Dimetal Center Research Articles

The complexed crystal structure of dihydropyrimidinase reveals a potential interactive link with the neurotransmitter γ-aminobutyric acid (GABA)

Dihydropyrimidinase (DHPase) plays a crucial role in pyrimidine degradation, showcasing a broad substrate specificity that extends beyond pyrimidine catabolism, hinting at additional roles for this ancient enzyme. In this study, we solved the crystal structure of Pseudomonas aeruginosa DHPase (PaDHPase) complexed with the neurotransmitter γ-aminobutyric acid (GABA) at a resolution of 1.97 Å (PDB ID 8WQ9). Our structural analysis revealed two GABA binding sites in each monomer of PaDHPase. Interactions between PaDHPase and GABA molecules, involving residues within a contact distance of <4 Å, were examined. In silico analyses via PISA and PLIP software revealed hydrogen bonds formed between the side chain of Cys318 and GABA 1, as well as the main chains of Ser333, Ile335, and Asn337 with GABA 2. Comparative structural analysis between GABA-bound and unbound states unveiled significant conformational changes at the active site, particularly within dynamic loop I, supporting the conclusion that PaDHPase binds GABA through the loop-out mechanism. Building upon this molecular evidence, we discuss and propose a working model. The study expands the GABA interactome by identifying DHPase as a novel GABA-interacting protein and provides structural insight into the interaction between a dimetal center in the protein's active site and GABA. Further investigations are warranted to explore potential interactions of GABA with other DHPase-like proteins and to understand whether DHPase may have additional regulatory and physiological roles in the cell, extending beyond pyrimidine catabolism.

Reactions of Heterometallic Phosphinidene-Bridged MoMn and MoRe Complexes with Sulfur and Selenium: From Chalcogenophosphinidene- to Trithiophosphonate-Bridged Derivatives.

Reactions of [MoReCp(μ-PR*)(CO)6] with S8 were strongly dependent on experimental conditions (R* = 2,4,6-C6H2tBu3). When using 1 equiv of sulfur, complex [MoReCp(μ-η2:κ1S-SPR*)(CO)6] was slowly formed at 313 K, with a thiophosphinidene ligand unexpectedly bridging the dimetal center in the novel μ-κ1S:η2 coordination mode, as opposed to the μ-κ1P:η2 mode usually found in related complexes. The latter underwent fast decarbonylation at 363 K to give [MoReCp(μ-η2:η2-SPR*)(CO)5], with a six-electron donor thiophosphinidene ligand rearranged into the rare μ-η2:η2 coordination mode. Depending on reaction conditions, reactions with excess sulfur involved the addition of two or three S atoms to the phosphinidene ligand to give new complexes identified as the dithiophosphinidene-bridged complex [MoReCp(μ-η2:κ2S,S'-S2PR*)(CO)5], its dithiophosphonite-bridged isomer [MoReCp(μ-κ2S,S':κ2S,S'-S2PR*)(CO)5], or the trithiophosphonate-bridged derivative [MoReCp(μ-κ2S,S':κ2S,S'-S3PR*)(CO)5], all of them displaying novel coordination modes of their PRS2 and PRS3 ligands, as determined by X-ray diffraction studies. In contrast, the related MoMn complex yielded [MoMnCp(μ-η2:η2-SPR*)(CO)5] under most conditions. A similar output was obtained in reactions with gray selenium for either MoRe or MoMn phosphinidene complexes, which under different conditions only gave the pentacarbonyl complexes [MoMCp(μ-η2:η2-SePR*)(CO)5] (M = Re, Mn), these providing a new coordination mode for selenophosphinidene ligands.

