HIGH-PRESSURE HOLE-BURNING STUDIES OF THE BACTERIOCHLOROPHYLL A ANTENNA COMPLEX FROM CHLOROBIUM-TEPIDUM

The dependence of the low-temperature Q(y) absorption and nonphotochem ical hole-burned spectra of the title complex (also known as the FMO c omplex) on pressure (less than or similar to 700 MPa) is reported. Pre ssure-induced structural changes of the complex were found to be elast ic. The linear pressure shifts at 4.2 K for the principal absorption b ands at 805, 814 and 825 nm are -0.08, -0.11, and -0.11 cm(-1)/MPa, re spectively. Importantly, the 825 and 814 nm absorption profiles (shape , intensity) are independent of pressure. The results establish that, even at the highest values used, pressure has only a weak effect on th e pairwise excitonic couplings of the bacteriochlorophyll (BChl) molec ules, inhomogeneous broadening, and electron-phonon coupling. The pres sure dependence of the Q, spectrum and zero-phonon holes (ZPH) burned in the 825 nm band can be rationalized in terms of dispersion interact ions when BChl occupation numbers for the exciton levels are taken int o account. These ZPH, which are assigned to the lowest level at 827 nm , carry a width of 0.6 cm(-1) at 4.2 K, which is independent of the pr essure at which the hole is burned. This width is ascribed to dephasin g, T-2 = 35 ps. Possible mechanisms for the dephasing are considered, and its pressure independence is discussed. Comparison of the Linear p ressure shifts for the FMO complex against those of ether complexes su ggests that pressure is a useful probe of strong coupling between BChl molecules.