ESEEM Studies of Alcohol Binding to the Manganese Cluster of the Oxygen Evolving Complex of Photosystem II

Abstract

An ESEEM (electron spin−echo envelope modulation) spectroscopic study employing a series of 2H-labeled alcohols provides direct evidence that small alcohols (methanol and ethanol) ligate to the Mn cluster of the oxygen evolving complex (OEC) of Photosystem II in the S2-state of the Kok cycle. A numerical method for calculating the through-space hyperfine interactions for exchange-coupled tetranuclear Mn clusters is described. This method is used to calculate hyperfine interaction tensors for protons [deuterons] in the vicinity of two different arrangements of Mn ions in a tetranuclear cluster: a symmetric cubane model and the EXAFS-based Berkeley “dimer-of-dimers” model. The Mn−H distances derived from the spectroscopically observed coupling constants for methanol and ethanol protons [deuterons] and interpreted with these cluster models are consistent with the direct ligation of these small alcohols to the OEC Mn cluster. Specifically, for methanol we can simulate the three-pulse ESEEM time domain pattern with three dipolar hyperfine interactions of 2.92, 1.33, and 1.15 MHz, corresponding to a range of maximal Mn−H distances in the models of 3.7−5.6 Å (dimer-of-dimers) and 3.6−4.9 Å (symmetric cubane). We also find evidence for limited access of n-propanol, but no evidence for 2-propanol or DMSO access. Implications for substrate accessibility to the OEC are discussed.

Effects of methanol on the S i -state transitions in photosynthetic water-splitting

“…Previous reports showed that methanol concentrations up to 10% (v/v; 2.5 M) do not affect the oxygen-evolution rates (Bernat et al 2002;Deak et al 1999;Force et al 1998), while at 20% (v/v; 5 M) a small, reversible decline was reported (Force et al 1998). The data in Fig.…”

“…In such experiments suspensions of PSII samples are illuminated for about 1 min with strong white light in the presence of artificial electron acceptors. These kinds of measurements have been performed by several groups and consistently did not reveal any inhibitory effects of methanol at concentrations of up to 5-10% (v/v) (Bernat et al 2002;Deak et al 1999;Force et al 1998). This finding raises the question as to whether or not methanol really acts as substrate analog in PSII.…”

“…For example, the S 3 state parallel mode EPR signal (Matsukawa et al 1999) and the g = 4.1 perpendicular mode S 2 state EPR signal (for review see, Haddy 2007) cannot be observed in presence of methanol or ethanol, while in PSII membrane fragments from higher plants the S 0 multiline signal can only be resolved after methanol addition (Messinger et al 1997a). An ESEEM study employing 2 H-labeled methanol clearly showed that methanol (and ethanol) binds at-or at least very close to-the Mn 4 O x Ca cluster (Force et al 1998). Consistent with this result it was recently demonstrated by molecular mechanics simulations that an access channel exists that is big enough in diameter to allow methanol (and of cause the smaller water) to diffuse from the lumen to the catalytic Mn 4 O x Ca cluster (Ho and Styring 2008;see, also Murray and Barber 2007).…”

Investigation of the interaction of the water oxidising manganese complex of photosystem II with the aqueous solvent environment

“…Models have been proposed suggesting di¡erent functions for the two dimers [17,18]. Our results indicate that the S 2 multiline signal is associated with a Mn dimer which is inaccessible to the solvent.…”

“…After the initial submission of this paper, a paper by Force et al [18] was published, describing ESEEM studies of the interaction of small alcohols, including methanol, with the S 2 state. As in the experiments reported here, weakly coupled deuterons were observed.…”