Essential role of a conserved aspartate for the enzymatic activity of plasmanylethanolamine desaturase

Plasmalogens are an abundant class of glycerophospholipids in the mammalian body, with special occurrence in the brain and in immune cell membranes. Plasmanylethanolamine desaturase (PEDS1) is the final enzyme of plasmalogen biosynthesis, which introduces the characteristic 1-O-alk-1′-enyl double bond. The recent sequence identification of PEDS1 as transmembrane protein 189 showed that its protein sequence is related to a special class of plant desaturases (FAD4), with whom it shares a motif of 8 conserved histidines, which are essential for the enzymatic activity. In the present work, we wanted to gain more insight into the sequence–function relationship of this enzyme and mutated to alanine additional 28 amino acid residues of murine plasmanylethanolamine desaturase including those 20 residues, which are also totally conserved—in addition to the eight-histidine-motif—among the animal PEDS1 and plant FAD4 plant desaturases. We measured the enzymatic activity by transient transfection of tagged murine PEDS1 expression clones to a PEDS1-deficient human HAP1 cell line by monitoring of labeled plasmalogens formed from supplemented 1-O-pyrenedecyl-sn-glycerol in relation to recombinant protein expression. Surprisingly, only a single mutation, namely aspartate 100, led to a total loss of PEDS1 activity. The second strongest impact on enzymatic activity had mutation of phenylalanine 118, leaving only 6% residual activity. A structural model obtained by homology modelling to available structures of stearoyl-CoA reductase predicted that this aspartate 100 residue interacts with histidine 96, and phenylalanine 118 interacts with histidine 187, both being essential histidines assumed to be involved in the coordination of the di-metal center of the enzyme.

Molecular Insights into How the Dimetal Center in Dihydropyrimidinase Can Bind the Thymine Antagonist 5-Aminouracil: A Different Binding Mode from the Anticancer Drug 5-Fluorouracil.

Dihydropyrimidinase (DHPase) is a key enzyme for pyrimidine degradation. DHPase contains a binuclear metal center in which two Zn ions are bridged by a posttranslationally carbamylated lysine. DHPase catalyzes the hydrolysis of dihydrouracil to N-carbamoyl-β-alanine. Whether 5-aminouracil (5-AU), a thymine antagonist and an anticancer drug that can block DNA synthesis and induce replication stress, can interact with DHPase remains to be investigated. In this study, we determined the crystal structure of Pseudomonas aeruginosa DHPase (PaDHPase) complexed with 5-AU at 2.1 Å resolution (PDB entry 7E3U). This complexed structure revealed that 5-AU interacts with Znα (3.2 Å), Znβ (3.0 Å), the main chains of residues Ser289 (2.8 Å) and Asn337 (3.3 Å), and the side chain of residue Tyr155 (2.8 Å). These residues are also known as the substrate-binding sites of DHPase. Dynamic loop I (amino acid residues Pro65-Val70) in PaDHPase is not involved in the binding of 5-AU. The fluorescence quenching analysis and site-directed mutagenesis were used to confirm the binding mode revealed by the complexed crystal structure. The 5-AU binding mode of PaDHPase is, however, different from that of 5-fluorouracil, the best-known fluoropyrimidine used for anticancer therapy. These results provide molecular insights that may facilitate the development of new inhibitors targeting DHPase and constitute the 5-AU interactome.

Allosteric cooperation in a de novo-designed two-domain protein

We describe the de novo design of an allosterically regulated protein, which comprises two tightly coupled domains. One domain is based on the DF (Due Ferri in Italian or two-iron in English) family of de novo proteins, which have a diiron cofactor that catalyzes a phenol oxidase reaction, while the second domain is based on PS1 (Porphyrin-binding Sequence), which binds a synthetic Zn-porphyrin (ZnP). The binding of ZnP to the original PS1 protein induces changes in structure and dynamics, which we expected to influence the catalytic rate of a fused DF domain when appropriately coupled. Both DF and PS1 are four-helix bundles, but they have distinct bundle architectures. To achieve tight coupling between the domains, they were connected by four helical linkers using a computational method to discover the most designable connections capable of spanning the two architectures. The resulting protein, DFP1 (Due Ferri Porphyrin), bound the two cofactors in the expected manner. The crystal structure of fully reconstituted DFP1 was also in excellent agreement with the design, and it showed the ZnP cofactor bound over 12 Å from the dimetal center. Next, a substrate-binding cleft leading to the diiron center was introduced into DFP1. The resulting protein acts as an allosterically modulated phenol oxidase. Its Michaelis-Menten parameters were strongly affected by the binding of ZnP, resulting in a fourfold tighter Km and a 7-fold decrease in kcat These studies establish the feasibility of designing allosterically regulated catalytic proteins, entirely from scratch.

