Analysis of photoinduced electron transfer in flavodoxin

Investigating the spectral and electrochemical properties of novel flavin‐pyrene dyads separated via variable spacer

The present manuscript describes the synthesis and the photophysical properties of a pair of novel flavin-pyrene dyads where the donor and the acceptor entities are separated via variable spacer. The dyads were well characterized using standard techniques and investigated for their photophysical and electrochemical nature. The observed absorption spectra of the dyads mainly display peaks corresponding to the individual pyrene and flavin units, with some contribution from the flavin entity in the pyrene region. While, strong emission quenching was observed for both the dyads if compared to its individual constituents. However, a careful analysis of the emission spectra and the solvent dependent studies reveals subtle difference between the two dyads. While no significant difference could be observed when excited in the flavin region; excitation at the pyrene region displays a weak and broad emission band in case of closely connected dyad. Further, the electrochemical properties were investigated by cyclic voltammetry and the reduction ability was observed to follow the trend as FlPy2 < FlPy1 < Fl.

Equivalence between inverted regions of the energy gap law and inverted regions of donor–acceptor distances in photoinduced electron transfer processes in flavoproteins

In the present work, we discuss about the relationship between the energy gap law and extended Dutton law in flavoproteins. The extend Dutton law is defined herein as the dependence of logarithmic rates (ln Rate) of photoinduced electron transfer (ET) from aromatic amino acids to excited isoalloxazine (Iso*) on donor–acceptor distances (Rcs). Both functions of ln Rate vs. negative values of the standard free energy gap and ln Rate vs. Rc display a parabolic behavior, when the ET rates are ultrafast. The negative values of the standard free energy gap at peaks of ln Rate [X_m(ES)] were obtained for FMN-binding protein, wild-type pyranose 2-oxidase, T169S (Thr169 is replaced by Ser) pyranose 2-oxidase, and medium-chain acyl-CoA dehydrogenase. The values of Rc at peaks of ln Rate [X_m(Rc)] were also obtained for these flavoproteins. The negative values of the standard free energy gap decreased with approximate linear functions of Rc. The negative values of standard free energy gap [X_m(ESRc)] at Rc = X_m(Rc) were evaluated using the linear functions of the negative standard free energy gap with Rc. The values of X_m(ESRc) were mostly in very good agreement with the values of X_m(ES). This implies that the energy gap law and the extend Dutton law are equivalent. X_m(ES) values in ET donors displaying the linear extend Dutton law with Rc were obtained by energy gap law, and then X_m(Rc) values were evaluated with the negative standard free energy gap. Thus, the obtained X_m(Rc) values were much smaller than the Rc range obtained by the method of molecular dynamics simulation. This suggests that ET processes with linear profiles of the extend Dutton law could be parabolic when Rc becomes much shorter than the Rc range obtained by the method of molecular dynamics simulation.

Relationship between EXDL and SEGL.

Comparative studies on picosecond-resolved fluorescence of d-amino acid oxidases from human with one from porcine kidney. Photoinduced electron transfer from aromatic amino acids to the excited flavin

Fluorescence dynamics of human d-amino acid oxidase (hDAAO) and its five inhibitors have been studied in the picoseconds time domain, and compared with one in d-amino acid oxidase from porcine kidney (pkDAAO) reported. The fluorescence lifetimes were identified as 47 ps in the dimer, 235 ps in the monomer, which are compared with those of pkDAAO (45 ps-185 ps). The fluorescence lifetimes of the hDAAO did not change upon the inhibitor bindings despite of modifications in the absorption spectra. This indicates that the lifetimes of the complexes are too short to detect with the picosecond lifetime instrument. Numbers of the aromatic amino acids are similar between the both DAAOs. The fluorescence lifetimes of hDAAO were analysed with an ET theory using the crystal structure. The difference in the lifetimes of the dimer and monomer was well described in terms of difference in the electron affinity of the excited isoalloxazine (Iso*) between the two forms of the protein, though it is not known whether the structure of the monomer is different from the dimer. Three fastest ET donors were Tyr314, Trp52 and Tyr224 in the dimer, while Tyr314, Tyr224 and Tyr55 in the monomer, which are compared to those in pkDAAO, Tyr314, Tyr224 and Tyr228 in the dimer, and Tyr224, Tyr314 and Tyr228 in the monomer. The ET rate from Trp55 in hDAAO dimer was much faster compared to the rate in pkDAAO dimer. A rise component with negative pre-exponential factor was not observed in hDAAO, which are found in pkDAAO.

Classification of the mechanisms of photoinduced electron transfer from aromatic amino acids to the excited flavins in flavoproteins

Emission wavelength-dependence of the relationship between logarithmic ET rate vs. donor–acceptor distance in pyranose 2-oxidase.

Structural heterogeneity among four subunits in pyranose 2-oxidase: A molecular dynamics simulation study

The homotetramer pyranose 2-oxidase (P2O) from Tetrametes multicolor contains flavin adenine dinucleotide (FAD) as a cofactor, and displays two conformers with different transient fluorescence spectra and lifetimes (ca. 0.1 ps and 360 ps). The ultrashort lifetimes of isoalloxazine (Iso) are ascribed to the photoinduced electron transfer (ET) from Trp168 to the excited Iso. Here, the structural heterogeneity among the four subunits in solution was studied by means of molecular dynamics simulation (MDS). The ET donor–acceptor distances in crystal and solution were compared. The distribution of the H-bond distances between Iso and the surrounding amino acids revealed appreciable differences among the four subunits. The structural fluctuations in two distant places were examined for the Iso-P and Iso-Q distances (where P and Q are Trp or Tyr) with the correlation coefficients between Iso-P and Iso-Q distances, revealing cooperative motions even though P and Q were more than 1 nm apart and located in different subunits. Moreover, distributions of the distances between Iso and its closest ionic amino acids markedly differed among the four subunits. Electrostatic (ES) energies between the Iso anion and the ionic amino acids in the entire protein were obtained using a static dielectric constant of 1. The ES energy in each subunit was strongly influenced by the other subunits, whilst the distributions of the ES energies greatly differed among the four subunits. This heterogeneous distribution of the ES energy between subunits may contribute to the large differences in the experimentally detected ET rates.

Photoinduced electron transfer modeling to simulate flavoprotein fluorescence decay

A method of analysis is described on the photoinduced electron transfer (PET) from aromatic amino acids as tryptophans (Trp) and tyrosines (Tyr) to the excited isoalloxazine (Iso*) in FMN-binding proteins (FBP) from Desulfovibrio vulgaris (strain, Miyazaki F). Time-dependent geometrical factors as the donor-acceptor distances are determined by means of a molecular dynamics simulation (MDS) of the proteins. Fluorescence decays of the single mutated isoforms of FBP are used as experimental data. The electrostatic (ES) energy between the photoproducts and ionic groups in the proteins is introduced into the Kakitani and Mataga (KM) model, which is modeled for an electron transfer process in solution. The PET parameters contained in the KM rate are determined by means of a nonlinear least square method, according to the Marquardt algorithm. The agreement between the observed and calculated decays is quite good, but not optimal. Characteristics on PET in flavoproteins, obtained by the present method, are described. Possible improvements of the method are discussed.