Pressure-induced spectral changes for the special-pair radical cation of the bacterial photosynthetic reaction center

摘要

The effect of pressure up to 6 kbars on the near to mid infrared absorption spectrum (7500-14 300 cm~(-1) or 1333-700 nm) of the oxidized reaction center of Rhodobacter sphaeroides is measured and interpreted using density-functional B3LYP, INDO, and PM5 calculations. Two weak electronic transition origins at ~8010 and ~10 210 cm~(-1) are unambiguously identified. The first transition is assigned to a Q_y tripdoublet band that involves, in the localized description of the excitation, a triplet absorption on one of the bacteriochlorophyll molecules (P_M) in the reaction center's special pair intensified by the presence of a radical cation on the other (P_L). While most chlorophyll transition energies decrease significantly with increasing pressure, the tripdoublet band is found to be almost pressure insensitive. This difference is attributed to the additional increase in the tripdoublet-band energy accompanying compression of the pi-stacked special pair. The second band could either be the anticipated second Q_y tripdoublet state, a Q_x tripdoublet state, or a state involving excitation from a low-lying doubly occupied orbital to the half-occupied cationic orbital. A variety of absorption bands that are also resolved in the 8300-9600 cm~(-1) region are assigned as vibrational structure associated with the first tripdoublet absorption. These sidebands are composites that are shown by the calculations to comprise many unresolved individual modes; while the calculated pressure sensitivity of each individual mode is small, the calculated pressure dependence of the combined sideband structure is qualitatively similar to the observed pressure dependence, preventing the positive identification of possible additional electronic transitions in this spectral region.