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Interprotein electron transfer (ET) reactions play an important role in biological energy conversion processes. One of these reactions, the ET between cytochrome [c.sup.2] (cyt) and reaction center from photosynthetic bacteria, is the focus of this theoretical study. The changes in the ET rate constant at fixed distances during the association process were calculated as the cyt moved from the electrostatically stabilized encounter complex to the bound state having short range van der Waals contacts in the tunneling region. Multiple conformations of the protein were generated by molecular dynamics simulations including explicit water molecules. For each of these conformations, the ET rate was calculated by using the Pathways model. The ET rate increased smoothly as the cyt approached from the encounter complex to the bound state, with a tunneling decay factor [beta] = 1.1 [[Angstrom].sup.-1]. This relatively efficient coupling between redox centers is due to the ability of interfacial water molecules to form multiple strong hydrogen bonding pathways connecting tunneling pathways on the surfaces of the two proteins. The ET rate determined for the encounter complex ensemble of states is only about a factor of 100 slower than that of the bound state ([tau] = 100 [micro]s, compared with 1 [micro]s), because of fluctuations of the cyt within the encounter complex ensemble through configurations having strong tunneling pathways. The ET rate for the encounter complex is in agreement with rates observed in mutant reaction centers modified to remove short-range hydrophobic interactions, suggesting that in this case, ET occurs within the solvent-separated, electrostatically stabilized encounter complex. encounter complex | protein complex | tunneling pathways

Studies done on the effect of replacing Glu-L212 with Asp and Asp-L213 with Glu in the Rhodobacter sphaeroides reaction center by monitoring the light-induced FTIR absorption changes associated with the photoreduction of QB is presented. The findings of the study indicate that there is a proton sharing among carboxylic acids in a system very similar to the native reaction center, which differs in the absorption frequency of protonating carboxylic acids and in the extent of proton sharing.

Research has been conducted on the proton transfer pathway to Q (sub)B. The study of this pathway in the reaction center from Rhodobacter sphaeroides has been carried out and the results demonstrate that the binding of Zn (super)2+ or Cd (super)2+ to the reaction center surface at His-H126, His-H128 and Asp-H124 inhibits the proton transfer rate to Q (sub)B.

Light in the photosynthetic bacterial reaction center (RC) induces electron and proton transfer reactions leading to double reduction and protonation of secondary quinone QB to QBH2. Current FTIR results from RCs with mutations at the H173 site and results from RCs with mutations at other L subunit sites, showed that Glu H173 and Asp L213 are mostly ionized in both QB and QB- states, making no significant contribution to proton uptake in native RCs at pH 7.

The interprotein electron transfer reaction between cytochrome [c.sub.2] (cyt) and the reaction center (RC) was explored to determine the mechanisms related to coupling association and electron transfer. The obtained results provide a quantitative model for coupling protein association with electron transfer and demonstrated the role of short-range interactions in determining the rate of electron transfer.

Fourier transform infrared (FTIR) spectroscopy and [super 18]O water substitution is used to identify water molecules perturbed by electron transfer to [Q.sub.A] and [Q.sub.B] in bacterial reaction center (RCs). The results revealed that the weakly hydrogen bonded water perturbed by quinone reduction might play a significant role in stabilizing the charge-separated state on the quinone and facilitate transfer of a proton to the [Q.sub.B] site.

The mutation to Ala of reaction center (RC) residues are performed to understand the role of short-range hydrogen bond interactions at the interface between electron transfer proteins cytochrome [c.sub.2] (cyt) and the RC from Rhodobacter sphaeroides. Based on the results it is concluded that short-range hydrogen bond interactions contribute to the close packing of residues in the central contact region between the cyt and RC near Asn m187 and Tyr L162.

The interaction between Ser-L223 and Q(sub B) and Q(sub B-) (where Q(sub B)=secondary quinone) isotope-edited Q(sub B-)/Q(sub B) FTIR difference spectra were measured in a mutant reaction center (RC) in which Ser-L223 is replaced with Ala and compared to the native RC. The result shows Ser-L223 is not hydrogen bonded to Q(sub B) or Q(sub B-) but forms a hydrogen bond to nearby acid group, Asp-L213.

Contacts between hydrophobic residues Tyr L162, Leu M191, and Val M192 on the RC and the surface of cyt are shown by the structure of the complex between cytochrome c2 (cyt) and the photosynthetic reaction center (RC) from Rhodobacter sphaeroides. The study correlates the changes in the binding and electron transfer rate.

