Isotope effect on electron transfer in reaction centers from Rhodopseudomonas sphaeroides

Abstract

Previous ENDOR studies on reaction centers from Rhodopseudomonas sphaeroides have shown the presence of two hydrogen-bonded protons associated with the primary, ubiquinone, acceptor QA. These protons exchange with deuterons from solvent 2H20. The effect of this deuterium substitution on the charge-recombination kinetics (BCh nsl)Q-(BChl)2QA has been studied with a sensitive kinetic difference technique. The electron-transfer rate was found to increase with deuterium exchange up to a maximum Ak/k of 5.7 + 0.3%. The change in rate was found to have an exchange time of 2 hr, which matched the disappearance of the ENDOR lines due to the exchangeable protons. These results indicate that these protons play a role in the vibronic coupling associated with electron transfer. A simple model for the isotope effect on electron transfer predicts a maximum rate increase of 20%, which is consistent with the experimental results.The primary photochemistry in bacterial photosynthesis involves light-induced electron transfer in a bacteriochlorophyll (BChl) protein complex called the reaction center (RC) (for reviews, see refs. 1-3). The absorbed photon excites a primary donor species, a BChl dimer, which transfers an electron to a bacteriopheophytin (BPh) molecule. Subsequent electron transfer occurs from BPh to a primary quinone, QA, and then to a secondary quinone, QB.The structural basis for electron transfer in bacterial RCs has been investigated by a wide variety of biochemical and biophysical techniques, including the recent x-ray crystallographic structure determination by Deisenhofer et al. (4,5) of the RC from Rhodopseudomonas viridis. The structure ofthe Rhodopseudomonas sphaeroides RC used in this study is assumed to be very similar to that of R. viridis. This assumption is based on the amino acid sequence homology between the two RCs (6-8) and the preliminary electron density mapping of the RC from R. sphaeroides by the molecular replacement method (9). The components of the electron-transfer chain (BChl dimer, BPh, and QA) are arranged in series across the membrane. The secondary quinone, QB, is related to QA by an approximately 2-fold symmetry axis. An Fe2+ coordinated to four histidine residues is located between the two quinones.The success of the structural studies of bacterial RCs has motivated us to investigate the structural basis for the electron-transfer reactions. Theories of electron transfer require the coupling of electron transfer to vibrational motion (intermolecular or intramolecular) of the molecules involved in the reaction (10)(11)(12)(13)(14). This coupling is necessary to match the energies of the product and reactant state (i.e., to form the transition state for electron transfer). The coupling enables the excess energy of the reactant state to be given off to the lattice. The goal of this study was to identify the vibrational modes that play a role in the electron transfer. The approach that was adopted is based on the effect of isotope substitution on the rate of electron transfer.Rece...

Primary photochemistry of reaction centers from the photosynthetic purple bacteria

“…sphaeroides reaction centers indicate both carbonyl groups maybe hydrogen bonded when QA is reduced [87]. When the two protons hydrogen bonded to QA are replaced with deuterons, the time constant for the P+ QA --* PQ back reaction is changed [107]. It is possible that motions involving these hydrogen-bonded protons could also be important for electron transfer from BPh-to QA.…”

Quantum effects in biology: golden rule in enzymes, olfaction, photosynthesis and magnetodetection

“…As is the nuclear environment (the nuclear spin) which is key to coupling to the D/A states (equation (13)) for the asymmetry in reaction rates. Intriguingly, although protein environments (e.g.…”

“…for deuterium versus hydrogen (D/H) and tritium over hydrogen (T/H). So isotopic substitution allows a test for how well the observed rates can be predicted in a KIE [11][12][13]:…”

Proton transfer in bioenergetic proteins

“…The rate of P + Q A − → PQ A electrontransfer, k PA was found to increase slightly with deuterium exchange up to a maximum k PA (D + )/k PA (H + ) = 1.06 [22]. The solvent isotope effect indicates that these protons play a role in the vibronic coupling associated with electron transfer of charge recombination.…”

“…where k' on is the bimolecular rate constant of proton binding (values of 2-6·10 10 M -1 s -1 are commonly found for neutralization of strong bases) 22 and k off is the rate constant of proton dissociation. The ratio K H = k off /k' on gives the proton dissociation constant.…”