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 transfe r between the two bacteriochlorophylls in the special pair is eliminat ed by the recent discovery of an even lower energy transition which is clearly of this (intervalence) type. Also, we find that suggestions t hat this band in fact corresponds to SHOMO-->HOMO absorption cannot ac count for the strong intermonomer-coupling dependence of the band inte nsity. In this work, this band is assigned to a triplet-coupled (''tri p-doublet'') Q(y) absorption, expected, and found to occur at the trip let absorption energy of the monomer chromophores. As is observed, thi s band is local to one monomer and hence produces no significant chang e in the molecular dipole moment, but its intensity is highly dependen t on the strength of the electronic coupling between the L and M bacte riochlorophylls of the special pair, as observed, becoming asymptotica lly spin-forbidden.