Split EPR Signals from Photosystem II Are Modified by Methanol, Reflecting S State-Dependent Binding and Alterations in the Magnetic Coupling in the CaMn₄ Cluster

Abstract

Methanol binds to the CaMn4 cluster in photosystem II (PSII). Here we report the methanol dependence of the split EPR signals originating from the magnetic interaction between the CaMn4 cluster and the Y(Z)* radical in PSII which are induced by illumination at 5 K. We found that the magnitudes of the "split S1" and "split S3" signals induced in the S1 and S3 states of PSII centers, respectively, are diminished with an increase in the methanol concentration. The methanol concentrations at which half of the respective spectral changes had occurred ([MeOH](1/2)) were 0.12 and 0.57%, respectively. By contrast, the "split S0" signal induced in the S0 state is broadened, and its amplitude is enhanced. [MeOH](1/2) for this change was found to be 0.54%. We discuss these observations with respect to the location and nature of the methanol binding site. Furthermore, by comparing this behavior with methanol effects reported for other EPR signals in the different S states, we propose that the observed methanol-dependent changes in the split S1 and split S0 EPR signals are caused by an increase in the extent of magnetic coupling within the cluster.

Probing tyrosine Z oxidation in photosystem II core complex isolated from spinach by EPR at liquid helium temperatures

“…Figure 4 shows that both the g = 2.03 and the weak g = 2.15 and g = 1.85 EPR signals disappear after addition of 6% (v/v) methanol in the PSII core complex. The disappearance of g = 2.03 signal upon addition of methanol is consistent with the previous report Su et al 2006). It is interesting that the weak g = 2.18 and 1.85 signals are also sensitive to the presence of methanol and disappear after addition of 6% methanol.…”

“…It is well known that methanol can interact with the Mn-cluster and significantly modify its EPR signals in different S states (Messinger et al 1997;Force et al 1998;Deak et al 1999;Su et al 2006;Å hrling et al 2006;Haddy 2007). Methanol can also affect the EPR signals arising from the magnetic interaction between Tyr Z • and the Mn-cluster in various S states .…”

The first tyrosyl radical intermediate formed in the S2–S3 transition of photosystem II

“…At these temperatures, electron donation to P680 + by Y Z can still occur in a minority of centers (o50%), but the subsequent electron transfer from the Mn 4 CaO 5 cluster to Y Z is blocked, arresting catalytic progression from S i Y Z to S i+1 Y Z . [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] Similar trapped intermediate states can be generated in chemically modified PSII, with examples seen in Ca 2+ depleted PSII, [30][31][32] acetate treated PSII, 13,33 and in PSII poised at high pH. 34,35 It is also noted that in the higher S-states (S 2 , S 3 ) infrared light excitation of the Mn 4 CaO 5 cluster also allows the ''backwards'' cycling, i.e.…”

Interaction of methanol with the oxygen-evolving complex: atomistic models, channel identification, species dependence, and mechanistic implications

“…The phenomenology of split EPR signals of both the S 1 Y Z ˙ and the S 0 Y Z ˙ states has been rationalized in terms of an increase in Δ E upon addition of methanol, 60 so a similar mode of interaction as for the S 1 and S 2 states (operating on the Mn3–Mn4 antiferromagnetic coupling) would be valid for the S 0 state as well. We prefer not to propose specific S 0 models at this point because of the more complex EPR phenomenology of this state 50,77,116,117 and remaining uncertainties in the protonation states of terminal water-derived ligands and oxo bridges.…”

“…47–65 A major effect is that it increases the energy separation between the lowest magnetic levels of the OEC, 48,51,60,62 stabilizing the S = 1/2 ground spin states of the S 0 and S 2 states and the diamagnetic ground state of S 1 . Other effects relate to the amplitude enhancement of specific spectral forms, for example of the g ≈ 2 multiline signal of the S 2 state in plants over the g ≥ 4 component.…”