High Pressure and Stark Hole-Burning Studies of Antenna Complexes of Purple Bacteria
Abstract
The light harvesting (LH) complexes, LH1 and LH2, of purple bacteria exhibit striking Cn symmetry with n=8,9,16. This leads to cyclic arrays which contain from 16 to 32 bacteriochlorophyll (BChl) molecules with nearest neighbor BChl-BChl a couplings being strong enough to require a Frenkel-type excitonic level description of excited state electronic structure. Phonon-assisted relaxation between exciton levels of the cyclic array can occur as fast as 100 fs while energy transfer from LH2 to LH1, which transfers to the reaction center, occurs in about 1 ps. These dynamical events are only weakly dependent on temperature as many recent hole-burning and time domain studies have shown. The problem of the dynamics becomes more intriguing when one realizes that the glass-like structural heterogeneity of proteins means that the effects of energy disorder on excitonic level structure must be taken into account. In this paper the first results from high pressure and Stark hole-burning experiments on the B800 and B850 bands of the LH2 complex and the B875 band of the LH1 complex are presented. The former experiments yield the linear pressure shifting and broadening rates of persistent spectral holes burned into selected exciton levels while the latter yield the permanent dipole moment changes. The high pressure data establish that electron-exchange interactions are important for understanding the excitonic level structures of the B850 and B875 BChl rings. It is argued that energy disorder, which leads to localization effects, is important for understanding the small dipole moment changes (less than 1 D) of these rings. The data can serve as stringent benchmarks for electronic structure calculations, the results of which are essential for detailed understanding of energy transfer dynamics.
- Publication:
-
APS March Meeting Abstracts
- Pub Date:
- March 1998
- Bibcode:
- 1998APS..MAR.S1001S