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

Abstract

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(I₀/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.