Critical evaluation of electron transfer kinetics in P-F/F, P-F, and P-A Photosystem I core complexes in liquid and in trehalose glass.

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 and A to F and charge recombination rates from A, A, A, F, and [F/F] to P were measured in P-F/F complexes, P-F cores, and P-A 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/A oxidation, and over 9 orders of time at 830 nm, a wavelength that reports P reduction and A 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 A, A and F were 53%, 16% and 22%, respectively, with only 10% of electrons reaching the terminal F/F clusters. The ~10 μs and ~150 μs components in both liquid and trehalose-embedded PS I were assigned to recombination between A and P and between A and P, 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 and A quinones to the electrochromic signal at 480 nm. Possible reasons for these effects are discussed.