Hm. Wu et al., HOLE-BURNING AND ABSORPTION STUDIES OF THE LH1 ANTENNA COMPLEX OF PURPLE BACTERIA - EFFECTS OF PRESSURE AND TEMPERATURE, JOURNAL OF PHYSICAL CHEMISTRY B, 102(20), 1998, pp. 4023-4034
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.