Switching sides—Reengineered primary charge separation in the bacterial photosynthetic reaction center

Abstract

We report 90% yield of electron transfer (ET) from the singlet excited state P* of the primary electron-donor P (a bacteriochlorophyll dimer) to the B-side bacteriopheophytin (HB) in the bacterial photosynthetic reaction center (RC). Starting from a platform Rhodobacter sphaeroides RC bearing several amino acid changes, an Arg in place of the native Leu at L185—positioned over one face of HB and only ∼4 Å from the 4 central nitrogens of the HB macrocycle—is the key additional mutation providing 90% yield of P+HB−. This all but matches the near-unity yield of A-side P+HA− charge separation in the native RC. The 90% yield of ET to HB derives from (minimally) 3 P* populations with distinct means of P* decay. In an ∼40% population, P* decays in ∼4 ps via a 2-step process involving a short-lived P+BB− intermediate, analogous to initial charge separation on the A side of wild-type RCs. In an ∼50% population, P* → P+HB− conversion takes place in ∼20 ps by a superexchange mechanism mediated by BB. An ∼10% population of P* decays in ∼150 ps largely by internal conversion. These results address the long-standing dichotomy of A- versus B-side initial charge separation in native RCs and have implications for the mechanism(s) and timescale of initial ET that are required to achieve a near-quantitative yield of unidirectional charge separation.

The origin of unidirectional charge separation in photosynthetic reaction centers: nonadiabatic quantum dynamics of exciton and charge in pigment–protein complexes

“…2d). This trend is qualitatively consistent with the experimental observations for the mutant PbRC, [47][48][49][50]52 where the calculated time constant ($118 ps) is quantitatively larger than the experimental values ($16 ps). 47 Here, only the local geometry of Phe-M210 was optimized in QM/MM, xing surrounding proteins at the original geometry of wild type, although the mutation may also affect the surrounding geometry.…”

“…The previous time-resolved spectroscopic measurements of mutant PbRC suggested that some specic residues especially contribute to unidirectional charge separation. [47][48][49][50]52 We have extensively analyzed the contribution of each residue to the potential shi on the pigments one by one, and concluded that Tyr-M210 near B L has the largest contribution to the stabilization of B L c À , 33,34 where Phe-L181 is located at the counterpart position near B M .…”

“…[5][6][7] Time-resolved spectroscopic measurements indicated that the excited electron in (P L P M )* is transferred to H L via B L along the L-branch on a time scale of a few ps. [39][40][41][42][43][44][45][46][47][48][49][50] Despite the pseudo-C 2 symmetric cofactor arrangement, the difference in the amino acid sequences between the L-and M-branches leads to the difference in the redox potentials of the pigments via electrostatic interactions and polarization. [51][52][53] A previous study using time-dependent density functional theory (TDDFT) with the quantum mechanics/molecular mechanics/polarizable continuum model (QM/MM/PCM) method indicated that the intermediate states of charge separation along the L-and M-branches, i.e., [P L P M ]c + B L c À and [P L P M ]c + B M c À , are lower and higher in energy than that of (P L P M )*, respectively.…”

Photosynthetic Reaction Center Variants Made via Genetic Code Expansion Show Tyr at M210 Tunes the Initial Electron Transfer Mechanism

“…S20 and Table S4). This biexponential P* decay and P + HAformation has been noted previously in the literature for WT RCs (6,10,15,18,19,44,45).…”

Photosynthetic reaction center variants made via genetic code expansion show Tyr at M210 tunes the initial electron transfer mechanism

“…An alternative model for ET has been proposed in which P* transfers an electron to HA directly through a superexchange mechanism, in which the BA chromophore mediates the electronic coupling between P* and HA. For wild-type (WT) RCs evidence favors 2-step ET at room temperature (1,12,13,15,16). It has been previously proposed that minor degrees of superexchange occur in WT RCs, likely arising from or enhanced by the inherent distribution in the energies of P*, P + BA -, and P + HAcaused by protein populations with slight variations in amino acid nuclear coordinates around chromophores (5,(17)(18)(19), but this has been difficult to study experimentally (1,12).…”