Theg-tensor anisotropy of the triplet state of the primary electron donor in the photosynthetic bacteriumRhodobacter sphaeroides by high-field (95 GHz) EPR

Abstract

The anisotropy of the g-tensor of the light-induced triplet state of the primary electron donor (D) of the photosynthetic bacterium Rhodobacter sphaeroides is determined by electron-spinecho-detected electron paramagnetic resonance at 95 GHz. Measurements on frozen solutions of quinone-depleted reaction centers yield g-values along the principal directions of the zero-field splitting tensor (Norris J.R., Budil D.E., Gast P., Chang C.H., ElKabbani 0., Schiffer M.: Proc. Natl.Acad. Sei. USA 86, [4335][4336][4337][4338][4339] 1989). g-Values determined are: 2.00308, 2.00238, and 2.00138. The deviation from axial symmetry and the z-component of the g-tensor are smaller than observed in the cation radical of D.

Low-Temperature Pulsed EPR Study at 34 GHz of the Triplet States of the Primary Electron Donor P865and the Carotenoid in Native and Mutant Bacterial Reaction Centers ofRhodobacter sphaeroides

“…We also use this approximation and the three g values amount to 2.0033, 2.0038, and 2.0023, respectively. These numbers are similar, but not identical to the ones found in measurements at 130 and 95 GHz ( , ), which are also not identical. The discrepancies, which stem from differences of only a few gauss, likely come from the contribution of 14 N hyperfine couplings to the width of the EPR spectrum, that become less important at high microwave frequencies.…”

“…The D parameter is 0.0188 cm -1 , identical to the one read directly from the spectrum. The different widths of the low-field and high-field signals are attributed to a small anisotropy of the g tensor of 3 P 865 (48,49). In the simulation in ref 48, the g tensor was assumed to be collinear with the ZFS tensor.…”

Temperature Dependence of the Primary Donor Triplet State g-Tensor in Photosynthetic Reaction Centers of Rhodobacter sphaeroides R-26 Observed by Transient 240 GHz Electron Paramagnetic Resonance

“…g-Tilt. Previous determinations of the 3 P principal g -values have made the assumption that the g -axes are coincident with the zero-field axes of 3 P. ,, There is no justification for this assumption on a physical basis, although it greatly simplifies the analysis and may have been necessary given the more limited resolution at 130 GHz and lower frequencies. It is known that the g -axes of the P + state are significantly tilted away from the dimer symmetry axes, , which are coincident with the 3 P zero-field axes to within 5° .…”

“…[4][5][6][7][8][9][10][11] Another paramagnetic state that has yielded much information about the photodynamics of the RC at conventional EPR frequencies is the primary donor triplet state 3 P*, [12][13][14][15][16] which is formed by back-reaction of the primary radical pair (P + BPh -) in RCs when secondary electron transfer beyond the bacteriopheophytin (BPh) acceptor is blocked. We have reported preliminary observations of 3 P* by 240 GHz EPR 17 and two groups have subsequently published spectra of 3 P* at the significantly lower frequencies of 95 GHz 18 and 130 GHz. 19 In this work we present high-frequency (240 GHz) EPR spectra of the primary donor triplet state 3 P from photosynthetic reaction centers of Rhodobacter sphaeroides R26.1 as a function of temperature.…”

New developments in high field electron paramagnetic resonance with applications in structural biology

“…The G-tensor anisotropy leads to spectra that are no more antisymmetric to their centre. Differences in the main G-tensor values compared to the cation state of the dimer could be interpreted by singlet-triplet spin-orbit coupling to a close lying excited singlet state or by a redistribution of the unpaired electron over the two dimer halves (Labahn andHuber 2001, Zeng et al 2003). In the same way the temperature dependence of the triplet state G-tensor values could be attributed to singlet-triplet spin-orbit coupling or to small structural changes upon freezing .…”