Fluorescence Lifetimes of Chromophores Interacting with Biomolecules

Extrinsic fluorophores have been widely used in recent years to study biological macromolecules or structures because their optical properties are often affected unequivocally by the physical or chemical properties of the biomolecules to which they bind [1]. These studies add to those utilizing fluorophores as labels of specific biomolecules for e.g.cytometric quantitations of biomolecules or membrane permeation measurements. In our perspective the specific affinity of the dye for the biomolecule to be studied is a pre-requisite, whereas the main point of our investigations will be the interaction of the fluorophore with the biosubstrate which can influence either the radiative or the radiationless decay pathways from its excited singlet state S1. For the sake of simplicity we will first consider the case of a dye exhibiting a first-order kinetics for its decay from the S1 state when it is free in solution. In other words, we disregard complicating effects such as self-association or the coexistence of various ground-state configurations which can cause the chromophore to depart from simple kinetics even in solution.