No abstract

Hydrogen bonding is generally thought to play an important role in tuning the electronic structure and reactivity of metal-sulfur sites in proteins. To develop a quantitative understanding of this effect, S K-edge X-ray absorption spectroscopy (XAS) has been employed to directly probe ligand-metal bond covalency, where it has been found that protein active sites are significantly less covalent than their related model complexes. Sulfur K-edge XAS data are reported here on a series of P450 model complexes with increasing H-bonding to the ligated thiolate from its substituent. The XAS spectroscopic results show a dramatic decrease in pre-edge intensity. DFT calculations reproduce these effects and show that the observed changes are in fact solely due to H-bonding and not from the inductive effect of the substituent on the thiolate. These calculations also indicate that the Hbonding interaction in these systems is mainly dipolar in nature. The −2.5 kcal/mole energy of the H-bonding interaction was small relative to the large change in ligand-metal bond covalency (30%) observed in the data. A bond decomposition analysis of the total energy is developed to correlate the pre-edge intensity change to the change in Fe-S bonding interaction on H-bonding. This effect is greater for the reduced than the oxidized state, leading to a 330 mV increase in the redox potential. A simple model shows that E° should vary approximately linearly with the covalency of the Fe-S bond in the oxidized state, which can be determined directly from S K-edge XAS.

Novel dioxo-tungsten(VI) bis(1,2-benzenedithiolate) complexes with neighboring amide groups, as models for tungsten enzymes, (NEt4)2[W(VI)O2{1,2-S(2)-3,6-(RCONH)2C6H2}2] (R = CH3, t-Bu), were designed and synthesized. The presence of the NH...S hydrogen bond was confirmed through IR spectrometry and X-ray crystallographic analysis. In the W(VI)O2 complexes, the NH...S hydrogen bond trans to the oxo ligand is stronger than that cis to oxo. On the basis of comparisons with [W(VI)O2(1,2-S2C6H4)2](2-), the NH...S hydrogen bond positively shifted the W(VI)/W(V) redox potentials and depressed the reduction by benzoin or triphenylphosphine. These results suggest that the NH...S hydrogen bond stabilizes the oxo ligand through trans influence and regulates O-atom transfer in tungsten and molybdenum enzymes.

Cytochrome P-450 contains a cysteine thiolate ligand at the axial position of (porphinato)iron(III) core. The axial thiolate has been shown to promote the heterolysis of the coordinated dioxygen upon sequential donation of electron and proton. 1-3 The crystallographic structure of cytochrome P-450 cam reported by Poulos et al. 4 has suggested the presence of weak double NH‚‚‚S hydrogen bonds between S of Cys 357 and two amide NHs of Gly 359 and Gln 360, as shown in Figure 1a. Recently, the presence of two clear NH‚‚‚S hydrogen bonds was reported for the crystal structure of chloroperoxidase. 5 Similar hydrogen bonds are found in the active site of P-450 BM-3 , 6 P-450 terp , 7 and P-450 eryF . 8 For P-450 cam , a molecular dynamics simulation from the reported crystallographic structure results in the shortening of the NH‚‚‚S hydrogen bond distances. 9 Furthermore, comparison of the Protein Data Bank crystallographical structural parameters at the active sites of substrate-bound P-450 cam with substrate-free P-450 cam 10 indicates shorter N‚‚‚S distance at the active site of the substrate-bound P-450 cam accompanied by a positive shift of the reported redox potential. 11 A remarkable effect on the electrochemical properties by NH‚‚‚S hydrogen bond have already been revealed by us for various metal thiolate complexes 12-14 as well as iron-sulfur proteins. [15][16][17] In order

Iron(III) porphinate complexes of arenethiolate having single or double NH···S hydrogen bonds at the axial position, [FeIII(OEP){S-2,6-(RCONH)2C6H3}] (R = CF3 (1), CH3 (2)) or [FeIII(OEP)(S-2-RCONHC6H4)] (R = CF3 (3), CH3 (4), t-Bu (5)), were synthesized as models of P-450 and chloroperoxidase. The presence of an NH···S hydrogen bond in these complexes is confirmed by their crystal structures in the solid state, the IR shift of the amide NH band and the direct through-bond contact-shift of the amide 2H NMR signal in benzene. The NH···S hydrogen bond elongates the Fe−S bond distance, stabilizes the Fe(III) state, protects the complexes from decomposition by air and moisture, and shifts the redox to more positive potentials. These functions by the hydrogen bond are more significant than the effect of steric hindrance.

