A model for multiexponential tryptophan fluorescence intensity decay in proteins

Abstract

Tryptophan fluorescence intensity decay in proteins is modeled by multiexponential functions characterized by lifetimes and preexponential factors. Commonly, multiple conformations of the protein are invoked to explain the recovery of two or more lifetimes from the experimental data. However, in many proteins the structure seems to preclude the possibility of multiple conformers sufficiently different from one another to justify such an inference. We present here another plausible multiexponential model based on the assumption that an energetically excited donor surrounded by N acceptor molecules decays by specific radiative and radiationless relaxation processes, and by transferring its energy to acceptors present in or close to the protein matrix. If interactions between the acceptors themselves and back energy transfer are neglected, we show that the intensity decay function contain 2N exponential components characterized by the unperturbed donor lifetime, by energy transfer rates and a probability of occurrence for the corresponding process. We applied this model to the fluorescence decay of holo- and apoazurin, ribonuclease T1, and the reduced single tryptophan mutant (W28F) of thioredoxin. Use of a multiexponential model for the analysis of the fluorescence intensity decay can therefore be justified, without invoking multiple protein conformations.

Time-resolved fluorescence analysis of the mobile flavin cofactor in p-hydroxybenzoate hydroxylase

“…The considerably lower amplitude of ultrafast quenching of PHBH in comparison with the enzymes mentioned above may reflect the increased freedom of the flavin cofactor: in the fraction of enzyme molecules in which close contact exists between the properly oriented flavin and tyrosine at the moment of excitation, ultrafast fluorescence quenching can occur. In the fraction of enzyme molecules responsible for the longer fluorescence lifetime components, quenching occurs through interactions with other residues, in a similar manner as observed for the PHBH Y222 mutants (this refers to the so-called multiple quenching sites model 1,49 ). Based on the distance to the flavin, Trp185 as well as Tyr201 and Tyr385 (the latter two residues are both involved in substrate binding) may also contribute to fluorescence quenching.…”

A long lifetime component in the tryptophan fluorescence of some proteins

“…The heterogeneity has even been extended to cover a distribution of lifetimes around each mean lifetime . Another interpretation has been given by neglecting different substates and instead invoking different mechanisms of energy transfer in one state (Bajzer and Prendergast 1993). The concept of substates, however, gains more and more support, both from experiment and from computer simulations.…”

Effect of external electric field on ground and singlet excited states of phenylalanine: A theoretical study

“…Among the naturally occurring amino acids; phenylalanine (Phe), tryptophan (Trp) and tyrosine (Try) possess fluorescent aromatic chromophores. The fluorescence spectra of aromatic amino acids depend on the surrounding environments and thus the measurement of their fluorescence may be used to study different aspects of protein structure [7][8][9]. Phenylalanine is the only aromatic amino acid present in many native biopolymers [10].…”