THE NATURE OF THE NEAR-INFRARED ELECTRONIC ABSORPTION AT 1250 NM IN THE SPECTRA OF THE RADICAL CATIONS OF THE SPECIAL PAIRS IN THE PHOTOSYNTHETIC REACTION CENTERS OF RHODOBACTER-SPHAEROIDES AND RHODOPSEUDOMONAS-VIRIDIS

The P+ electronic spectra of bacterial photosynthetic reaction centers contain a band at ca. 1250 nm (8000 cm(-1)). This has received little attention: the possibility that it represents an optical hole transfer between the two bacteriochlorophylls in the special pair is eliminated by the recent discovery of an even lower energy transition which is clearly of this (intervalence) type. Also, we find that suggestions that this band in fact corresponds to SHOMO-->HOMO absorption cannot account for the strong intermonomer-coupling dependence of the band intensity. In this work, this band is assigned to a triplet-coupled (''trip-doublet'') Q(y) absorption, expected, and found to occur at the triplet absorption energy of the monomer chromophores. As is observed, this band is local to one monomer and hence produces no significant change in the molecular dipole moment, but its intensity is highly dependent on the strength of the electronic coupling between the L and M bacteriochlorophylls of the special pair, as observed, becoming asymptotically spin-forbidden.