Flash-induced enhancements in the proton NMR relaxation rate of Photosystem II particles: Response to flash trains of 1-5 flashes

Abstract

Flash-induced enhancements of the proton NMR relaxation rate R1 of the solvent in suspensions of PS II particles have been recorded for sequences of 1-5 saturating flashes. The one-flash relaxation transient is a positive R1 enhancement of 0.0080 s-1, which relaxes to the preflash baseline with a half-time of 25 s. The two-flash response is a positive relaxation transient of nearly identical amplitude to the one-flash response, but with a slower decay (t1/2 [approximate] 40 s). The appearance of a strongly relaxing paramagnetic center after one flash is consistent with the expected properties of an Mn(III) --> Mn(IV) oxidation. In contrast, the absence of a further R1 enhancement after the second flash shows that a strongly relaxing paramagnetic center is not formed by S2 --> S3 as is expected for manganese redox chemistry involving oxidation of Mn(III); the NMR experiment gives no indication that manganese redox chemistry occurs on the S2 --> S3 transition. The three-flash R1 response is a positive R1 transient of 0.0066 s-1, which decays with a half-time of approx. 50 s to a stable value of 0.002 s-1 above the preflash baseline. Positive R1 enhancements have been observed in the presence of carbonylcyanide m-chlorophenylhydrazone (CCCP), an agent known to accelerate the decay of S2 and S3, at concentrations which very effectively suppress the one-flash and two-flash decays. These enhancements indicate that the formation of the S0 state from S1 involves the production of a strongly relaxing center, and the sign of the R1 response is consistent with the expected properties of an Mn(III) --> Mn(II) reduction. The kinetic behavior of the three-flash R1 transient is indicative of slow redox equilibria involving the strongly relaxing center of the S0 state. Flash transients across a five-flash cycle exhibit oscillatory behavior with a local minimum on the fourth flash. Simulations of the flash profile indicate the presence of an R1 contribution from a minor fraction of centers which are capable of transition to S2 and S3, but are inhibited at S3 --> S0.