Critical evaluation of electron transfer kinetics in P700-FA/FB, P700-FX, and P700-A1 Photosystem I core complexes in liquid and in trehalose glass

Abstract

This work aims to fully elucidate the effects of a trehalose glassy matrix on electron transfer reactions in cyanobacterial Photosystem I (PS I). Forward and backward electron transfer rates from A_1A^- and A_1B^- to F_X, and charge recombination rates from A₀^-, A_1B^-, A_1A^-, F_X^-, and [F_A/F_B]^- to P₇₀₀⁺ were measured in P₇₀₀-F_A/F_B complexes, P₇₀₀-F_X cores, and P₇₀₀-A₁ cores, both in liquid and in a trehalose glassy matrix at 11% humidity. By comparing CONTIN-resolved kinetic events over 6 orders of time in increasingly simplified versions of PS I at 480 nm, a wavelength that reports primarily A_1A^-/A_1B^- oxidation, and over 9 orders of time at 830 nm, a wavelength that reports P₇₀₀⁺ reduction and A₀^- oxidation, assignments could be made for nearly all of the resolved kinetic phases. Trehalose-embedded PS I samples demonstrated partially arrested forward electron transfer. The fractions of complexes in which electron transfer did not proceed beyond A₀, A₁ and F_X were 53%, 16% and 22%, respectively, with only 10% of electrons reaching the terminal F_A/F_B clusters. The ~10 μs and ~150 μs components in both liquid and trehalose-embedded PS I were assigned to recombination between A_1B^- and P₇₀₀⁺ and between A_1A^- and P₇₀₀⁺, respectively. The kinetics and amplitudes of these resolved kinetic phases in liquid and trehalose-embedded PS I samples could be well-fitted by a kinetic model that allowed us to calculate the asymmetrical contribution of the A_1A^- and A_1B^- quinones to the electrochromic signal at 480 nm. Possible reasons for these effects are discussed.

Keywords: Charge recombination kinetics; Forward electron transfer reactions; Photosystem I; Primary charge separation; Trehalose glassy matrices.