Michael J. Baldwin

A proposal for water oxidation in photosystem II

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).

Mixed-donor, alpha-hydroxy acid-containing chelates for binding and light-triggered release of iron.

A series of five new alpha-hydroxy acid-containing chelates inspired by photoactive marine siderophores, along with their Fe(III) complexes, have been synthesized and characterized. These chelates, designated X-Sal-AHA, each contributes a bidentate salicylidene moiety (X-Sal, X = 5-NO(2), 3,5-diCl, H, 3,5-di-tert-butyl, or 3-OCH(3) on the phenolate ring) and a bidentate alpha-hydroxy acid moiety (AHA). The X-ray crystal structure of Na[Fe(3)(3,5-diCl-Sal-AHA)(3)(mu(3)-OCH(3))] shows an Fe(III) trimer with the triply deprotonated, trianionic ligands each spanning two Fe(III)s that are bridged by the hydroxyl group of the ligand. Additionally, a mu(3)-methoxy anion caps the Fe(III)(3) face. Electrospray ionization mass spectra demonstrate that this structure is representative of the Fe(III) complexes of all five derivatives in methanol solution, with the exception of the X = 3,5-di-t-Bu derivative having a mu(3)-OH bridge rather than a methoxy bridge. Stability constants determined from reduction potentials range from 10(34) for the 5-NO(2) derivative to >10(40) for the 3,5-di-tBu derivative. All five complexes are photoactive when irradiated by sunlight, with the relative rate of photolysis as monitored by Fe(II) transfer correlating with the Hammett sigma(+) parameter for the phenolate ring substituents.

The effect of protonation on [Mn(IV)(μ2-O)]2 complexes

The series of complexes [Mn(IV)(X-SALPN)(μ2-O)]2, 1: X=5-OCH3; 2: X=H; 3: X=5-Cl; 4: X=3,5-diCl; 5: X=5-NO2, contain [Mn2O2]4+ cores with Mn-Mn separations of 2.7 Å. These molecules can be protonated to form [Mn(IV)(X-SALPN)(μ2-O,OH)]2+ in which a bridging oxide is protonated. The pKa values for the series of [Mn(IV)(X-SALPN)(μ2-O,OH)]2+ track linearly versus the shift in redox potential with a slope of 84 mV/pKa. This observation suggests that the [Mn2O2]4+ core can be considered as a unit in which the free energy of protonation is directly related to the ability to reduce the Mn(IV) ion. The marked sensitivity of the reduction potential to the presence of protons presents a mechanism in which an enzyme can control the oxidizing capacity of an oxo manganese cluster by the degree and timing of oxo bridge protonation.

Photochemical reactivity of the iron(III) complex of a mixed-donor, α-hydroxy acid-containing chelate and its biological relevance to photoactive marine siderophores.

The trimeric clusters [Fe(III)3(X-Sal-AHA)3(μ3-OCH3)](-), where X-Sal-AHA is a tetradentate chelate incorporating an α-hydroxy acid moiety (AHA) and a salicylidene moiety (X-Sal with X being 5-NO2, 3,5-diCl, all-H, 3-OCH3, or 3,5-di-t-Bu substituents on the phenolate ring), undergo a photochemical reaction resulting in reduction of two Fe(III) to Fe(II) for each AHA group that is oxidatively cleaved. However, photolysis of structurally analogous mixed Fe/Ga clusters demonstrate that a similar photolysis reaction will occur with only a single Fe(III) in the cluster. Quantum yields of iron reduction for the series of [Fe(III)3(X-Sal-AHA)3(μ3-OCH3)](-) complexes measured by monitoring Fe(II) production are twice those for ligand oxidation, measured by loss of the CD signal for the complex due to cleavage of the chiral AHA group.The quantum yields, 2-13% in the UVA and UVB ranges, are higher for complexes with electron-withdrawing X groups than for those with electron-donating X groups [corrected]. The observed final photolysis product of the chelate is different if irradiation is done in the air than if it is done under Ar. The first observed photochemical product is the aldehyde resulting from decarboxylation of the AHA. This is the final product under anaerobic conditions. In air, this is followed by an Fe- and O2-dependent reaction oxidizing the aldehyde to the corresponding carboxylate, then a second Fe- and light-dependent decarboxylation reaction giving a product that is two carbons smaller than the initial ligand. These reactivity studies have important biological implications for the photoactive marine siderophores. They suggest that different types of photochemical products for different siderophore structure types do not result from different initial photochemical steps, but rather from different susceptibility of the initial photochemical product to air oxidation.

Modeling the Chemistry and Properties of Multinuclear Manganese Enzymes

Dinuclear manganese complexes have been prepared using two similar ligands 2-OHsalpn and salpn. These complexes span the Mn-oxidation states from II/II to IV/IV. The [Mn(2-OHsalpn)]2 complexes have been demonstrated to be efficient functional models for the manganese catalases, and suggest a mechanism for inactivation of the enzymes. The high valent system [MnIV(salpn)(μ-O]2 can be successively protonated on the oxo bridges, which leads to changes in Mn-Mn distance, magnetic exchange and reaction chemistry. The second protonation leads to reductive decomposition of the dimer to MnIII monomers, likely accompanied by oxidation of water.

