Interpretation of Fluorescence Decays using a Power-like Model

• Published in: Biophysical journal Vol. 85; no. 1; pp. 589 - 598
• Main Authors: Włodarczyk, Jakub, Kierdaszuk, Borys
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.07.2003; Biophysical Society
• Subjects: Biochemistry; Computer Simulation; Fluorescence; Formycins - analysis; Formycins - chemistry; Formycins - metabolism; Half-Life; Light; Models, Chemical; Proteins; Spectrometry, Fluorescence - methods; Spectroscopy, Imaging, Other Techniques; Tryptophan - analogs & derivatives; Tryptophan - analysis; Tryptophan - chemistry; Tryptophan - radiation effects; Tyrosine - analysis; Tyrosine - chemistry; Tyrosine - radiation effects
• ISSN: 0006-3495; 1542-0086
• DOI: 10.1016/S0006-3495(03)74503-2

A power-like decay function, characterized by the mean excited-state lifetime and relative variance of lifetime fluctuation around the mean value, was applied in analysis of fluorescence decays measured with the aid of time-correlated single photon counting. We have examined the fluorescence decay, in neutral aqueous medium, of tyrosine ( l-tyrosine and N-acetyl- l-tyrosinamide), and of the tyrosine residues in a tryptophan-free protein, the enzyme purine nucleoside phosphorylase from Escherichia coli in a complex with formycin A (an inhibitor), and orthophosphate (a co-substrate). Tryptophan fluorescence decay was examined in neutral aqueous medium for l-tryptophan, N-acetyl- l-tryptophanamide, and for two tryptophan residues in horse liver alcohol dehydrogenase. To detect solvent effect, fluorescence decay of Nz-acetyl- l-tryptophanamide in aqueous medium was compared with that in dioxan. Hitherto, complex fluorescence decays have usually been analyzed with the aid of a multiexponential model, but interpretation of the individual exponential terms (i.e., pre-exponential amplitudes and fluorescence lifetimes), has not been adequately characterized. In such cases the intensity decays were also analyzed in terms of the lifetime distribution as a consequence of an interaction of fluorophore with environment. We show that the power-like decay function, which can be directly obtained from the gamma distribution of fluorescence lifetimes, is simpler and provides good fits to highly complex fluorescence decays as well as to a purely single-exponential decay. Possible interpretation of the power-like model is discussed.