We report light-induced electron paramagnetic resonance triplet spectra from samples of chloroplasts or digitonin photosystem I particles that depend upon the dark redox state of the bound acceptors of photosystem I. If the reaction centers are prepared in the redox state P-700 A X- FdB-FdA-, then upon illumination at 11K we observe a polarized chlorophyll triplet species which we interpret as arising from radical pair recombination between P-700+ and A-. This chlorophyll triplet is apparently the analog of the PR state of photosynthetic bacteria [Parson, W.W. & Cogdell, R.J. (1975) Biochim. Biophys. Acta 416, 105-149]. If the reaction centers are prepared in the dark redox state P-700 A X FdB-FdA-, then upon illumination at 11K we observe a different triplet species of uncertain origin, possibly pheophytin or carotenoid. This species is closely associated with the photosystem I reaction center and it traps excitation when P-700 is oxidized.

SUMMARYThe formation of chlorophyll triplet states during illumination of photosystem I reaction center samples depends upon the redox states -of P700, X and ferredoxin Centers A and B.When the reaction centers are in the + --states P700 A 1 X Fd 8 FdA and P700 A 1 X Fd 8 -FdA-prior to ill~mination, we observe electron paramagnetic resonance (EPR) spectra from a triplet species which has zero field splitting parameters(IDI and lEI) larger than those of either chlorophyll a or chlorophyll .!!_monomer triplet, and a polarization which results from population of the triplet spin sublevels by an intersystem crossing mechanism. We interpret this triplet as arising from photoexcited chlorophyll antenna species associated with reaction centers in the states P700+FdA-and P700 +x-, respectively, which undergo de-excitation via intersystem crossing. When the reaction centers are in the states P700 A 1 X Fd 8 -FdA-and P700 A 1 X-Fd 8 -FdAprior to illumination, we observe a triplet species with a polarization which results from population of the triplet spin _ sublevels by radical pair recombination, and which has a ,,0,1 value similar to that of chlorophyll a monomer. We interpret this triplet {the RPP triplet) as arising from 3 P700 which has been populated by the process P700+A 1 ----? 3 P700 A 1 • We observe both the RPP triplet and the chlorophyll antenna triplet when the reaction centers are in the state P700 A 1 X Fd 8 -FdA-' presumably because the processes P700+ A 1 -x ---7 P700+ A 1 X-and P700+ A 1 -x ---t 3 P700 A 1 X have similar rate constants when Centers A and B are reduced,. ' .... '• .. ,, 3 i.e., the forward electron transfer time ·from A 1 -to X is apparently much slower in the redox state P700 A 1 X Fd 8 -FdA-than it is in the state P700 A 1 X Fd 8 FdA.The amplitude of the RPP triplet does not decrease in the presence of a 13.5 Gauss wide EPR signal centered at g = 2.0 which was recorded in the dark prior to triplet measurements in samples previously frozen under intense illumination. This g = 2.0 signal, which has been attributed to phototrapped A 1 determined to have a halftime of 120 ms at 5 K, was attributed to the charge recombination between P700 and X. The faster phase has a halftime of about 1 ms at 5 K and was attributed to charge recombination between P700 and an additional acceptor, denotedA 1 • The. simplest interpretation of these data is that when X is not reduced prior to measurement, one observes the process ~=120 ms at 5 K whereas when X is reduced prior to illumination, one observes . ". . '• .• the P700 A 1 back reaction. They observed an EPR triplet from P700 whose polarization indicates that it has been populated during primary photochemistry from a radical pair precursor [5].In addition, the kinetic data are suggesti~e that photo-reduced A 1 -does not always transfer its electron to X when X is not reduced prior to photoexcitation. Sauer et al [3] reported that the 1 ms phase wa~ observed regardless of whether X was reduced prior to measurement, and Shuvalov et ~ [4] ob...