Structure-function correlations in copper clusters in proteins

A coupled binuclear copper active site is present in a wide variety of proteins and enzymes which perform different biological functions utilizing 02. The hemocyanins (Hc) reversibly bind 02, the tyrosinases (Ty) are monoxygenases which hydroxylate monophenols to o-diphenols and oxidize these to o-quinones, and the multicopper oxidases (laccase (Lc), ascorbate oxidase and ceruloplasmin), which contain additional copper centers (Type 1 and Type 2) catalyze the four electron reduction of 02 to water. Our original chemical and spectroscopic studies over a series of protein active site derivatives of Hc and Ty demonstrated (ref. 1,2) that these proteins have very similar active sites. The key features of this spectroscopically effective model for the coupled binuclear copper active site, reproduced in Fig. 1, are that dioxygen binds as peroxide, bridging two tetragonal Cu(1I)s in a cis p-l,2 fashion, with an additional endogenous bridge between the coppers (RO-). The endogenous bridge is likely hydroxide based on the crystal structure (ref. 3) of deoxy Hc. The major difference between the Hc and Ty sites is the high accessibility of the Ty active site to exogenous ligands (ref. 4). We found that substrate analogues bind directly to the copper in Ty and compete with peroxide for the same binding site (ref, 5). These studies resulted in the proposed structural mechanism for hydroxylation and oxidation catalysis given in Fig. 2. Alternatively, chemical and spectroscopic studies of a series of protein active site derivatives of Lc (which is the simplest multicopper oxidase, containing one Type 1, one Type 2, and one coupled binuclear Type 3 center) showed the Type 3 site to be strikingly different from the coupled binuclear site in Hc and Ty (ref. 6). Low-temperature magnetic circular dichroism (LTMCD) spectroscopy was found to be a powerful probe of the different copper centers in the multicopper oxidases, allowing a correlation of excited state spectral features with the ground state magnetic properties. From LTMCD studies, the Type 3 site was in fact found to be part of a trinuclear comer cluster with the Type 2 center (ref. 7). This new class of copper site has recently been supported by crystallographic studies on ascorbate oxidase (ref. 8). Our recent studies have focussed on I) Spectroscopy of model complexes to further develop our understanding of the unique spectral features of oxy Hc; 11) A detailed comparison of the coupled binuclear site in Hc and the Type 3 site in Lc; 111) Evaluation of the metal centers required for the 02 reactivity of the multicopper oxidases; and IV) Detailed LTMCD spectral studies of the trinuclear copper cluster to establish its geometric and electronic structure and interactions with exogenous ligands as related to the mechanism of multielectron reduction of 02. These studies are summarized below.

Photochemistry and Anion-Controlled Structure of Fe(III) Complexes with an α-Hydroxy Acid-Containing Tripodal Amine Chelate

The tripodal amine chelate with two pyridyl groups and an α-hydroxy acid (AHA) group, Pyr-TPA-AHA, was synthesized. Different Fe(III) complexes form with this chelate depending upon the counterion of the Fe(III) source used in the synthesis. A dinuclear complex, Fe(III)2(Pyr-TPA-AHA)2(μ-O), 1, and mononuclear complexes Fe(III)(Pyr-TPA-AHA)X (X = Cl- or Br-, 2 and 3, respectively) were synthesized. 2 can be easily converted to 1 by addition of silver nitrate or a large excess of water. The structure of 1 was solved by X-ray crystallography (C32H34N6O7Fe2·13H2O, a = 14.1236(6) Å, b = 14.1236(6) Å, c = 21.7469(15) Å, α = β = γ = 90°, tetragonal, P42212, Z = 4). 2 and 3 each have simple quasireversible cyclic voltammograms with E1/2 (vs aqueous Ag/AgCl) = +135 mV for 2 and +470 for 3 in acetonitrile. The cyclic voltammogram for 1 in acetonitrile has a quasireversible feature at E1/2 = -285 mV and an irreversible cathodic feature at -1140 mV. All three complexes are photochemically active upon irradiation with UV light, resulting in cleavage of the AHA group and reduction of the iron to Fe(II). Photolysis of 1 results in reduction of both Fe(III) ions in the dinuclear complex for each AHA group that is cleaved, while photolysis of 2 and 3 results in reduction of a single Fe(III) for each AHA cleavage. The quantum yields for 2 and 3 are significantly higher than that of 1.

The linear MnII complex Mn3(5-NO2-salimH)2(OAc)4 provides an alternative structure type for the carboxylate shift in proteins

The title complex {where 5-NO2-salimH2= 4-[2-(5-NO2-salicylideneimine)ethyl]imidazole} has been structurally characterized and is the first µ-phenolato, µ-carboxylato-O, µ-carboxylato-O,O′ type model for the ‘carboxylate shift’ in polymanganese and polyiron enzymes.

Aqua­(pyridine N-oxide-κO)[tris(2-hydroxy­amino­propyl)­amine-κ4N]­nickel(II) dinitrate

The title complex, [Ni(C5H5NO)(C9H18N4O3)(H2O)](NO3)2, is a six-coordinate pseudo-octahedral nickel(II) complex containing a tripodal amine ligand framework with three oxime donors, as well as pyridine N-oxide and aqua ligands. This complex displays crystallographic mirror symmetry and O—H⋯O hydrogen bonding between the cation and anion.

Tris(1‐propan‐2‐onyl oxime)­amine

The title compound, Ox3H3 or C9H18N4O3, adopts an open extended structure which does not restrict solvent accessibility or metallation reactivity. An intermolecular O—H⋯N hydrogen bond is observed.