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-only mutant of Rhodobacter sphaeroides. Zero-phonon hole (ZPH) action spectra lead to values of 120 (WT) and 140 cm(-1) (mutant) for the separation (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 for most of the B875 band＇s absorption intensity. Values for the inhomogeneous broadening of the relatively weak B896 absorption band are given. High-pressure hole-burning data for the B896 band yielded a very large linear pressure shifting of -0.67 cm(-1)/MPa, about 10% higher than the shift rate for the B875 absorption band. These shifts are about a factor of 7 times higher than those of weakly coupled chlorophyll molecules in protein complexes and isolated chromophores in polymers and glasses. A theoretical model is presented which leads to the conclusion that electron-exchange coupling between nearest neighbor BChl a molecules 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 follows that a firm understanding of the excitonic structure of the B875 ring is unachievable by consideration of only electrostatic interactions. These conclusions also apply to the B850 BChl a ring of the LH2 complex. The data from the pressure studies can be used as benchmarks for electronic structure calculations which take into account both electrostatic and CT interactions. Thermal broadening and shifting data for the B875 band establish that the LH1 complex undergoes a nondenaturing structural change at similar to 150 K in a glycerol/water glass, as has been reported for the LH2 complex. The structural change occurs for chromatophores and isolated complexes. The ability to detect the subtle structural change via the B875 and B850 bands is a consequence of strong coupling between BChl a molecules of the B875 and B850 rings. The results of the high-pressure experiments indicate that CT is an important contributor to the coupling and that the structural change may not be detectable by X-ray diffraction at typical similar to 2 Angstrom resolution. Theoretical models from Wu et al. (J. Phys. Chem. B 1997, 101, 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-frequency, intermolecular promoting model(s) which modulate nearest neighbor couplings : The nearest neighbor BChl a-BChl a couplings for the low-temperature structure of the B875 ring are estimated to be 30% stronger than those of the high-temperature structure.