Hm. Wu et al., DIRECT OBSERVATION AND HOLE-BURNING OF THE LOWEST EXCITON LEVEL (B870) OF THE LH2 ANTENNA COMPLEX OF RHODOPSEUDOMONAS-ACIDOPHILA (STRAIN-10050), JOURNAL OF PHYSICAL CHEMISTRY B, 101(4), 1997, pp. 651-656
Results of 4.2 K absorption and hole-burning experiments on the B850 a
bsorption band of isolated LH2 (B800-850) complexes from Rhodopseudomo
nas acidophila are presented for samples that exhibited a B850 absorpt
ion band of sufficient narrowness (200 cm(-1)) to allow for direct obs
ervation of the lowest exciton level of the B850 ring of dimers as a w
eak but distinct shoulder at the red edge of the B850 band. This shoul
der correlates perfectly with the zero-phonon hole action spectrum of
B870 which has been assigned as the lowest exciton level of B850. The
action spectrum reveals that the B870 band carries an inhomogeneous wi
dth of 120 +/- 10 cm(-1), is characterized by weak electron-phonon cou
pling, and carries 3% of the total intensity of the B850 absorption ba
nd. The B870 exciton level lies 200 cm(-1) below the B850 band maximum
. Based on the X-ray structure of LH2 and under the assumption of perf
ect C-9 symmetry (absence of diagonal and/or off-diagonal energy disor
der) for the B850 ring, the B850 maximum should be determined by the s
trongly allowed E(1) level of the C-9-array of bacteriochlorophyll a d
imers and B870 (the A level) should be adjacent, forbidden in absorpti
on and the lowest energy level. However, the location of the A level 2
00 cm(-1) below the E(1) level appears to require a coupling between t
he nearest bacteriochlorophyll a monomers of adjacent dimers that is l
arger than the coupling between the monomers of the special pair of th
e reaction center of Rhodobacter sphaeroides. As a result, it is concl
uded that diagonal and/or off-diagonal energy disorder within the B850
ring must be taken into account in order to understand B870 and the Q
(y) electronic structure of B850. However, the temperature dependence
(4.2-270 K) of the LH2 absorption spectrum reveals that the coupling b
etween neighboring B850 molecules strengthens upon formation of the gl
ycerol: H2O glass near 150 K. This is important for electronic structu
re calculations that utilize the room temperature structure of LH2 and
low-temperature spectroscopic data. Finally, data pertaining to the p
ure dephasing of the B870 exciton level are presented and interpreted
in terms of scattering due to imperfections in the B850 ring.