The electrostatic interactions between Rhodobacter sphaeroides that function in intermolecular electron transfer in photosynthesis are examined. Results show that, although electrostatic steering is present, other short-range interactions must be present to contribute to the binding energy and to determine the structure of the complex.

The initial advances in the theoretical understanding of the interprotein electron transfer between cytochrome c2 and the photosynthetic reaction center in Rhodobacter sphaeroides is described from calculations based on the x-ray crystal structure. The results revealed the major impact of the charged residues at the RC surface region in binding.

The trapping and characterization of the semiquinon biradical Q(sub A)(super .-)Q(Sub B)(super .-), which forms an intermediate state in the photocycle of the bacterial photosynthetic reaction center from Rhodobacter sphaseroides is described. EPR study of this biradical at 9.6, 35 and 94 GHz allows to determine an accurate value of the dipole-dipole interaction and to estimate the exchange interaction J(sub o) between the radicals.

The reaction center from Rhodobacter sphaeroides uses light energy for the reduction and protonation of a quinone molecule, [Q.sub.B]. This process involves the transfer of two protons from the aqueous solution to the protein-bound [Q.sub.B] molecule. The second proton, [H.sup.+](2), is supplied to [Q.sub.B] by Glu-L212, an internal residue protonated in response to formation of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] In this work, the pathway for [H.sup.+](2) to Glu-L212 was studied by measuring the effects of divalent metal ion binding on the protonation of Glu-L212, which was assayed by two types of processes. One was proton uptake from solution after the one-electron reduction of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], studied by using pH-sensitive dyes. The other was the electron transfer [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. At pH 8.5, binding of [Zn.sup.2+], [Cd.sup.2+], or [Ni.sup.2+] reduced the rates of proton uptake upon [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] formation as well as [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] by [approximately equals] an order of magnitude, resulting in similar final values, indicating that there is a common rate-limiting step. Because [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] is formed [10.sup.5]-fold faster than the induced proton uptake, the observed rate decrease must be caused by an inhibition of the proton transfer. The Glu-L212 [right arrow] Gln mutant reaction centers displayed greatly reduced amplitudes of proton uptake and exhibited no changes in rates of proton uptake or electron transfer upon [Zn.sup.2+] binding. Therefore, metal binding specifically decreased the rate of proton transfer to Glu-L212, because the observed rates were decreased only when proton uptake by Glu-L212 was required. The entry point for the second proton [H.sup.+](2) was thus identified to be the same as for the first proton [H.sup.+](1), close to the metal binding region Asp-H124, His-H126, and His-H128.

A new study uses EPR to investigate the photocycle of bacterial photosynthetic reaction centers from Rhodobacter sphaeroides.

Quinones are essential cofactors in many physiologicalprocesses,among them proton-coupled electron transfer (PCET) in photosynthesisand respiration. A key intermediate in PCET is the monoprotonatedsemiquinone radical. In this work we produced the monoprotonated benzosemiquinone(BQH•) by UV illumination of BQ dissolved in 2-propanolat cryogenic temperatures and investigated the electronic and geometricstructures of BQH•in the solid state (80 K) usingEPR and ENDOR techniques at 34 GHz. The g-tensorof BQH•was found to be similar to that of the anionicsemiquinone species (BQ•–) in frozen solution.The peaks present in the ENDOR spectrum of BQH•wereidentified and assigned by 1H/2H substitutions.The experiments reconfirmed that the hydroxyl proton (O–H)on BQH•, which is abstracted from a solvent molecule,mainly originates from the central CH group of 2-propanol. They alsoshowed that the protonation has a strong impact on the electron spindistribution over the quinone. This is reflected in the hyperfinecouplings (hfc’s) of the ring protons, which dramatically changedwith respect to those typically observed for BQ•–. The hfc tensor of the O–H proton was determined by a detailedorientation-selection ENDOR study and found to be rhombic, resemblingthose of protons covalently bound to carbon atoms in a π-system(i.e., α-protons). It was found that the O–H bond liesin the quinone plane and is oriented along the direction of the quinoneoxygen lone pair orbital. DFT calculations were performed on differentstructures of BQH•coordinated by four, three, orzero 2-propanol molecules. The O–H bond length was found tobe around 1.0 Å, typical for a single covalent O–H bond.Good agreement between experimental and DFT results were found. Thisstudy provides a detailed picture of the electronic and geometricstructures of BQH•and should be applicable to othernaturally occurring quinones. [ABSTRACT FROM AUTHOR]