Analyses of donor–acceptor distance-dependent rates of photo-induced electron transfer in flavoproteins with three kinds of electron transfer theories Fumio Tanaka a, * , Rong Rujkorakarn b , Haik Chosrowjan c , Seiji Taniguchi c , Noboru Mataga c a SC1-413, Department of Chemistry, Faculty of Science, Mahasarakham University, Mahasarakham 44150, Thailand b Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand c Institute for Laser Technology, Utsubo-Hommachi 1-8-4, Nishiku, Osaka 550-0004, Japan article info Article history: Received 23 February 2007 Accepted 5 March 2008 Available online 10 March 2008 Keywords: Photo-induced electron transfer Flavoproteins Analysis Electron transfer theories Donor–acceptor distance-dependent rate . abstract Reported donor–acceptor distance-dependent rates of photo-induced electron transfer from tryptophan (Trp), tyrosine (Tyr), and benzoate (Bz) to the excited isoalloxazine in ten flavoprotein systems were ana- lyzed with three kinds of electron transfer theories by Marcus, by Bixon and Jortner, and also by Kakitani, Yoshimori, and Mataga. Average donor–acceptor distances that were obtained from X-ray structures of flavoproteins were used for the analysis, rather than the edge-to-edge distance. The observed photo- induced electron transfer rates were best reproduced by the Kakitani, Yoshimori, and Mataga theory. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Photo-induced electron transfer (ET) phenomena are very important in physics, chemistry, and biology [1–5]. A number of works have tested ET theories [6–14], mostly from the standpoint of the free energy gap law [5,15,16]. Regarding other aspects, the donor–acceptor distance-dependent ET rates have been rarely ana- lyzed by these theories, especially in femtoseconds/picoseconds time domain, because of the difficulty to obtain molecular systems with various donor–acceptor distances in bulk solution. Flavoproteins play an important role in the oxidation–reduction reactions of microorganisms, fish, mammals, and plants [17]. Some flavoproteins function as photoreceptors [18]. Bright green fluores- cence of free flavins in aqueous solution is often significantly quenched as they combine with proteins. In these flavoproteins, aromatic amino acids such as tryptophan (Trp) or tyrosine (Tyr) are located near the isoalloxazine ring (Iso) of flavins. It was found by means of picosecond [19,20] and femtosecond-resolved [21] transient absorption spectroscopy that ET processes from Trp or Tyr to the excited Iso in flavoproteins were responsible for the quenching. Fluorescence decays of several flavoproteins, which were believed to be non-fluorescent, were observed in the time do- main of femtoseconds/picoseconds by a fluorescence up-conver- sion technique [22–27]. An empirical equation for ET rate was derived as a function of average distance, R, rather than edge to edge distance, between Iso and nearby Trp or Tyr in flavoproteins [25]. Angle between aromatic planes of Iso and Trp or Tyr did not seem to play essential role for the ET rates [25]. Flavoprotein systems are considered to be good models for the analyses of the ET theories, because the three-dimensional structures are known, and also because geometry between the donor and acceptor may be evaluated from them. In the present work, we analyzed the ob- served donor–acceptor distance-dependent ET rates in ten flavo- protein systems with three kinds of ET theories [6–14], and compared the results among them. 2. Method of analysis 2.1. Experimental data of ET rates in flavoproteins Fluorescence decay parameters at 22 °C, obtained by fluores- cence up-conversion method, except for D-amino acid oxidase dimer (DAO), and geometries between the donor and acceptor in ten flavoproteins reported, are summarized in Table 1. 0301-0104/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2008.03.005 Abbreviations: Iso, isoalloxazine; Trp, tryptophan; Tyr, tyrosine; Bz, benzoate; ET, photo-induced electron transfer; M theory, Marcus theory; BJ theory, Bixon and Jortner theory; KYM theory, Kakitani, Yoshimori and Mataga theory.  Corresponding author. Tel./fax: +66 43 754246. E-mail address: fukoh2003@yahoo.com (F. Tanaka). Chemical Physics 348 (2008) 237–241 Contents lists available at ScienceDirect Chemical Physics journal homepage: www.elsevier.com/locate/chemphys