From discrete multi-exponential model to lifetime distribution model and power law fluorescence decay function

Abstract

Experimental and theoretical studies of the fluorescence intensity decays in biomolecular systems showed that under constraints of typical experiment fluorescence lifetime distribution is given by gamma function, which led to a power-like decay function. The latter well fits complex (heterogeneous) as well as simple mono-exponential decays. Fluorescence decay kinetics is described by mean value of lifetime distribution characterizing the average rate of the excited-state decay, and one new parameter of heterogeneity (1 < q < 3/2) describing the relative variance of distribution, and objectively reflecting physical heterogeneity of the system. In the classical limit, when q → 1, the gamma distribution becomes the Dirac delta function, and decay function converges from power-like form to the single-exponential form. Numerous examples illustrate successful applications of this model to rational analysis of complex fluorescence decays of biomacromolecules, e.g., complexes ofE. colipurine nucleoside phosphorylase (PNP-I) with formycin A (FA, inhibitor) and orthophosphate (Pi, cosubstrate), which led to identification of the mechanism of deactivation of excited state and the N(2)H tautomeric form of FA selectively bound by PNP-I. The latter is of great importance for the studies of the mechanism of protein (enzyme) action as well as for more rational drug design.