Spectral and kinetic characterization of electron acceptor A₁ in a Photosystem I core devoid of iron-sulfur centers F_X, F_B and F_A

The kinetic and spectroscopic properties of the secondary electron acceptor A₁ were determined by flash absorption spectroscopy at room and cryogenic temperatures in a Photosystem I (PS I) core devoid of the iron-sulfur clusters F_X, F_B and F_A. It was shown earlier (Warren, P.V., Golbeck, J.H. and Warden, J.T. (1993) Biochemistry 32: 849-857) that the majority of the flash-induced absorbance increase at 820 nm, reflecting formation of P700⁺, decays with a t_1/2 of 10 μs due to charge recombination between P700⁺ and A₁^-. Following A₁^- directly around 380 nm, where absorbance changes due to the formation of P700⁺ are negligible, two major decay components were resolved in this study with t_1/2 of ≈ 10 μs and 110 μs at an amplitude ratio of ≈ 2.5:1. The difference spectra between 340 and 490 nm of the two kinetic phases are highly similar, showing absorbance increases from 340 to 400 nm characteristic of the one-electron reduction of the phylloquinone A₁. When measured at 10 K, the flash-induced absorbance changes around 380 nm can be fitted with two decay phases of t_1/2 ≈ 15 μs and 150 μs at an amplitude ratio ≈ 1:1. The difference spectra of both kinetic phases from 340 to 400 nm are similar to those determined at 298 K and are therefore attributed to charge recombination in the pair P700⁺A₁^-. These results indicate that the backreaction between P700⁺ and A₁^- is multiphasic when F_X, F_B and F_A are removed, and only slightly temperature dependent in the range of 298 K to 10 K.