Subpicosecond formation of the intradimer charge transfer state [BChl_LP⁺BPh_MP^-] in reaction centers from the His^M200 → Leu mutant of Rhodobacter capsulatus

Transient difference spectra acquired 600 fs after excitation of reaction centers (RCs) from the His^M200→Leu mutant of Rhodobacter capsulatus display a distinct absorption band near 650 nm that is characteristic of a reduced bacteriochlorin pigment. We assign this band to the BPh^- component of the [BChl_LP⁺BPh^MP-] intradimer charge transfer state, which appears to form within 350 fs of excitation of the BChl/BPh heterodimer (D). This [BChl_LP⁺BPh_MP^-] intradimer charge transfer state appears to be the immediate precursor to state D⁺BPh_L^- in the His^M200→Leu RC. The time-dependent polarization of the 650 nm band supports this assignment.