Efficient Synthesis and Multisite Reactivity of a Phosphinidene-Bridged Mo-Re Complex. A Platform Combining Nucleophilic and Electrophilic Features.

The heterometallic complex [MoReCp(μ-PR*)(CO)6] (3) was prepared in 60% overall yield from syn-[MoCp(PHR*)(CO)2] via a three-step procedure involving complexes syn-[MoCp(PClR*)(CO)2] and [MoReCp(μ-PR*)(CO)7] as intermediate species (R* = 2,4,6-C6H2tBu3). The PR* ligand in 3 displays a novel asymmetric interaction with the dimetal center, involving a double bond with one atom (Mo) and a dative single bond with the other one (Re). Compound 3 underwent thermal isomerization involving a C-H bond cleavage to yield the hydride [MoReCp(μ-H){μ-P(CH2CMe2)C6H2tBu2}(CO)6] and reacted with I2 to give [MoReCpI2(μ-PR*)(CO)6], which displays a symmetrical phosphinidene bridge. Its reaction with methyl propiolate at 293 K proceeded with [2 + 2] cycloaddition of the alkyne and decarbonylation to yield the phosphapropenylidene-bridged complex [MoReCp{μ-κ2P,C:η3-PR*CHC(CO2Me)}(CO)5] as the major product, whereas its reaction with excess CN(4-C6H4OMe) at 273 K proceeded with formal [2 + 1] cycloaddition of the isocyanide and further isocyanide addition at the Re site to yield the complex [MoReCp{μ-η2P,C:κ1P-PR*CN(4-C6H4OMe)}(CO)6{CN(4-C6H4OMe)}], which displays an azaphosphaallene ligand in a novel bridging coordination mode.

Crystal structure of dihydropyrimidinase in complex with anticancer drug 5-fluorouracil

Dihydropyrimidinase (DHPase) catalyzes the reversible cyclization of dihydrouracil to N-carbamoyl-β-alanine in the second step of the pyrimidine degradation pathway. Whether 5-fluorouracil (5-FU), the best-known fluoropyrimidine that is used to target the enzyme thymidylate synthase for anticancer therapy, can bind to DHPase remains unknown. In this study, we found that 5-FU can form a stable complex with Pseudomonas aeruginosa DHPase (PaDHPase). The crystal structure of PaDHPase complexed with 5-FU was determined at 1.76 Å resolution (PDB entry 6KLK). Various interactions between 5-FU and PaDHPase were examined. Six residues, namely, His61, Tyr155, Asp316, Cys318, Ser289 and Asn337, of PaDHPase were involved in 5-FU binding. Except for Cys318, these residues are also known as the substrate-binding sites of DHPase. 5-FU interacts with the main chains of residues Ser289 (3.0 Å) and Asn337 (3.2 Å) and the side chains of residues Tyr155 (2.8 Å) and Cys318 (2.9 Å). Mutation at either Tyr155 or Cys318 of PaDHPase caused a low 5-FU binding activity of PaDHPase. This structure and the binding mode provided molecular insights into how the dimetal center in DHPase undergoes a conformational change during 5-FU binding. Further research can directly focus on revisiting the role of DHPase in anticancer therapy.

