Photochemical Dissociation in Optically Dense Solutions: Applications to Photolyzed Carboxymyoglobin (Mb⋆CO)

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Photodissociation of ligands has made important contributions to the understanding of function and structure of heme proteins. Here we present a theory for photochemical dissociation that is not limited by the assumption of previous analyses of optically thin samples, and apply it to interpretation of the photodissociated state of carboxymyoglobin (Mb⋆CO). Equations are derived and presented in terms of the effects of absorbance, [log10(I 0/I) = A, the probability of absorption of light quanta per unit surface area], and the potential for dissociation, D (maximum probability of photodissociation per unit surface area; a linear function in time of photolysis), for both monochromatic and polychromatic light sources. When monochromatic light is used, we show that for large absorbances (A > 2) the fractional photolysis increases as (log D)/A, and may appear to "saturate" even though well below completion. For polychromatic light intensities and absorbances, the theory predicts that the near-infrared tail of the absorbance band of carboxymyoglobin should be sufficiently transparent to allow the radiation to penetrate the sample, yet still have a significant absorptivity such that complete photodissociation is possible. An optically thick myoglobin-CO sample illuminated with a tungsten lamp was observed to behave somewhere between these two theories. These theoretical relations may be useful in the analysis of photolysis data from optically dense solutions and as a guide for future experimental design.

Keywords: Beer-Lambert relations; Infrared; Mathematical modeling, photolysis; Methods, analytical; Quantum efficiency; Techniques, spectroscopic; UV-Visible spectroscopy

Document Type: Research Article


Affiliations: 1: Department of Physiological Chemistry, Ohio State University, Columbus, Ohio 43210 2: Department of Physiological Chemistry, Ohio State University, Columbus, Ohio 43210; present address: Department of Physiological Chemistry, Ohio State University College of Medicine, 1645 Neil Avenue, Columbus, OH 43210

Publication date: January 1, 1985

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