HOLE-BURNING AND ABSORPTION STUDIES OF THE LH1 ANTENNA COMPLEX OF PURPLE BACTERIA - EFFECTS OF PRESSURE AND TEMPERATURE

Spectral hole-burning and absorption spectroscopies were combined with pressure and temperature in studies of the light harvesting 1 (LH1 or B875) antenna complex of wild-type (WT) chromatophores and an LH1-onl y mutant of Rhodobacter sphaeroides. Zero-phonon hole (ZPH) action spe ctra lead to values of 120 (WT) and 140 cm(-1) (mutant) for the separa tion (Delta E) between the lowest energy exciton level of A symmetry, B896, and the adjacent, strongly absorbing E-1 level of the C-16 ring of 32 bacteriochlorophyll a molecules. The E-1 level is responsible fo r most of the B875 band's absorption intensity. Values for the inhomog eneous broadening of the relatively weak B896 absorption band are give n. High-pressure hole-burning data for the B896 band yielded a very la rge linear pressure shifting of -0.67 cm(-1)/MPa, about 10% higher tha n the shift rate for the B875 absorption band. These shifts are about a factor of 7 times higher than those of weakly coupled chlorophyll mo lecules in protein complexes and isolated chromophores in polymers and glasses. A theoretical model is presented which leads to the conclusi on that electron-exchange coupling between nearest neighbor BChl a mol ecules of the B875 ring is largely responsible for the large pressure shifts. (Such coupling produces charge-transfer (CT) states which mix with the neutral (1) pi pi states of the BChl a molecules.) It follow s that a firm understanding of the excitonic structure of the B875 rin g is unachievable by consideration of only electrostatic interactions. These conclusions also apply to the B850 BChl a ring of the LH2 compl ex. The data from the pressure studies can be used as benchmarks for e lectronic structure calculations which take into account both electros tatic and CT interactions. Thermal broadening and shifting data for th e B875 band establish that the LH1 complex undergoes a nondenaturing s tructural change at similar to 150 K in a glycerol/water glass, as has been reported for the LH2 complex. The structural change occurs for c hromatophores and isolated complexes. The ability to detect the subtle structural change via the B875 and B850 bands is a consequence of str ong coupling between BChl a molecules of the B875 and B850 rings. The results of the high-pressure experiments indicate that CT is an import ant contributor to the coupling and that the structural change may not be detectable by X-ray diffraction at typical similar to 2 Angstrom r esolution. Theoretical models from Wu et al. (J. Phys. Chem. B 1997, 1 01, 7641) provide a satisfactory explanation for the thermal shifting and broadening of the B875 band. The broadening is dynamic, the result of interexciton level downward relaxation triggered by a low-frequenc y, intermolecular promoting model(s) which modulate nearest neighbor c ouplings: The nearest neighbor BChl a-BChl a couplings for the low-tem perature structure of the B875 ring are estimated to be 30% stronger t han those of the high-temperature structure.