J . Phys. Chem. 1991, 95, 1323-1328 C'. Itrrplic~trtioirsj b r thc Strirctirre of the Special Pair in Bacterial Photosynthetic Reaction Centers. The primary electron donor in bacterial photosynthetic reaction centers consists of a dimer of bacteriochlorophyll molecules which are separated by approximately 3.4 A, not covalently linked, and slipped with rcspcct to onc another such that only the ring I pyrroles of the two molcculcs significantly ~verlap.~'-'~This arrangement dictates th:it thc cxtcnt of X K intcrnction in the special pair be less than that in thc sandwich complcxcs. Ncvertheless, oxidation of the priiiiary donor results in thc appearance of a near-IR band ca. I300 niii".33 an energy quite close to that of the near-lR feature of the single-holc I.n(TPP)? complexes.Isc In the electronic ground state of the special pair, there is no net bond order due to K A interactions of the type found in either single- or double-hole dimcrs because all clcctrons are paired (this does not, however, preclude other types of X K interactionsIib). However, the lowest cncrgy K X * excited state of the special pair, from which charge separation occurs. results from promotion of an electron from an orbital which is antibonding in the strong interaction picture to one which is bonding.I4 Consequently, in this excited state, the K K interactions should be qualitatively similar to those which would occur in the ground state of a double-hole complex of similar geometry. Our studies of single- and double-hole sandwiches have shown that thcrc is ;I largc diffcrcncc in the magnitude of K A ( 2 8 ) (a) Dicmhofcr, J.; Epp. 0.:Miki, K.: Huber. R.; Michel, H. J . Mol. B i d . 1984. 180. 385-398. (b) Diesenhofer. J.: Epp. 0.;Miki. K.: Huber, R.; Michcl, H. Notrrrc, 1985. 318. 618-624. (29) Chang. C.-H.: Ticdc. D.: Tang. J.; Smith, U.; Norris. J.; Schiffer, M. FEBS L ~ t t 1986. . 20.7. 82-86. (30) Allcn, J. P.; Fchcr, G.; Ycatcs, T. 0.;Rees, D. C.; Diesenhofer, J.; Hubcr, R. Proi'. Notl. .4cod. Sci. U.S.A. 1986, 83, 8589-8593. (31) Dutton. P. L.: Knuffmann, K. J.: Chance, B.; Rentzepis, P. M. FEBS Lett. 1975. 60. 275-280. (32) Fajcr, J.; Burnc. D. C.; Davis, M . S.; Forman, A,: Spaulding, L. D. Pro
