Distinction between different competing kinetic models of irreversible intramolecular two-state excited-state processes with added quencher: global compartmental analysis of the fluorescence decay surface

Abstract

In this report we investigate how, even in the absence of any a priori information, one can distinguish between competing irreversible intramolecular two-state excited-state processes. Only two different kinetic models are possible for irreversible intramolecular two-state excited-state processes: (i) one with a unidirectional excited-state process and (ii) one without an excited-state process. The distinction between these models is based on the collection of a fluorescence decay surface with at least three quencher concentrations and the use of standard global biexponential and global compartmental analyses. Standard global biexponential analysis provides estimates for the preexponential factors and decay times which, together with the steady-state fluorescence spectrum, allow the construction of decay-associated emission spectra. Two series of global compartmental analyses have to be performed whereby (i) k01 is kept constant at various preset values while k12 is held fixed at zero and whereby (ii) k02 is scanned while k21 is held constant at zero. kol denotes the rate constant of deactivation of excited species i*, klz represents the rate constant of transformation of excited species 2* into 1*, and k21 describes the opposite process. If negative preexponentials-indicative of a unidirectional excited-state process-are obtained in the global biexponential analysis, the statistical goodnessof- fit criteria of the repetitive global compartmental analyses allows one to determine the direction of the unidirectional excited-state process. When only positive preexponentials are obtained in the global biexponential analysis, the decay-associated emission spectra should be compared to the species-associated emission spectra. If both sets of spectra coincide, no excited-state process occurs. Otherwise, two solutions in which one rate constant of interconversion is zero are mathematically possible. It is impossible to distinguish between these two nontrivial alternative solutions.