Pulse Electron Paramagnetic Resonance Studies of the Interaction of Methanol with the S₂State of the Mn₄O₅Ca Cluster of Photosystem II

Abstract

The binding of the substrate analogue methanol to the catalytic Mn4CaO5 cluster of the water-oxidizing enzyme photosystem II is known to alter the electronic structure properties of the oxygen-evolving complex without retarding O2-evolution under steady-state illumination conditions. We report the binding mode of 13C-labeled methanol determined using 9.4 GHz (X-band) hyperfine sublevel-correlation (HYSCORE) and 34 GHz (Q-band) electron spin–echo electron nuclear double resonance (ESE-ENDOR) spectroscopies. These results are compared to analogous experiments on a mixed-valence Mn(III)Mn(IV) complex (2-OH-3,5-Cl2-salpn)2Mn(III)Mn-(IV) (salpn = N,N′-bis(3,5-dichlorosalicylidene)-1,3-diamino-2-hydrox-ypropane) in which methanol ligates to the Mn(III) ion (Larson et al. (1992) J. Am. Chem. Soc., 114, 6263). In the mixed-valence Mn(III,IV) complex, the hyperfine coupling to the 13C of the bound methanol (Aiso = 0.65 MHz, T = 1.25 MHz) is appreciably larger than that observed for 13C methanol associated with the Mn4CaO5 cluster poised in the S2 state, where only a weak dipolar hyperfine interaction (Aiso = 0.05 MHz, T = 0.27 MHz) is observed. An evaluation of the 13C hyperfine interaction using the X-ray structure coordinates of the Mn4CaO5 cluster indicates that methanol does not bind as a terminal ligand to any of the manganese ions in the oxygen-evolving complex. We favor methanol binding in place of a water ligand to the Ca2+ in the Mn4CaO5 cluster or in place of one of the waters that form hydrogen bonds with the oxygen bridges of the cluster.

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

“…65 W3-1 , together with W6-1 discussed below, show the best overall agreement with the experimental values. On the other hand, both W3-1 and W4-1 were excluded in the previous section because they cannot explain the increase in the energy gap Δ E , thus these models cannot satisfy both constraints.…”

“…Still, this criterion is not sufficient to definitively distinguish between direct binding of methanol to Ca (model W4-1d ) or second-sphere interaction in the vicinity of O4 (model W6-1 ). In the following section this distinction is achieved conclusively by comparison of the computed 13 C hyperfine coupling parameters with the data of Oyala et al 65 …”

Ammonia Binds to the Dangler Manganese of the Photosystem II Oxygen-Evolving Complex

“…We have recently used ENDOR and HYSCORE to probe the hyperfine coupling of 13 C-labeled methanol to the S 2 -state of the OEC, and we interpret the observed weak 13 C hyperfine coupling to indicate that methanol does not bind directly to any of the Mn ions of the cluster. 28 The small coupling is consistent with ligation methanol to the Ca 2+ of the OEC, or in place of one of the nearby waters that hydrogen bond to bridging oxygens of the cluster.…”

Five-coordinate Mn IV intermediate in the activation of nature’s water splitting cofactor

“…Both these molecules associate with the water channels of PSII leading to the catalyst but do not inhibit catalysis, although there is evidence that they render S-state advancement less efficient (37,38). In the case of methanol, it associates with the channel that terminates at Mn4 (Asp61 channel) and possibly also the channel that terminates at the Ca 2+ (39) (Fig. 1A).…”