Metal-free class Ie ribonucleotide reductase from pathogens initiates catalysis with a tyrosine-derived dihydroxyphenylalanine radical

All cells obtain 2'-deoxyribonucleotides for DNA synthesis through the activity of a ribonucleotide reductase (RNR). The class I RNRs found in humans and pathogenic bacteria differ in (i) use of Fe(II), Mn(II), or both for activation of the dinuclear-metallocofactor subunit, β; (ii) reaction of the reduced dimetal center with dioxygen or superoxide for this activation; (iii) requirement (or lack thereof) for a flavoprotein activase, NrdI, to provide the superoxide from O2; and (iv) use of either a stable tyrosyl radical or a high-valent dimetal cluster to initiate each turnover by oxidizing a cysteine residue in the α subunit to a radical (Cys•). The use of manganese by bacterial class I, subclass b-d RNRs, which contrasts with the exclusive use of iron by the eukaryotic Ia enzymes, appears to be a countermeasure of certain pathogens against iron deprivation imposed by their hosts. Here, we report a metal-free type of class I RNR (subclass e) from two human pathogens. The Cys• in its α subunit is generated by a stable, tyrosine-derived dihydroxyphenylalanine radical (DOPA•) in β. The three-electron oxidation producing DOPA• occurs in Escherichia coli only if the β is coexpressed with the NrdI activase encoded adjacently in the pathogen genome. The independence of this new RNR from transition metals, or the requirement for a single metal ion only transiently for activation, may afford the pathogens an even more potent countermeasure against transition metal-directed innate immunity.

Charge Transfer Properties of Triarylamine Integrated Dimolybdenum Dyads.

Three quadruply bonded dimolybdenum complexes equipped with a triarylamine pendant, [(DAniF)3Mo2(μ-O2CC6H4N(C6H4CH3)2] (DAniF = N,N'-di(p-anisyl)formamidinate; [OO-ph-N]), [(DAniF)3Mo2(μ-OSCC6H4N(C6H4CH3)2] ([OS-ph-N]), and [(DAniF)3Mo2(μ-S2CC6H4N(C6H4CH3)2] ([SS-ph-N]), have been synthesized and characterized by single crystal X-ray diffraction. In electrochemical measurements, the redox couple for the organic amine group becomes irreversible, reflecting the substantially strong electronic interaction between the dimetal center and organic redox site. The potential difference for the two successive redox events, ca. ΔE1/2(E1/2(2)(N/N•+) - E1/2(1)(Mo2IV/V)), falls in the range of 0.5-0.8 V as estimated from the differential pulse voltammograms. For the monocation radicals [OO-ph-N]+, [OS-ph-N]+, and [SS-ph-N]+, obtained by chemical oxidation of the neutral precursor, a broad ligand (amine) to metal (Mo2) charge transfer (LMCT) absorption band is observed in the near-IR region. Interestingly, analogous to the intervalence charge transfer (IVCT) bands for mixed-valence complexes, the LMCT absorption bands, which are solvent dependent, decrease in energy and bandwidth as the electronic coupling between the two redox sites increases in an order of increasing S content in the chelating group. The electronic coupling matrix elements (Hab) are determined by optical analyses from the generalized Mulliken-Hush (GMH) theory, falling in the range of 400-800 cm-1 in CH2Cl2. These results indicate that in these radical cations the charge is localized. Time-dependent DFT calculations show that the frontier molecular orbitals for these asymmetrical donor-acceptor systems have unbalanced distribution of electron density, and the LMCT bands arise from an electronic transition from the pendant ligand-based to metal-based molecular orbitals, corresponding to donor (N)-acceptor (Mo2) charge transfer.

Crystal structure of dihydropyrimidinase from Pseudomonas aeruginosa PAO1: Insights into the molecular basis of formation of a dimer

Dihydropyrimidinase, a tetrameric metalloenzyme, is a member of the cyclic amidohydrolase family, which also includes allantoinase, dihydroorotase, hydantoinase, and imidase. In this paper, we report the crystal structure of dihydropyrimidinase from Pseudomonas aeruginosa PAO1 at 2.1 Å resolution. The structure of P. aeruginosa dihydropyrimidinase reveals a classic (β/α)8-barrel structure core embedding the catalytic dimetal center and a β-sandwich domain, which is commonly found in the architecture of dihydropyrimidinases. In contrast to all dihydropyrimidinases, P. aeruginosa dihydropyrimidinase forms a dimer, rather than a tetramer, both in the crystalline state and in the solution. Basing on sequence analysis and structural comparison of the C-terminal region and the dimer–dimer interface between P. aeruginosa dihydropyrimidinase and Thermus sp. dihydropyrimidinase, we propose a working model to explain why this enzyme cannot be a tetramer.

