Results from the first Stark hole-burning spectroscopic studies of pho
tosynthetic complexes are reported. External electric held effects wer
e determined as a function of laser burn frequency and polarization fo
r the B800 band and the B870 band of Rhodobacter sphaeroides, where B8
70 is the lowest exciton level of the strongly coupled bacteriochlorop
hyll a molecules of the B850 ring. For either parallel or perpendicula
r polarization of the burn laser relative to the external Stark field,
only broadening of the zero-phonon holes which is linear in field str
ength was observed. Near the center of the B870 and B800 bands, f(Delt
a mu(parallel to), Delta mu(perpendicular to)) = (1.1, 1.4) and (1.1,
0.8) Debye, respectively, where f is the local field correction factor
. However, a dependence of the Delta mu-values on location of the hole
within the inhomogeneously broadened absorption bands is observed. Fo
r B870 they increase by 10-20% from the blue to red edge while for B80
0 an opposite trend is observed. Results for B870 of Rhodopseudomonas
acidophila and Rhodospirillum molischianum are similar. A detailed dis
cussion of the results and a comparison of them with those from classi
cal Stark modulation spectroscopic studies of the entire B800 and B850
absorption bands are given. It is concluded for the B850 ring that mi
xing of charge transfer states with the Q(y)-exciton levels is of mino
r importance. The hole burning data, together with consideration of th
e C-n-symmetry and the effects of energy disorder from structural hete
rogeneity, suggest that localization effects from energy disorder are
probably important for understanding the effects of external electric
field on the B850 band which is comprised of several components. (C) 1
998 Elsevier Science B.V. All rights reserved.