Primary donor triplet states of Photosystem I and II studied by Q-band pulse ENDOR spectroscopy

Abstract

The photoexcited triplet state of the “primary donors” in the two photosystems of oxygenic photosynthesis has been investigated by means of electron-nuclear double resonance (ENDOR) at Q-band (34 GHz). The data obtained represent the first set of 1H hyperfine coupling tensors of the 3P700 triplet state in PSI and expand the existing data set for 3P680. We achieved an extensive assignment of the observed electron-nuclear hyperfine coupling constants (hfcs) corresponding to the methine α-protons and the methyl group β-protons of the chlorophyll (Chl) macrocycle. The data clearly confirm that in both photosystems the primary donor triplet is located on one specific monomeric Chl at cryogenic temperature. In comparison to previous transient ENDOR and pulse ENDOR experiments at standard X-band (9–10 GHz), the pulse Q-band ENDOR spectra demonstrate both improved signal-to-noise ratio and increased resolution. The observed ENDOR spectra for 3P700 and 3P680 differ in terms of the intensity loss of lines from specific methyl group protons, which is explained by hindered methyl group rotation produced by binding site effects. Contact analysis of the methyl groups in the PSI crystal structure in combination with the ENDOR analysis of 3P700 suggests that the triplet is located on the Chl aʹ (PA) in PSI. The results also provide additional evidence for the localization of 3P680 on the accessory ChlD1 in PSII.

Triplet Delocalization over the Reaction Center Chlorophylls in Photosystem II

“…5,24,27−31 The Chl molecules responsible for this triplet delocalization have not been clearly identified yet, although the possibility of P D1 , P D2 , Chl D2 , and even Pheo D1 has been suggested. 5,15,19,24,27,30,31 In this study, we identified the Chl molecules on which the triplet state is distributed in PSII by means of FTIR different spectroscopy in combination with site-directed mutagenesis in Synechocystis sp. PCC 6803.…”

Water oxidation in oxygenic photosynthesis studied by magnetic resonance techniques

“…Electron paramagnetic resonance and ENDOR have been used extensively in photosynthesis research since radicals, radical pairs and also triplet states are created in the light-induced single electron transfer that can ideally be investigated by these techniques [131][132][133][134][135][136] even in solution at physiological temperatures [137]. EPR on transition metals like manganese requires, however, cryogenic temperatures due to fast relaxation causing severe line broadening at higher temperatures.…”

Magnetophotoselection in the Investigation of Excitonically Coupled Chromophores: The Case of the Water-Soluble Chlorophyll Protein

“…These angular parameters allow us to precisely determine the orientation of ZFS axes in the molecular frame: a rotation of 8 • with respect to the axis passing through the methine groups has been found(see Figure 4C). Assignment of the ZFS tensor directions in the molecular frame is an important task to be fulfilled for exploiting the full potential of orientation-selective triplet-state EPR techniques [77,78]. As a further result, fixing the ZFS directions, this leads to a final orientation of the Q y TDM axes in the Chl a monomers, which translated to MeTHF and EtOH:MeOH mixture gives a deviation of 12 • and 3 • from the molecular y axis, respectively.…”

“…As a further result, fixing the ZFS directions, this leads to a final orientation of the Q y TDM axes in the Chl a monomers, which translated to MeTHF and EtOH:MeOH mixture gives a deviation of 12 • and 3 • from the molecular y axis, respectively. The assumption that the ZFS tensor directions are the same in the solvents and in WSCP is based on the fact that the ZFS axes orientations are not particularly affected by the environment, as demonstrated by the comparative ENDOR investigations on 3 Chl a in glassy matrix, in WSCP, as well as in photosynthetic reaction centres [27,46,66,78]. Thus, exploiting the excitonic interaction, we were also able to give the orientation of the Chl a Q y TDM, resulting in it being slightly rotated with respect to the molecular y axis.…”