A monooxomolybdenum(IV) complex containing two intramolecular NH···S hydrogen bonds, (NEt(4))(2)[Mo(IV)O(1,2-S(2)-3-t-BuNHCOC(6)H(3))(2)], was synthesized. The trans isomer was crystallized as the major product, and the molecular structure was determined by X-ray analysis. The trans isomer was isomerized by heating in solution to give a 1:1 mixture of trans and cis isomers. Oxidation of these isomers by Me(3)NO afforded (NEt(4))(2)[Mo(VI)O(2)(1,2-S(2)-3-t-BuNHCOC(6)H(3))(2)]. (1)H NMR analysis revealed that the dioxomolybdenum(VI) complex existed as a single isomer where both oxo ligands were trans to each of the two hydrogen-bonded thiolate ligands. The Mo(VI)═O bond was effectively stabilized by the NH···S hydrogen bond via trans influence, which was determined using resonance Raman spectroscopy. These results were supported by preliminary density functional theory calculations.

Three cis-dioxomolybdenum(VI) complexes with dithiolene-like benzenedithiolate ligands, Q2[MoVIO2(1,2-benzenedithiolato)2] (Q = NEt4 (1a), PPh4 (1b)), [MoVIO2(3-triphenylsilyl-1,2-benzenedithiolato)2]2- (2), and [MoVIO2(4-methyl-1,2-benzenedithiolato)2]2- (3), were synthesized as models of oxidized sulfite oxidase and compared with the structure of (PPh4)2[MoVIO2(S2C2(CN)2)2]·2MeOH (4). The crystal data for 1b, chemical formula C60H48O2P2S4Mo, are a = 13.938(6) Å, b = 16.65(1) Å, c = 22.645(5) Å, β = 103.81(2)°, V = 5104(5) Å3, Z = 4, and space group P21/c and the data for 4·2MeOH, chemical formula C58H48O2P2S4Mo, are a = 20.273(3) Å, b = 15.632(5) Å, c = 17.685(4) Å, V = 5604(3) Å3, Z = 4, and space group Pbcn. 1b and 4 have a slightly distorted octahedral structure with two long MoS bonds (2.588(3) Å and 2.608(3) Å for 1b and 2.635(2) Å for 4) trans to each of the MoO groups and with two short MoS bonds (2.417(3) Å and 2.434(3) Å for 1b and 2.440(2) for 4) cis to them. The short SC bond distances of the thiolate trans to the MoO groups in 4 suggest the presence of partial double bonding at the SC (thioketone-like) bond, which contributes to the stabilization of the dioxomolybdenum(VI) species by weakening π-bonding in the MoS bonds trans to the MoO groups. The electron-donating or -withdrawing substituents, e.g. 4-CH3 or 3-SiPh3, on the 1,2-benzenedithiolate Mo(VI) complexes accelerate or retard its oxo-transfer reaction, respectively. The formation of any binuclear Mo(V) complex does not occur in the reaction between [MoVIO2(1,2-benzenedithiolato)2]2- and [MoIVO(1,2-benzenedithiolato)2]2- when the cis-dioxomolybdenum(VI) complex is reduced by triphenylphosphine or benzoin.

Carboxylate-containing polyamides were synthesized from 2,6-di(amino)benzoic acid and dimethylmalonyl dichloride, isophthaloyl dichloride, and fumaryl dichloride. These polymers, {NHC6H3-(COO)NHCOC(CH3)2CO}n, {NHC6H3(COO)NHCO-m-C6H4CO}n, and {NHC6H3(COO)NHCO-trans-CHd CHCO}n, have seven to eight carboxylate units in a polymer chain, and their Ca(II) complexes possess NH‚‚‚O hydrogen bonds, which prevents the Ca-O dissociation by hydrolysis. CaCO3 was obtained from aqueous Ca 2+ and CO3 2-in the presence of the polyamide or the Na salt of these polyamides ([Ca 2+ ]/ [monomer unit] ) 100) to give calcite, vaterite, and/or their mixture. We were able to observe polymers in CaCO3 utilizing the 13 C cross-polarization/magic angle spinning solid-state NMR. The binding of Ca(II) by carboxylate appears to be an important factor in these studies and may be relevant to biomineralization of CaCO3.