Origin of the Two Bands in the B800 Ring and Their Involvement in the Energy Transfer Network of Allochromatium vinosum

Abstract

Bacterial photosynthesis features robust and adaptable energy-harvesting processes in which light-harvesting proteins play a crucial role. The peripheral light-harvesting complex of the purple bacterium Allochromatium vinosum is particularly distinct, featuring a double peak structure in its B800 absorption band. Two hypotheses-not necessarily mutually exclusive-concerning the origin of this splitting have been proposed; either two distinct B800 bacteriochlorophyll site energies are involved, or an excitonic dimerization of bacteriochlorophylls within the B800 ring takes place. Through the use of two-dimensional electronic spectroscopy, we present unambiguous evidence that excitonic interaction shapes the split band. We further identify and characterize all of the energy transfer pathways within this complex by using a global kinetic fitting procedure. Our approach demonstrates how the combination of two-dimensional spectral resolution and self-consistent fitting allows for extraction of information on light-harvesting processes, which would otherwise be inaccessible due to signal congestion.

The light-harvesting complex 2 of Allochromatium vinosum: B800 absorption band splitting and exciton relaxation

“…In fact it is closer to the spectrum reported in Ref. [27], also measured in glycerol, but at 77 K (see Fig. 3).…”

“…In Fig. 3a the experimental absorption spectrum [27] is compared with a simulation using the original model of Ref. [23], supplemented by the excitonic phase relaxation.…”

“…Also shown is the experimental spectrum (black dashed line) from Ref. [27]. Model parameters are given in Tab.…”

Simulating Fluorescence-Detected Two-Dimensional Electronic Spectroscopy of Multichromophoric Systems

“…The energy transfer between these two bands is efficient despite weak coupling between chromophores in the two bands. 35−40 Variations of this system exist in different bacteria and also depend on the growth conditions. 41−48 In this study, we will consider a bacterial system with 27 bacteriochlorophylls, modelled after the wild-type, high-light LH2 from Rhodopseudomonas palustris .…”

Interpreting Fluorescence Detected Two-Dimensional Electronic Spectroscopy

“…11 Even though 2DES and F-2DES are analogous coherent techniques providing detailed information on ultrafast dynamics in two spectral dimensions, the methods have important differences and the measured spectra do not look the same. 12,13,14,15 While the "anatomy" of the 2DES has been studied in numerous articles 16,17,18 and is quite well understood, interpretation of the F-2DES spectra 19,20,21,22,23 has not reached similar maturity and is currently being debated in literature. In this Viewpoint we briefly summarize the various approaches to interpreting the F-2DES and present our view with the intension to resolve at least some of the existing controversial issues.…”