The Open, the Closed, and the Empty: Time-Resolved Fluorescence Spectroscopy and Computational Analysis of RC-LH1 Complexes from Rhodopseudomonas palustris

Abstract

ABSTRACT:We studied the time-resolved fluorescence of isolated RC-LH1 complexes from Rhodopseudomonas palustris as a function of the photon fluence and the repetition rate of the excitation laser. Both parameters were varied systematically over 3 orders of magnitude. On the basis of a microstate description we developed a quantitative model for RC-LH1 and obtained very good agreement between experiments and elaborate simulations based on a global master equation approach. The model allows us to predict the relative population of RC-LH1 complexes with the special pair in the neutral state or in the oxidized state P + and those complexes that lack a reaction center. ■ INTRODUCTIONPhotosynthetic purple bacteria have evolved a wonderful modular principle for the light-harvesting (LH) apparatus that captures the solar radiation. These modules consist of pairs of hydrophobic, low molecular weight polypeptides that noncovalently bind a small number of bacteriochlorophyll (Bchl a) and carotenoid (Car) molecules and that self-assemble to produce the native pigment−protein complexes. 1−6 Most purple bacteria have two main types of complexes, the core complex, RC-LH1, and the peripheral complex, LH2. In the photosynthetic membrane the LH2 complexes are arranged in a two-dimensional network around the perimeter of the RC-LH1 complexes and the light energy absorbed by LH2 is transferred via LH1 to the reaction center (RC) where it is used to separate charge across the membrane. 7 For many years, high-resolution structural information has been available for the RC, 8−11 and the peripheral light-harvesting complexes 1−3 for a couple of species, whereas the first higher-resolution X-ray structure for a RC-LH1 complex has been determined only recently. 6 The current view is that there are at least two distinct classes of RC-LH1 complexes: Those that are monomeric, i.e., consisting of one RC surrounded by one LH1 complex, and those that are dimeric. 12,13 For RC-LH1 from Rps. palustris exists a lowresolution X-ray structure. 4 Accordingly, the RC is enclosed by an overall elliptically shaped LH1 consisting of 15 αβ-subunits that feature a gap, which is also consistent with data from optical single-molecule experiments. 14,15 Each subunit accommodates two light harvesting BChl a molecules giving rise to an absorption band at 875 nm. The reaction center complex accommodates a BChl a dimer (special pair, primary donor, P), two accessory BChl a molecules (B A , B B ), two bacteriopheophytin a (BPheo a) molecules (H A , H B ), and two ubiquinones (Q A , Q B ) bound to two protein subunits denoted L and M. The cofactors are arranged in two nearly identical branches, called A and B, which share the primary donor, P. 9,11,16,17 In particular the RC from Rhodobacter (Rb.) sphaeroides has served as a cornerstone for elucidating structure−function relationships employing a large variety of spin resonance 18−23 and optical spectroscopies. 24−31 From these studies it is well established that in the RC the absorbed photon-energy is used to dri...

Exciton Transport in Molecular Aggregates – From Natural Antennas to Synthetic Chromophore Systems

“…Finally, the energy is transfered from the LH1 complex to the RC (35 ps) where it initiates a charge separation. [293][294][295][296] b) Spatial organization of the B800 (yellow) and B850 (red) BChl a molecules in LH2 from Rps. acidophila.…”

“…Upon excitation of the LH2 complexes the excitation energy equilibrates within a single complex on an ultrafast timescale (<1 ps) followed by energy transfer (red arrows) between the LH2 complexes (10 ps) and subsequently between the LH2 and LH1 complex (5 ps). Finally, the energy is transfered from the LH1 complex to the RC (35 ps) where it initiates a charge separation . b) Spatial organization of the B800 (yellow) and B850 (red) BChl a molecules in LH2 from Rps.…”

Photoprotection through ultrafast charge recombination in photochemical reaction centres under oxidizing conditions

“…The likely reason is that ultrafast processes are easier to study in a uniform sample and partial P oxidation can easily be corrected with mild reductants. Previous studies of dynamics in oxidized RCs have employed high concentrations (5-500 mM) of potassium ferricyanide to oxidize P [25][26][27][28][29] or used a high excitation rate to photo-accumulate P þ [37], whereas studies of excitation trapping in closed RC-LH1 complexes have used a high excitation intensity and/or background illumination to photo-accumulate P þ [21,22].…”

“…Such RCs are described as being 'closed' and, in principle, this circumstance could result in the formation of triplet excited states and photodamage. However, a remarkable feature of purple bacterial photosystems is that closed RCs accept energy from the LH1 antenna with a lifetime of around 200 ps, only fourfold slower than in the case for 'open' RCs with P reduced [21,22]. This implies a mechanism whereby P þ can accept energy from other pigments.…”

Spatial Patterns of Light-Harvesting Antenna Complex Arrangements Tune the Transfer-to-Trap Efficiency of Excitons in Purple Bacteria

“…The hop constant k h in eq is set to the experimentally measured value of 0.06173 ps –1 . The rate of fluorescence is assumed to be constant for each type of protein and is consistent with experimentally measured fluorescence lifetimes of the corresponding complexes: k F LH1 = 0.005 ps –1 and k F LH2 = 0.0025 ps –1 . The only free parameter left is the transfer rate from LH1 to the RC, which is obtained as k RC LH1 = 0.015625 ps –1 by fitting the exciton lifetime in the membrane with an LH1:LH2 ratio of 1:2 to 60 ps .…”