Bacteriopheophytin triplet state in Rhodobacter sphaeroides reaction centers

Abstract

It is well established that photoexcitation of Rhodobacter sphaeroides reaction centers (RC) with reduced quinone acceptors results in the formation of a triplet state localized on the primary electron donor P with a significant yield. The energy of this long-lived and therefore potentially damaging excited state is then efficiently quenched by energy transfer to the RC spheroidenone carotenoid, with its subsequent decay to the ground state by intersystem crossing. In this contribution, we present a detailed transient absorption study of triplet states in a set of mutated RCs characterized by different efficiencies of triplet formation that correlate with lifetimes of the initial charge-separated state P+HA−. On a microsecond time scale, two types of triplet state were detected: in addition to the well-known spheroidenone triplet state with a lifetime of ~4 μs, in some RCs we discovered a bacteriopheophytin triplet state with a lifetime of ~40 μs. As expected, the yield of the carotenoid triplet increased approximately linearly with the lifetime of P+HA−, reaching the value of 42 % for one of the mutants. However, surprisingly, the yield of the bacteriopheophytin triplet was the highest in RCs with the shortest P+HA− lifetime and the smallest yield of carotenoid triplet. For these the estimated yield of bacteriopheophytin triplet was comparable with the yield of the carotenoid triplet, reaching a value of ~7 %. Possible mechanisms of formation of the bacteriopheophytin triplet state are discussed.

Temperature dependence of nanosecond charge recombination in mutant Rhodobacter sphaeroides reaction centers: modelling of the protein dynamics

“…At a longer time scale after excitation there is also a small component, non-decaying on a 200-ns time scale, which for the WT RC varies from 4% of the initial signal at room temperature (RT) to 7.5% at 78 K. In the FLY and FLA RCs this component varies, respectively, from 0 to 4% or oscillates at about 0%. This small component is assigned to the triplet state of the primary electron donor, 3 P, which subsequently is transferred to the nearby carotenoid molecule [40] and then decays within ~ 4 µs [41]. The low signal from triplet state correlates with the results of our previous room temperature broad-band transient absorption studies on WT, FLA and FLA RCs showing the triplet yield of only 14% for WT and 5-10% for the two mutants under study [41].…”

“…The low yield of triplet formation and its poor detection at 690 nm in the mutants did not allow solid conclusions to be drawn on the obtained values of the k t rate constant, which exceeded (100 ns) −1 at all temperatures even for WT RCs that show a higher yield of triplet formation (Figs. 5F and 6) [41]. We may speculate that the small value of k t may be related with the reduced state of Q A and specific mutation effects.…”

Insight into Electron Transfer from a Redox Polymer to a Photoactive Protein

“…The addition of sodium ascorbate to the solution of RCs causes their closing by the formation of reduced Q A (Q A − ). The occurrence of this was evidenced (Figure S5) by the appearance of faster decay components attributable to P + H A − charge recombination (on the order of ∼10 ns 54,55 ) and T Car formation and decay (nanoseconds and microseconds, respectively 56,57 ).…”

“…Moreover, it is conceivable that access of Q 0 to the Q-side of the protein in some RCs may have been locked by the adsorbed polymer (see below). Accordingly, the fastest ΔDADS is attributed to a mixture of T Car decay (expected lifetime ∼2.5–4 μs) , and P-Os → P + electron transfer. However, the main amplitude of this decay lies in the >750 nm region, where the signal comes almost exclusively from P/P + , and thus, P-Os → P + electron transfer has the dominating contribution to this lifetime.…”

Modelling of the cathodic and anodic photocurrents from Rhodobacter sphaeroides reaction centres immobilized on titanium dioxide

“…In “closed” RCs, where Q A is already reduced, the most probable event is recombination of P + H A − to the ground state. In a smaller percentage of RCs, recombination occurs to a long-lived triplet excited state of P, termed P T (Woodbury and Allen 1995 ), which is usually efficiently quenched either by the carotenoid or by the BPhes (Arellano et al 2004 ; Białek et al 2016 ). The quantum yield of primary charge separation in open RCs is near 100% (Wraight and Clayton 1974 ), while the yield of P T triplet formation in closed RCs is approximately 15% (Blankenship et al 1995 ).…”