Charge Recombination between P700⁺ and A₁⁻ Occurs Directly to the Ground State of P700 in a Photosystem I Core Devoid of F_X, F_B, and F_A

The charge recombination between P700⁺ and electron acceptor A₁⁻ was studied by flash kinetic spectroscopy in a photosystem I core devoid of iron-sulfur centers F_X, F_B, and F_A. We showed previously that the majority of the flash-induced absorption change at 820 nm decayed with a 10-µs half-time, which we assigned to the disappearance of the P700 triplet formed from the backreaction of P700⁺ with A₁⁻ [Warren, P. V., Parrett, K. G., Warden, J. T., & Golbeck, J. H. (1990) Biochemistry 29, 6545–6550]. We have reinvestigated this assignment in the near-UV, blue, and near-IR wavelength regions. The difference spectrum from 380 to 480 nm and from 720 to 910 nm shows that the P700⁺ A₁⁻ charge recombination is dominated by the P700 cation rather than the P700 triplet. Accordingly, the 10-µs kinetic transient represents the direct backreaction of P700⁺ with A₁⁻, which repopulates the ground state of P700. This is unlike a P700-F_A/F_B complex where, in the presence of reduced F_X⁻, F_B⁻, and F_A⁻, the P700⁺ A₁⁻ charge recombination populates the P700 triplet state [Sétif, P., & Bottin, H. (1989) Biochemistry 28, 2689–2697]. The A₁ acceptor is highly susceptible to disruption by detergents in the absence of iron-sulfur center Fx. The addition of 0.1% Triton X-100 to the P700-A₁ core leads to a ∼2.5-fold increase in the magnitude of the flash-induced absorption change at 780 nm; thereafter, 85% of the absorption change decays with a 25-ns half-time and 15% decays with a 3-µs half-time. The spectrum of the 25-ns phase is a convolution of contributions from both P700⁺ and A₀⁻. When the P700⁺ spectrum is subtracted, the spectrum of A₀⁻ displays a maximum at 760 nm and doublet minima at 412 and 438 nm and has an extinction coefficient 1.4 and 1.8 times that of P700⁺ at 438 and 790 nm, respectively. The spectrum of the 3-µs kinetic phase shows a broad absorption increase between 730 and 820 nm accompanied by a broad bleaching between 390 and 450 nm, consistent with the decay of the P700 triplet formed in low quantum yield from the backreaction of P700⁺ with A₀⁻. The rise time of the P700 triplet was measured to be ∼25 ns, a value identical to that of the P700⁺ A₀⁻ charge recombination.