Thermodynamics of Proton and Electron Transfer in Tetranuclear Clusters with Mn–OH₂/OH Motifs Relevant to H₂O Activation by the Oxygen Evolving Complex in Photosystem II

Abstract

We report the synthesis of site-differentiated heterometallic clusters with three Fe centers and a single Mn site that binds water and hydroxide in multiple cluster oxidation states. Deprotonation of FeIII/II3MnII–OH2 clusters leads to internal reorganization resulting in formal oxidation at Mn to generate FeIII/II3MnIII–OH. 57Fe Mӧssbauer spectroscopy reveals that oxidation state changes (three for FeIII/II3Mn–OH2 and four for FeIII/II3Mn–OH clusters) occur exclusively at the Fe centers; the Mn center is formally MnII when water is bound and MnIII when hydroxide is bound. Experimentally determined pKa (17.4) of the [FeIII2FeIIMnII–OH2] cluster and the reduction potentials of the [Fe3Mn–OH2] and [Fe3Mn–OH] clusters were used to analyze the O–H bond dissociation enthalpies (BDEO–H) for multiple cluster oxidation states. BDEO–H increases from 69, to 78, and 85 kcal/mol for the [FeIIIFeII2MnII-OH2], [FeIII2FeIIMnII-OH2], and [FeIII3MnII-OH2] clusters, respectively. Further insight of the proton and electron transfer thermodynamics of the [Fe3Mn–OHx] system was obtained by constructing a potential–pKa diagram; the shift in reduction potentials of the [Fe3Mn–OHx] clusters in the presence of different bases supports the BDEO–H values reported for the [Fe3Mn–OH2] clusters. A lower limit of the pKa for the hydroxide ligand of the [Fe3Mn–OH] clusters was estimated for two oxidation states. These data suggest BDEO–H values for the [FeIII2FeIIMnIII–OH] and [FeIII3MnIII–OH] clusters are greater than 93 and 103 kcal/mol, which hints to the high reactivity expected of the resulting [Fe3Mn=O] in this and related multinuclear systems.

S = 3 Ground State for a Tetranuclear MnIV4O4 Complex Mimicking the S3 State of the Oxygen-Evolving Complex

“…Despite significant efforts to prepare tetra-and pentanuclear complexes as models of the OEC, relevant complexes in terms of structure, redox state, spectroscopy, and reactivity are rare. [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] On the basis of EPR, MCD, and X-ray spectroscopic studies, the electronic structure of the S 3 state has been assigned to a homovalent Mn IV 4 core with an S = 3 spin ground state. 5,[39][40][41] Two structural isomers S 3 A (dimer-ofdimers) and S 3 B (trimer-monomer), both with S = 3 spin ground states, have been invoked for the S 3 state ( Figure 1a).…”

What Are We Missing by Not Measuring the Net Charge of Proteins?

“…[1,4] Deoxyribonuclease and ovalbumin, for example, have identical isoelectricp oints of pI = 5.1, but the formal and measured net charge of both proteins differ by approximately 7u nits at pH 8.4. [4] The systematic absence of experimentally determinedv alues of Z has likely impeded ar igorous understanding of most chemicalp rocesses in which proteinsa re involved including aggregation and self-assembly, [20][21][22][23][24][25][26] ligand binding, [27][28][29][30][31][32][33][34] catalysis, [35][36][37][38][39] electron transfer, [3,6,[40][41][42][43][44][45][46][47] protein crystallization, [14,48] analytical separation, [49,50] and protein engineering. [51][52][53][54][55][56] It is tempting to assume that the formal net chargeo faprotein predicted from generalized residue pK a values (Z seq )issosimilar to the actual net charge that any difference is irrelevant, and the isoelectric point tells us all we need to know about ap rotein's net charge.…”

Remote Ligand Modifications Tune Electronic Distribution and Reactivity in Site-Differentiated, High-Spin Iron Clusters: Flipping Scaling Relationships

“…Two reasonable simulations were obtained, both of which afford Mössbauer parameters for one subsite (50% total iron) which are consistent with the presence of two high spin, six-coordinate Fe II centers (δ ~ 1.1 mm/s, |ΔEQ| ~ 3.2 mm/s) within the triiron core. 5,52,[64][65][66][67] The relative intensity of the sharp resonance near 1 mm/s indicates the presence of one ferric ion whose isomer shift and quadrupole splitting depend on how the Lorentzian feature near -0.5 mm/s is modelled, with δ bounded between 0.34-0.47 mm/s. Isomer shifts in this range are common for high spin, six-coordinate ferric centers in O/N rich ligand environments, 35,[54][55][56]68 suggesting a [Fe II 2Fe III ] redox level for the triiron core identical to that inferred from the solid state structure.…”