On Light-Induced Photoconversion of B800 Bacteriochlorophylls in the LH2 Antenna of the Purple Sulfur Bacterium Allochromatium vinosum
The B800-850 LH2 antenna from the photosynthetic purple sulfur bacterium Allochromatium vinosum exhibits an unusual spectral splitting of the B800 absorption band; i.e., two bands are well-resolved at 5 K with maxima at 805 nm (B800R) and 792 nm (B800B). To offer more insight into the nature of the B800 bacteriochlorophyll (BChl) a molecules, high-resolution hole-burning (HB) spectroscopy is employed. Both white light illumination and selective laser excitations into B800R or B800B lead to B800R → B800B phototransformation. Selective excitation into B800B leads to uncorrelated excitation energy transfer (EET) to B800R and subsequent B800R → B800B phototransformation. The B800B → B800R EET time is 0.9 ± 0.1 ps. Excitation at 808.4 nm (into the low-energy side of B800R) shows that the lower limit of B800R → B850 EET is about 2 ps, as the B800R → B800B phototransformation process could contribute to the corresponding zero-phonon hole width. The phototransformation of B800R leads to a ~ 200 cm–1 average blue-shift of transition energies, i.e., B800R changes into B800B. We argue that it is unlikely that B800-B850 excitonic interactions give rise to a splitting of the B800 band. We postulate that the latter is caused by different protein conformations that can lead to both strong or weak hydrogen bond(s) between B800 pigments and the protein scaffolding. Temperature-dependent absorption spectra of B800, which revealed a well-defined isosbestic point, support a two-site model, likely with strongly and weakly hydrogen-bonded B800 BChls. Thus, BChls contributing to B800R and B800B could differ in the position of the proton in the BChl carbonyl-protein hydrogen bond, i.e., proton dynamics along the hydrogen bond may well be the major mechanism of this phototransformation. Yet, the effective tunneling mass is likely larger than the proton mass.
- Research Organization:
- Washington Univ., St. Louis, MO (United States). Energy Frontier Research Center (EFRC) Photosynthetic Antenna Research Center (PARC)
- Sponsoring Organization:
- USDOE Office of Science (SC), Basic Energy Sciences (BES)
- Grant/Contract Number:
- SC0001035
- OSTI ID:
- 1469903
- Journal Information:
- Journal of Physical Chemistry. B, Condensed Matter, Materials, Surfaces, Interfaces and Biophysical Chemistry, Vol. 121, Issue 43; Related Information: PARC partners with Washington University in St. Louis (lead); University of California, Riverside; University of Glasgow, UK; Los Alamos National Laboratory; University of New Mexico; New Mexico Corsortium; North Carolina State University; Northwestern University; Oak Ridge National Laboratory; University of Pennsylvania; Sandia National Laboratories; University of Sheffield, UK; ISSN 1520-6106
- Publisher:
- American Chemical SocietyCopyright Statement
- Country of Publication:
- United States
- Language:
- English
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