There has been much speculation concerning the mechanism for water oxidation by Photosystem 11. Based on recent work on the biophysics of Photosystem I1 and our own work on the reactivity of synthetic manganese complexes, we propose a chemically reasonable mechanistic model for the water oxidation function of this enzyme. An essential feature of the model is the nucleophilic attack by calcium-ligated hydroxide on an electrophilic 0x0 group ligated to high-valent manganese to achieve the critical 0-0 bond formation step. We also present a model for S-state advancement as a series of proton-coupled electron transfer steps, which has been proposed previously [Hoganson et. al., Photosynth. Res. 46, 177 (1995); Gilchrist et. al. Proc. Nat. Acad. Sci, USA. 92, 9545 (1995)], but for which we have developed model systems that allow us to probe the thermodynamics in some detail.One of the great unsolved mysteries in bioinorganic chemistry is the mechanism of water oxidation by the oxygen evolving complex (OEC) of Photosystem I1 (PS 11). This reaction is responsible for nearly all of the dioxygen on our planet and conceptually is the reverse reaction of respiration where dioxygen is converted back to water. Plants use an expansive airay of photopigments in Photosystem 11, four manganese ions, calcium and chloride to carry out these reactions. While intensively studied for many years, only now is a picture emerging as to how this fascinating and essential chemistry may result. The scope of this article is far too limited to allow for a detailed summary of previous studies in the field: therefore, interested readers are directed to recent reviews of this topic( ref. 1,2).In this contribution, we will present studies that are aimed at evaluating the chemical mechanism for water oxidation that is proposed in that proposed by G.T. Babcock(ref. 3, 4) but has significant chemical differences in the high and low S states. Important features of our proposal include: 1) oxidation of the catalytic center through a coupled protodelectron transfer from the manganese cluster to a redox active tyrosyl radical, 2) the generation in the S, state of a strongly electophilic manganyl 0x0 [Mn(V)=O] that can couple to a strongly nucleophilic hydroxyl group making a peroxide inteimediate and 3) oxidation of the transiently formed peroxide by a second 0x0 bridged dimer. Additionally, we p -1 consider the theirnodynamics of the system in order to evaluate implications for the energetics of water oxidation on cluster structure and reactivity. Figure 1 transitions require proton coupled electron transfer from the manganese cluster to a redox active tyrosine that is in close proximity to thc metal center. Functionally, this process is a hydrogen atom abstraction from a manganese bound water (hydroxide) hgand to a neutral tyrosyl radical. It is estimated that the homolytic bond dissociation energy (HBDE) for a tyrosine radical is 86.5 kcal/mol(ref. 6, 7). Thus, for H atom abstraction to be thermodynamically viable in this system, waterhydr...

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Interactions of water and methanol with a mixed valence Mn(III)Mn(IV) complex are explored with 1H electron spin echo (ESE)-electron nuclear double resonance (ENDOR) and 1H and 2H ESE envelope modulation (ESEEM). Derivatives of the (2-OH-3,5-Cl2-SALPN)2 Mn(III)Mn(IV) complex are ideal for structural and spectroscopic modeling of water binding to multinuclear Mn complexes in metalloproteins, specifically photosystem II (PSII) and manganese catalase (MnCat). Using ESE-ENDOR and ESEEM techniques, 1H hyperfine parameters are determined for both water and methanol ligated to the Mn(III) ion of the complex. The protons of water directly bound to Mn(III) are inequivalent and exhibit roughly axial dipolar hyperfine interactions (T dip = 8.4 MHz and T dip = 7.4 MHz), permitting orientations and radial distances to be determined using a model where the proton experiences a point dipole interaction with each Mn ion. General equations are given for the components of the rhombic dipolar hyperfine interaction between a proton and a spin coupled dinuclear metal cluster. The observed ENDOR pattern is from water protons 2.65 and 2.74 Å from the Mn(III) which make an Mn(IV)−Mn(III)−H angle of ∼160°. For the alcohol proton in the analogous methanol bound complex, a 2.65 Å Mn(III)−H distance is observed. Three pulse 2H ESEEM gives best fit Mn(III)−2H(1H) radial distances of 3.0, 3.5, and 4.0 Å for the three methyl deuterons in this complex.

The proton-initiated dissociation kinetics and equilibria of the mono, bis, and tris complexes of iron(III) with TV-methylacetohydroxamic acid (NMHA) were studied under conditions of 2.0 M NaC104/HC104 at 25 °C. The proton-dependent rate constants k3, k2, and k¡ for dissociation of the tris, bis, and mono complexes are 8.6 X 103 M"* 1 s"1,1.02 X 102 M"1 s-1, and 3.2 X 10~3 M"1 s-1, respectively. The corresponding equilibrium constants log K3, log K2, and log K\ are 1.06, -0.9, and -2.75, respectively. An acid-independent dissociation pathway is observed in the dissociation of the mono complex with a rate constant k\ =7.1 X 10~3 s_1. The solution NMR spectrum of the ligand shows split methyl peaks indicating hindered rotation about the C-N bond. The equilibrium ratio for the C-N rotation was found to be 3.5 in favor of the Z isomer and the minimum lifetime of the rotation was estimated to be 0.3 s. The rates and mechanism of Fe(NMHA);3"' dissociation are compared to corresponding processes observed for dissociation of (acetohydroxamato)iron(III) complexes and the natural trihydroxamate siderophore ferrioxamine B. Differences in rate and mechanism between the model systems and ferrioxamine B are discussed in terms of solvent effects, electrostatic effects, and C-N bond rotation in the hydroxamate group.