Heme proteins: effect of an intermediate on photochemical behavior

This paper analyzes, in terms of a triangular kinetic scheme, the possible effects of an intermediate in the photochemical behavior of simple heme proteins. Both steady-state and transient phenomena are analyzed. The magnitude of the quantum yield for a given system is determined by a competition between quenching and ligand detachment, measured by the two rate constants, kappa(-1) and kappa(2), respectively; as a first approximation, it can be described by a single parameter, namely the ratio kappa(-1)/kappa(2). The individual values of kappa(-1) and kappa(2) can only be determined at light intensities so high that the measured quantum yield becomes itself a function of light intensity. Under these conditions the relaxation time for transient approach to the steady-state can be complex, corresponding to heavily damped chemical oscillations. It is pointed out that when the model is extended to include more than one intermediate, multiple oscillations are possible. In this more general case the system in the steady-state may also show pseudocooperativity or anticooperativity.