Stark effect spectroscopy of Rhodobacter sphaeroides and Rhodopseudomonas viridis reaction centers

Abstract

The nature of the initially excited state of the primary electron donor or special pair has been investigated by Stark effect spectroscopy for reaction centers from the photosynthetic bacteria Rhodopseudomonas viiis and Rhodobacter sphaeroides at 77 K. The data provide values for the magnitude of the difference in permanent dipole moment between the ground and excited state, IAuI and the angle C between ApL and the transition dipole moment for the electronic transition.IAdA and C for the lowest-energy singlet electronic transition associated with the special pair primary electron donor were found to be very similar for the two species. IA|I for this transition is substantially larger than for the Qy transitions of the monomeric pigments in the reaction center or for pure monomeric bacteriochlorophylls, for which Stark data are also reported. We conclude that the excited state of the special pair has substantial charge-transfer character, and we suggest that charge separation in bacterial photosynthesis is initiated immediately upon photoexcitation of the special pair. Data for Rhodobacter sphaeroides between 340 and 1340 nm are presented and discussed in the context of the detection of chargetransfer states by Stark effect spectroscopy.The recent elucidation of the organization of the reactive components in reaction centers (RCs) from photosynthetic bacteria by x-ray diffraction (1)(2)(3)(4) makes possible an analysis of the initial charge-separation mechanism from first principles. The first step in such an analysis is the calculation of the spectroscopic properties of the RC from several species, including the absorption, circular dichroism, and linear dichroism spectra (5-9). In a recent report we demonstrated that a quantitative analysis of the effect of an electric field on the absorption spectrum (the Stark effect spectrum) in Rhodobacter sphaeroides RCs at room temperature provides additional observables for comparison with theory (10). This work followed up on an earlier preliminary report by deLeeuv et al. (11). In the present communication we extend these measurements to a different species, Rhodopseudomonas viridis, and to low temperatures (12).Quantitative analysis of the Stark effect spectrum provides information on the magnitude of the difference in permanent dipole moment between the ground and excited state, JAput, and on the angle, (, between Ap and the transition dipole moment.Since the orientation of the transition dipole moment can be determined relative to the molecular axes from single-crystal polarized absorption measurements, the angle (can be related to the movement of charge associated with excitation of the chromophores in the RC. Stark effect data have also been obtained for monomeric photosynthetic chromophores, and this information should be useful for testing the reliability ofthe wave functions that are used for the calculation of RC spectral properties and electron transfer pathways. EXPERIMENTAL Rps. viridis RCs were prepared as previously described for Rb. sphaeroides (...

Mixed Exciton–Charge-Transfer States in Photosystem II: Stark Spectroscopy on Site-Directed Mutants

“…S2), indicating that, to a first approximation, these Stark spectra are dominated by a change in dipole moment, Dm, between the ground and excited states associated with the electronic transitions. A similar Stark effect was observed in the bacterial RC (26)(27)(28) and in the previously reported Stark spectrum of higher plant PSII RC (spinach) (17). In the PSII RC, the Stark spectra are dominated by the excitonically coupled (and coupled to CT states) cofactors located in the active branch (P D1 , Chl D1 , and Phe D1 ) and inactive branch (P D2 ).…”

“…S4), indicating that to a first approximation, these Stark spectra are dominated by a change in dipole moment, Dm, between the ground and excited states associated with the electronic transitions. A similar Stark effect was observed in the bacterial RC (26,27) and the higher plant PSII RC (17) (Fig. 2).…”

Dielectric Asymmetry in the Photosynthetic Reaction Center

“…If these bands had large first-derivative contributions (due primarily to Aa, the difference polarizability tensor), it is likely that F(P+Q,-) would have induced a change in Ap. The Stark line shapes for the Q , band of P and the Q , bands of all of the chromophores are more complex, as discussed in detail elsewhere (21,34); these bands are not relevant to the present discussion. 40.…”

“…The value of A p for each probe chromophore can be obtained by measuring the effect of an external applied electric field, F,,,, on the absorption spectrum, also called the Stark effect spectrum (18)(19)(20)(21)(22). The magnitude and direction of the internal electric field at any position produced by P+ and QA-in vacuum (E = 1, where E is the dielectric constant), FCaIc(PfQA-, E = I), can be calculated from Coulomb's law by using the x-ray crystal structure coordinates and information on the Pf and QA-charge distributions from electronnuclear double resonance (ENDOR) spectroscopy and theory (23) where qi is the partial charge on atom i, r, is the vector between the charge qi and the probe charge, and Pi is a unit vector in the direction of ri.…”

Spectral hole burning of the primary electron donor state of Photosystem I

“…The P700 a b s o r p t i o n profile m a x i m u m occurs at ~ 702 n m with the differences between this wavelength a n d 710 n m due to the reorganization energy (Scorn) associated with the optical excitation. The strong linear e l e c t r o n -p h o n o n coupling for P700* suggests that it m a y possess significant charge-transfer character as appears to be the case for P870" (Braun et al 1987, Lockhart and Boxer 1987, Lrsche et al 1987, Lockhart and Boxer 1988) a n d P960" (Braun et al 1987, Lrsche et al 1987, Lockhart a n d Boxer 1988 Johnson DG, Seibert M and Govindjee (1989) Determination of the primary charge separation rate in isolated photosystem II reaction centers with 500-fs time resolution. Proc Natl Acad Sci USA 86:524--528…”