O<sub>2</sub> deactivation within the protein were determined.
The first-order radiative rate constant for <sup>1</sup>O<sub>2</sub> 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 <sup>1</sup>O<sub>2</sub> 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