Temporally and spectrally resolved subpicosecond energy transfer within the peripheral antenna complex (LH2) and from LH2 to the core antenna complex in photosynthetic purple bacteria.

Abstract

We report studies ofenergy transfer from the 800-nm absorbing pigment (B800) to the 850-nm absorbing pigment (B850) of the LH2 peripheral antenna complex and from LH2 to the core antenna complex (LH1) in Rhodobacter (Rb.) sphaeroides. The B800 to B850 process was studied in membranes from a LH2-reaction center (no LH1) mutant of Rb. sphaeroides and the LH2 to LH1 transfer was studied in both the wild-type species and in LH2 mutants with blueshifted B850. The measurements were performed by using '100-fs pulses to probe the formation of acceptor excitations in a two-color pump-probe measurement. Our experiments reveal a B800 to B850 transfer time of -0.7 ps at 296 K and energy transfer from LH2 to LH1 is characterized by a time constant of -3 ps at 296 K and -5 ps at 77 K. In the blue-shifted B850 mutants, the transfer time from B850 to LH1 becomes gradually longer with increasing blue-shift of the B850 band as a result of the decreasing spectral overlap between the antennae. The results have been used to produce a model for the association between the ring-like structures that are characteristic of both the LH2 and LH1 antennae.Organization and function of the core antenna complex (LH1) and the peripheral antenna complex (LH2) of purple bacteria have been extensively studied in the past (1, 2, 29). A major step forward in this work was taken very recently when the threedimensional structure of a complex of the peripheral antenna absorbing at 800 nm (B800) and at 850 nm (B850) from Rhodopseudomonas (Rps.) acidophila was solved to high resolution showing a ninefold circular symmetry of a13 pairs (3). From this structure of LH2 and the similar circular structure of LH1 (4), it has become clear that the pigment density in these light harvesting pigments is very high, leading to short intermolecular distances and strong dipole-dipole interactions. Presently, two modes of energy transfer are considered, incoherent F6rster transfer or exciton state relaxation (5-8), and it is a challenge for future research to establish the levels of organization at which the two modes of energy transfer are operative. Until experiments and theory have produced a unified description of the energy transfer dynamics, we have chosen to describe the energy transfer steps within LH2 and LH1 and between the complexes as incoherent Forster hopping. Early work on energy transfer dynamics in photosynthetic purple bacteria (9-16) yielded information about the overall exciton lifetime in the antenna and provided a time scale for energy equilibration within individual complexes (9, 10) and over the whole antenna (11,16). In particular, the LH2 antenna is probably the most extensively studied complex. Picosecond absorption and fluorescence studies performed at room and low temperature on the LH2 pigmentprotein complex of Rhodobacter (Rb.) sphaeroides revealed that B800 -> B850 energy transfer is a very fast and temperature-The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereb...

Tracking energy transfer between light harvesting complex 2 and 1 in photosynthetic membranes grown under high and low illumination

“…3B), we find that the population initially resides predominantly on B850, confirming our selective pumping condition. The spectral weight then shifts to B875, indicating forward B850 → B875 ET, within a few picoseconds, a timescale comparable to similar systems (24,25). However, even after 350 ps, a considerable amount of B850 population remains, indicating a backward B850 ← B875 ET, leading to a stationary B850∕B875 ratio after an equilibration dynamics.…”

“…We use a specially designed femtosecond pumpprobe spectrometer, combining tunable narrowband pump with broadband white-light probe pulses, and introduce an analytical method based on derivative spectroscopy for disentangling the congested transient absorption spectra of LH1 and LH2 complexes. In previous femtosecond studies (24,25), the contributions of forward and backward ET to the observed LH2 ↔ LH1 equilibration dynamics could not be resolved, and therefore information on backward ET has only been obtained in an indirect and qualitative manner (20,26,27). With our approach, we are able to track the equilibration dynamics, determine both forward and backward ET rate constants, and quantify the final population equilibrium between B850 and B875 excitons (see Fig.…”

Overall energy conversion efficiency of a photosynthetic vesicle

“…For reviews see (van Grondelle and Novoderezhkin, 2006a;Sener et al, 2011;Strümpfer et al, 2012). Excitation transfer kinetics in the chromatophore was reported experimentally in (Woodbury and Parson, 1984;Visscher et al, 1989;Crielaard et al, 1994;Hess et al, 1994Hess et al, , 1995. Exciton migration across the network of light harvesting complexes in the chromatophore can be described by a rate matrix K which is constructed from inter-complex exciton transfer rates k IJ , the latter given by Equation (S6), as follows (Sener et al, 2010(Sener et al, , 2007 …”

Photoprotection in intact cells of photosynthetic bacteria: quenching of bacteriochlorophyll fluorescence by carotenoid triplets

“…1). This assures efficient capture of light energy and rapid energy transfer from LH2 to LH1 and then to the reaction center (RC) (Hess et al 1995). Each light-harvesting complex is known to consist of oligomeric pairs of transmembrane proteins, termed α and β, to which BChls (a and b) and accessory Car pigments are noncovalently attached (Hawthornthwaite and Cogdell 1991).…”