Dynamics and Mechanisms of Ultrafast Fluorescence Quenching Reactions of Flavin Chromophores in Protein Nanospace

Abstract

We have studied excited-state dynamics of “nonfluorescent” flavoproteins including riboflavin binding protein (RBP), d-amino acid oxidase benzoate complex (DAOB), and others by means of femtosecond fluorescence up-conversion method and have observed ultrafast fluorescence quenching dynamics for the first time. We have interpreted the fluorescence quenching mechanisms of these flavoproteins as due to the ultrafast electron transfer (ET) to flavin chromophore (F) in the excited electronic state from nearby tryptophan (Trp.NH) or tyrosine (Tyr.OH) residues placed in the protein nanospace (PNS), on the basis of their X-ray structures. Extremely fast fluorescence quenching in RBP (τf ∼ 90−100 fs) could be attributed to the compact stacked arrangement, Trp.NH.....F.....Tyr.OH, supremely favorable for the ultrafast ET reaction dynamics. Comparisons of fluorescence time profiles and spectral characteristics of F in solution with those in PNS have indicated the existence of extremely fast FC (Franck−Condon) → Fl (fluorescence) state conversion in PNS within the time resolution of the apparatus. The ultrafast FC → Fl conversion may be a coherent process coupled with intra-chromophore high-frequency modes leading to formation of vibrationally nonrelaxed or only partially relaxed Fl state, from which barrierless ET seems to occur. Fluorescence dynamics of DAOB have indicated faster initial decay in both blue and red sides of the spectrum contrary to other flavoproteins which showed practically wavelength-independent fluorescence dynamics. This result of DAOB is similar to those of photoactive yellow protein and visual rhodopsin although their reaction mechanism (twisting) is different from DAOB (ET). We have proposed a possible mechanism for this fluorescence dynamics of DAOB on the basis of an extremely compact stacked configuration of F...benzoate-...Tyr.OH which seems to undergo moderate frequency intermolecular vibration coupled with intra-chromophore high-frequency modes of F in the course of ET from Tyr.OH to excited F.