Magnetic resonance spectroscopy of the primary state, P ^F , of bacterial photosynthesis

Abstract

We have obtained the magnetic resonance spectrum of the radical pair state pF by using reaction yield detected magnetic resonance spectroscopy. The magnetic resonance spectrum is quite sensitive to the local environment of pF. The data place limits on the lifetime of triplet pF and the distance of charge separation.(24). Optical measurements of the pR yield upon recombination show large magnetic field effects and have become one method of studying the radical pair state. In addition, we have found by optical measurements that the pR yield also is influenced by resonant microwaves applied to the radical pair. This is a convenient method to record the EPR spectrum of pF. Most of the transient states involved in the primary events of photosynthesis are paramagnetic, making magnetic resonance and magnetic field effects two convenient methods of studying the photosynthetic apparatus. Magnetic resonance spectros-'copy has been used extensively to help identify the radical intermediates and products of photosynthesis. In addition, the magnetic resonance spectrum and its electron spin polarization and the magnetic field dependence ofvarious reaction products have yielded some information about electron-electron dipolar interactions between radicals and about the exchange interactions that are so important for electron transfer. As attempts are made to see increasingly earlier intermediates, experimental difficulties arise due to the steadily decreasing lifetimes ofthese states. For example, in bacterial photosynthetic reaction centers, one of the earliest states is a radical pair consisting of the cation of the primary donor, P+, and the anion of a bacteriopheophytin intermediate acceptor, I-(1, 2). From the EPR spectrum of this radical pair, both the electron exchange interaction, J, and the electron-electron dipolar interaction (and hence distance of separation) can in principle be determined. However, the short lifetime of this radical pair state, pF (3) (which even in blocked reaction centers is only 10 nsec) has prevented its direct observation by conventional EPR techniques, and interactions within pF have been studied only indirectly by optical spectroscopy (3-10) or through magnetic field effects (11-16). We report here the observation ofthe magnetic resonance spectrum of this radical pair state pF at physiological temperatures by using reaction yield detected magnetic resonance (RYDMAR) spectroscopy (17-21).This radical pair state forms within about 5 psec (3, 4, 10) and decays within 200 psec or about 10 nsec (7, 8), depending on whether or not further electron transfer is blocked.When the transfer of an electron from I-to the iron-ubiquinone acceptor (22, 23), Fe-Q, is prevented by removal or chemical reduction of the quinone, charge recombination of a triplet radical pair produces the lowest excited triplet state of p, pR (7,8,(24)(25)(26)(27)(28)(29)(30), while recombination ofa singlet radical pair produces both P and I in their singlet ground states (15, 24). The relative yields of pR and ground ...

Anisotropic magnetic interactions in the primary radical ion-pair of photosynthetic reaction centers

“…1) (1,2). In the present paper we demonstrate that the quantum yield of this triplet state, (DT, depends on the orientation of the RCs in a magnetic field.…”

“…The effect of varying the polarization of the excitation pulse, ,1e is much smaller. I(H, the o 710) = S(H, tnei rio)/S(0, 'te' 'T0) [1] (H. e) I(H, nef0) -I(Hne900) [2] I(H,n1000) + 2 I(H, ne,900) [3] Because the relative yields, I(H,nieno), are defined as ratios, the effect of the field-independent photoselection is not in- If absorption at 532 nm were isotropic, the observation anisotropy, ao(H,q1e), would be independent of Be and the excitation anisotropy, ae(H,nr0), would be zero at all fields. Both of these conditions are approximately satisfied by the data in Table 1.…”

Magnetic characterization of the primary state of bacterial photosynthesis

“…The data to be evaluated in this paper were obtained by a previously described experimental technique (5) In addition, because the RYDMR linewidth is due only to a short lifetime for triplet pF (essentially our conclusion in this paper), then, if the electron spin-spin interactions were less than the hyperfine interactions, it would logically follow that the RC PF system would optically display a lifetime much shorter than 10-20 nsec. We note that the isotropic exchange interactions are known to be quite weak from previous work (10)(11)(12)(13)(14)(15)(16)(17).…”

Magnetic Interactions Between Photosynthetic Reactants

“…To investigate this, a low power, low magnetic field RYDMAR method was devised (Mohl et al, 1985). In addition, in this technique continuous illumination was employed, leading to a much higher signal-to-noise ratio than in the time-resolved technique of Bowman et al (1981). Lineshapes of the R Y D M A R resonance were observed that conformed very well to those theoretically predicted (Lersch et al, 1982; cessful RYDMAR experiment on photosynthetic R C , Michel-Beyerle, 1983;Tang and Norris, 1983).…”

“…Ito> remains degenerate with IS>, but it can be shown that in contrast to the Itk> levels the It,> level has zero I To> character, because its axis of quantization is perpendicular to that of the IT,> level ( B , I I?,) (Lersch and Michel-Beyerle, 1983 yield, compared to the value at B, for B 1 = 0, has been observed (Bowman et al, 1981). When B1 < ]Dl,/JI the situation is much more complex, the shape and sign of the RYDMAR signal being very sensitive to J (Lersch and Michel-Beyerle, 1983).…”