Studies of Iron(III) Porphyrinates Containing Silanethiolate Ligands

The chemistry of several iron­(III) porphyrinates containing silanethiolate ligands is described. The complexes are prepared by protonolysis reactions of silanethiols with the iron­(III) precursors, [Fe­(OMe)­(TPP)] and [Fe­(OH)­(H₂O)­(TMP)] (TPP = dianion of <i>meso-</i>tetraphenylporphine; TMP = dianion of <i>meso-</i>tetramesitylporphine). Each of the compounds has been fully characterized in solution and the solid state. The stability of the silanethiolate complexes versus other iron­(III) porphyrinate complexes containing sulfur-based ligands allows for an examination of their reactivity with several biologically relevant small molecules including H₂S, NO, and 1-methylimidazole. Electrochemically, the silanethiolate complexes display a quasi-reversible one-electron oxidation event at potentials higher than that observed for an analogous arenethiolate complex. The behavior of these complexes versus other sulfur-ligated iron­(III) porphyrinates is discussed

Studies of Iron(III) Porphyrinates Containing Silanethiolate Ligands

The chemistry of several iron­(III) porphyrinates containing silanethiolate ligands is described. The complexes are prepared by protonolysis reactions of silanethiols with the iron­(III) precursors, [Fe­(OMe)­(TPP)] and [Fe­(OH)­(H₂O)­(TMP)] (TPP = dianion of <i>meso-</i>tetraphenylporphine; TMP = dianion of <i>meso-</i>tetramesitylporphine). Each of the compounds has been fully characterized in solution and the solid state. The stability of the silanethiolate complexes versus other iron­(III) porphyrinate complexes containing sulfur-based ligands allows for an examination of their reactivity with several biologically relevant small molecules including H₂S, NO, and 1-methylimidazole. Electrochemically, the silanethiolate complexes display a quasi-reversible one-electron oxidation event at potentials higher than that observed for an analogous arenethiolate complex. The behavior of these complexes versus other sulfur-ligated iron­(III) porphyrinates is discussed

Vibrational Analysis of Mononitrosyl Complexes in Hemerythrin and Flavodiiron Proteins: Relevance to Detoxifying NO Reductase

Flavodiiron proteins (FDPs) play important roles in the microbial nitrosative stress response in low-oxygen environments by reductively scavenging nitric oxide (NO). Recently, we showed that FMN-free diferrous FDP from <i>Thermotoga maritima</i> exposed to 1 equiv NO forms a stable diiron-mononitrosyl complex (deflavo-FDP­(NO)) that can react further with NO to form N₂O [Hayashi, T.; Caranto, J. D.; Wampler, D. A; Kurtz, D. M., Jr.; Moënne-Loccoz, P. Biochemistry 2010, 49, 7040−7049]. Here we report resonance Raman and low-temperature photolysis FTIR data that better define the structure of this diiron-mononitrosyl complex. We first validate this approach using the stable diiron-mononitrosyl complex of hemerythrin, Hr­(NO), for which we observe a ν­(NO) at 1658 cm^–1, the lowest ν­(NO) ever reported for a nonheme {FeNO}⁷ species. Both deflavo-FDP­(NO) and the mononitrosyl adduct of the flavinated FPD (FDP­(NO)) show ν­(NO) at 1681 cm^–1, which is also unusually low. These results indicate that, in Hr­(NO) and FDP­(NO), the coordinated NO is exceptionally electron rich, more closely approaching the Fe­(III)­(NO^–) resonance structure. In the case of Hr­(NO), this polarization may be promoted by steric enforcement of an unusually small FeNO angle, while in FDP­(NO), the Fe­(III)­(NO^–) structure may be due to a semibridging electrostatic interaction with the second Fe­(II) ion. In Hr­(NO), accessibility and steric constraints prevent further reaction of the diiron-mononitrosyl complex with NO, whereas in FDP­(NO) the increased nucleophilicity of the nitrosyl group may promote attack by a second NO to produce N₂O. This latter scenario is supported by theoretical modeling [Blomberg, L. M.; Blomberg, M. R.; Siegbahn, P. E. J. Biol. Inorg. Chem. 2007, 12, 79−89]. Published vibrational data on bioengineered models of denitrifying heme-nonheme NO reductases [Hayashi, T.; Miner, K. D.; Yeung, N.; Lin, Y.-W.; Lu, Y.; Moënne-Loccoz, P. Biochemistry 2011, 50, 5939−5947] support a similar mode of activation of a heme {FeNO}⁷ species by the nearby nonheme Fe­(II)

Spectroscopy and DFT Calculations of a Flavo-diiron Enzyme Implicate New Diiron Site Structures

Flavo-diiron proteins (FDPs) are non-heme iron containing enzymes that are widespread in anaerobic bacteria, archaea, and protozoa, serving as the terminal components to dioxygen and nitric oxide reductive scavenging pathways in these organisms. FDPs contain a dinuclear iron active site similar to that in hemerythrin, ribonucleotide reductase, and methane monooxygenase, all of which can bind NO and O₂. However, only FDP competently turns over NO to N₂O. Here, EPR and Mössbauer spectroscopies allow electronic characterization of the diferric and diferrous species of FDP. The exchange-coupling constant <i>J</i> (H_ex = <i>J</i><b>S</b>₁·<b>S</b>₂) was found to increase from +20 cm^–1 to +32 cm^–1 upon reduction of the diferric to the diferrous species, indicative of (1) at least one hydroxo bridge between the iron ions for both states and (2) a change to the diiron core structure upon reduction. In comparison to characterized diiron proteins and synthetic complexes, the experimental values were consistent with a dihydroxo bridged diferric core, which loses one hydroxo bridge upon reduction. DFT calculations of these structures gave values of <i>J</i> and Mössbauer parameters in agreement with experiment. Although the crystal structure shows a hydrogen bond between the iron bound aspartate and the bridging solvent molecule, the DFT calculations of structures consistent with the crystal structure gave calculated values of <i>J</i> incompatible with the spectroscopic results. We conclude that the crystal structure of the diferric state does not represent the frozen solution structure and that a mono-μ-hydroxo diferrous species is the catalytically functional state that reacts with NO and O₂. The new EPR spectroscopic probe of the diferric state indicated that the diferric structure of FDP prior to and immediately after turnover with NO are flavin mononucleotide (FMN) dependent, implicating an additional proton transfer role for FMN in turnover of NO