On light-induced photoconversion of B800 bacteriochlorophylls in the LH2 antenna of the purple sulfur bacterium Allochromatium vinosum

Kell, A., Jassas, M., Hacking, K., Cogdell, R. J. and Jankowiak, R. (2017) On light-induced photoconversion of B800 bacteriochlorophylls in the LH2 antenna of the purple sulfur bacterium Allochromatium vinosum. Journal of Physical Chemistry B, 121(43), pp. 9999-10006. (doi:10.1021/acs.jpcb.7b06185) (PMID:29017010)

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Abstract

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 provide 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 propose 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. However, the effective tunneling mass is likely larger than the proton mass.

Item Type:Articles
Additional Information:This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC-0006678 (to R.J.). R.J.C. and K.H. acknowledge support from the Photosynthetic Antenna Research Center (PARC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC-0001035.
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Hacking, Dr Kirsty and Cogdell, Professor Richard
Authors: Kell, A., Jassas, M., Hacking, K., Cogdell, R. J., and Jankowiak, R.
College/School:College of Medical Veterinary and Life Sciences > Institute of Molecular Cell and Systems Biology
College of Medical Veterinary and Life Sciences > School of Life Sciences
Journal Name:Journal of Physical Chemistry B
Publisher:American Chemical Society
ISSN:1520-6106
ISSN (Online):1520-5207
Published Online:10 October 2017

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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
502612Photosynthetic Antenna Research Center (PARC)Richard CogdellUS Department of Energy (USDOE)DE-SC0001035RI MOLECULAR CELL & SYSTEMS BIOLOGY