On Light-Induced Photoconversion of B800 Bacteriochlorophylls in the LH2 Antenna of the Purple Sulfur Bacterium Allochromatium vinosum

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 (B800) and 792 nm (B800). 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 B800 or B800 lead to B800 → B800 phototransformation. Selective excitation into B800 leads to uncorrelated excitation energy transfer (EET) to B800 and subsequent B800 → B800 phototransformation. The B800 → B800 EET time is 0.9 ± 0.1 ps. Excitation at 808.4 nm (into the low-energy side of B800) shows that the lower limit of B800 → B850 EET is about 2 ps, as the B800 → B800 phototransformation process could contribute to the corresponding zero-phonon hole width. The phototransformation of B800 leads to a ∼ 200 cm average blue-shift of transition energies, i.e., B800 changes into B800. 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 B800 and B800 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.

Dichotomous Disorder versus Excitonic Splitting of the B800 Band of Allochromatium vinosum

“…Frame B of Figure 3 shows similar B800 absorption spectra (curves a−d) obtained for a heterodimer with the same coupling matrix elements (as in frame A) but different site energies (here Δ = 200 cm −1 ); note that such a photoproduct was observed experimentally. 1,2 The corresponding calculated photochemical holes assuming a photoproduct blue-shifted by 200 cm −1 are shown as curves a′−d′. Although the absorption spectrum with V B800−B800 = −26 cm −1 and Δ = 200 cm −1 nicely describe the B800 sub-bands, the calculated HB spectra poorly describe the experimentally observed phototransformation.…”

“…Our previous work provided clear evidence that the two B800 subbands (B800 R and B800 B ) are largely due to different BChl site energies and these bands were assigned to weakly and strongly hydrogen-bonded BChl chromophores. 1 Below, we examine the nature of the B800 band split in more detail and show that, while a weak excitonic coupling between pigments contributing to the B800 R and B800 B sub-bands is present, distinct siteenergies of weakly and strongly hydrogen-bonded BChls contributing to the B800 band are largely responsible for the observed B800 band split.…”

“…vinosum also did not reveal any obvious bleach of both B800 sub-bands upon selective excitation; however, under these conditions light-induced photoconversion of B800 R → B800 B occurs (vide infra), which could mask a simultaneous bleach within the B800 B band. 1,2 However, it is more likely that, for a coupled dimer, the weak coupling strength (compared to reorganization energy) leads to dynamic localization of the transitions, and no excitonic response is present in the HB spectra. Our previous work provided clear evidence that the two B800 subbands (B800 R and B800 B ) are largely due to different BChl site energies and these bands were assigned to weakly and strongly hydrogen-bonded BChl chromophores.…”

“…This behavior is consistent with previous data showing that by 70 K the holes are annealed and the photoconversion process does not produce persistent holes above this temperature. 1 Absorption spectra (curves a and d) are shown here for an easy comparison.…”

“…vinosum assuming C 12 symmetry is used in our modeling studies. 1,2 Both models are based on a non-Markovian Redfield theory with Monte Carlo disorder averaging of site energies.…”