Energy transfer dynamics in LH2 complexes of Rhodopseudomonas acidophila containing only one B800 molecule

Nonphotochemical hole burning (NPHB) and femtosecond pump-probe experiments have previously shown that following their excitation, the B800 molecules of the LH2 complex of Rhodobacter sphaeroides and Rhodopseudomonas (Rps.) a cidophila relax by two channels. The decay channel observed for excitation on the low energy side of the B800 absorption band is due to B800 --> B850 excitation energy transfer which occurs in similar to2 ps at 4 K. The addit ional decay channel becomes detectable for excitation on the high energy si de of the B800 band. The mechanisms that have been proposed for this channe l are: (i) vibrational relaxation following excitation of vibronic levels a ssociated with the Qy-states of either the B800 or B850 bacteriochlorophyll a molecules; (ii) direct excitation of upper B850 exciton levels, followed by downward relaxation within the B850 manifold; (iii) intraband B800-B800 energy transfer involving only the B800 molecules; and (iv) downward energ y transfer of initially excited mixed B800-B850 states. Presented here are NPHB results for intact LH2 and B800-deficient LH2 complexes of Rps. acidop hila (strain 10050). The latter contain only one B800 molecule, rather than nine. In sharp contrast with the intact LH2 complex, the zero-phonon holew idths for the B800-deficient LH2 complex were observed to be independent of the location of the burn frequency within the B800 band, 3.2 +/- 0.3 cm(-1 ) at 4 K. This finding and others eliminate the first two of the above mech anisms. It is argued that the third mechanism is highly unlikely. It is pro posed that mixed B800-B850 states are mainly responsible for the additional decay channel. These mixed states could undergo downward relaxation to B80 0 levels that are mainly "B800" in character and to levels that are mainly "B850" in character.