The electronic structure of the primary electron donor (D) of the photosynthetic bacterium Rhodobacter sphaeroides is investigated by ENDOR in the photoinduced triplet state of D. Hyperfine (hi') splittings of the triplet state measured on frozen solutions are given and compared to the results obtained earlier (Lendzian F., van Willigen H., Sastry S., M6bius K., Scheer H., Feick R.: Chem. Phys. Lett. 118, 145 (1985)). The hf splittings found are consistent with a model of ma asymmetric spin density distribution over the two baeteriochlorophyll moleeules which constitute D, suggesting a mirror image symmetry of HOMO and LUMO coefficients. This could be relevant for electron transfer, in particular tmidirectionality in the reaction center (RC). The first triplet state ENDOR experiments on single crystals of RC's are also reported.

Abstraet. ENDOR spectra of triplet state molecules have a characteristic line fxom degenerate NMR transitions within the zero level (ZL) electron spin manifold M s = 0. The ZL line, observed at the free nuclear Larmor frequency, dominates spectra when the number of nuelei is large. This line was found to be substantially reduced in intensity at low temperature. The strong variation of the ZL line intensity is analyzed within the frame of an electric-circuit analogy modeling. The result is as follows: At low temperature the electron and nuclear spin-lattice relaxation rates become small, and the nuclear-nuclear spin flip-flop transitions between degenerate substates, M 1 = coust within the ZL manifold, become relatively strong to compete for population redistribution. This reduces the population differences between nuclear sublevels. Additional NMR irradiation can thus do very little to reduce these differences even more, and the ENDOR effect becomes suppressed. A certain enhancement of the relaxationally suppressed ZL line occurring at increased EPR saturation is explained by the coherent action of the microwave field on the selected substate within the ZL manifold that shifis its energy out of the other degenerate states thereby closing the flip-flop relaxation channel.

Abstraet. An EPR and ENDOR probehead with a loop-gap resonator for X-band is described. The novel feature of the construction is that an iris-type coupling of the resonator is used instead of the conventional antenna coupling. The ENDOR coil combines the role of creating the radio frequency field and that of a shield for the microwave loop-gap structure. Hence, in order to accommodate the iris and waveguide, a pair of RF coils is used in conjunction with a reduced waveguide with dielectric filling. This arrangement simplifies matching the resonator to the microwave bridge, and standard EPR cryostats can be used making sample manipulation more convenient.One of the main possibilities of increasing the effŸ of Electron Paramagnetic Resonance (EPR) and Electron Nuclear Double Resonance (ENDOR) experiments, still open for development, is to design alternative resonator eonfigurations. The major innovations in this area have been to reduce the dimensions of the resonance mode region with the highest microwave (MW) field, B~, as compared to those of conventional, cylindrical of rectangular, EPR cavities. Thereby, the filling factor can be improved for small samples increasing the signal-to-noise (S/N) ratio. Moreover, this also increases the B1/~/-P (Pis MW power) ratio at typically lower Q, allowing for easier saturation in continuous-wave (cw) EPR, a necessary condition for cw ENDOR, and faster MW pulsing for pulsed EPR.Several types of sueh "concentrating" structures have been proposed: the loopgap resonator (LGR) [1-6] and the bridged loop-gap resonator (BLGR) [7-10]; the slotted tube resonator (STR) [11][12][13]; the stripline resonator [14,15]; the transmission-line resonator [16]; and the dielectric resonator [17]. The probehead also includes the MW-screening shield, typically cylindrical in shape, which encloses these structures. Therefore, these resonator constructions may be consid-

The specific ENDOR line at the free Larmor frequency Vp in the low temperature spectra of triplet state molecules is caused by degenerate NMR transitions within the M s = 0 zero-level (ZL) electron spin manifold. This ZL line was found to be orientationally dependent for the diradical complex Zn(3,6-di-tert-butyl-o-semiquinone)2Zn(DBSQ)2: the ZL line dominates the ENDOR spectrum if itis detected at the perpendicular canonical components of the EPR spectrum, and vanishes if the complex is oriented with its ZFS z-axis parallel to the direction of the magnetic field, i.e., if detected at the parallel canonical EPR components. This effect is shown to result from the interaction between nuclear spin substates of the S and T o manifolds, their levels being close to each other for the Zn(DBSQ) 2 complex. Such an interaction mixes the states and shifts energy levels. Consequently, it cancels the degeneracy of the nuclear substates within the ZL manifold and reduces the rate of nuclear flip-flop relaxation. This specific relaxation mechanism has been shown to substantially affect the amplitude of the ZL line (Doubinskii A.A., Lebedev Ya.S., M6bius K.: Appl. Magn. Reson. 13, 439 (1997)). The nuclear flip-flop relaxation effect is expected to be orientationally dependent since the S-T 0 separation depends upon the orientation of the diradical with respect to the external magnetic fiel&