The Crystal Structure of an Integral Membrane Fatty Acid α-Hydroxylase

Neuronal electrical impulse propagation is facilitated by the myelin sheath, a compact membrane surrounding the axon. The myelin sheath is highly enriched in galactosylceramide (GalCer) and its sulfated derivative sulfatide. Over 50% of GalCer and sulfatide in myelin is hydroxylated by the integral membrane enzyme fatty acid 2-hydroxylase (FA2H). GalCer hydroxylation contributes to the compact nature of the myelin membrane, and mutations in FA2H result in debilitating leukodystrophies and spastic paraparesis. We report here the 2.6 Å crystal structure of sphingolipid α-hydroxylase (Scs7p), a yeast homolog of FA2H. The Scs7p core is composed of a helical catalytic cap domain that sits atop four transmembrane helices that anchor the enzyme in the endoplasmic reticulum. The structure contains two zinc atoms coordinated by the side chains of 10 highly conserved histidines within a dimetal center located near the plane of the cytosolic membrane. We used a yeast genetic approach to confirm the important role of the dimetal-binding histidines in catalysis and identified Tyr-322 and Asp-323 as critical determinants involved in the hydroxylase reaction. Examination of the Scs7p structure, coupled with molecular dynamics simulations, allowed for the generation of a model of ceramide binding to Scs7p. Comparison of the Scs7p structure and substrate-binding model to the structure of steroyl-CoA desaturase revealed significant differences in the architecture of the catalytic cap domain and location of the dimetal centers with respect to the membrane. These observations provide insight into the different mechanisms of substrate binding and recognition of substrates by the hydroxylase and desaturase enzymes.

X-ray structure of a mammalian stearoyl-CoA desaturase

Stearoyl-CoA desaturase (SCD) is conserved in all eukaryotes and introduces the first double bond into saturated fatty acyl-CoAs1–4. Since the monounsaturated products of SCD are key precursors of membrane phospholipids, cholesterol esters, and triglycerides, SCD is pivotal in fatty acid metabolism. Humans have two SCD homologs (SCD1 and SCD5), while mice have four (SCD1–SCD4). SCD1-deficient mice do not become obese or diabetic when fed a high-fat diet because of improved lipid metabolic profiles and insulin sensitivity5,6. Thus, SCD1 is a pharmacological target in the treatment of obesity, diabetes, and other metabolic diseases7. SCD1 is an integral membrane protein located in the endoplasmic reticulum, and catalyzes the formation of a cis-double bond between the 9th and 10th carbons of stearoyl- or palmitoyl-CoA8,9. The reaction requires molecular oxygen, which is activated by a diiron center, and cytochrome b5, which regenerates the diiron center10. To better understand the structural basis of these characteristics of SCD function, we crystallized and solved the structure of mouse SCD1 bound to stearoyl-CoA at 2.6 Å resolution. The structure shows a novel fold comprising four transmembrane helices capped by a cytosolic domain, and a plausible pathway for lateral substrate access and product egress. The acyl chain of the bound stearoyl-CoA is enclosed in a tunnel buried in the cytosolic domain, and the geometry of the tunnel and configuration of the bound acyl chain provide a structural basis for the regioselectivity and stereospecificity of the desaturation reaction. The dimetal center is coordinated by a unique configuration of nine conserved histidine residues that implies a potentially novel metal center and mechanism for oxygen activation. The structure also illustrates a possible route for electron transfer from cytochrome b5 to the diiron center.

X-Ray Structure of a Mammalian Stearoyl-Coa Desaturase-1

Stearoyl-CoA desaturase (SCD) is conserved in all eukaryotes and introduces the first double bond into saturated fatty acyl CoAs. Since the monounsaturated products of SCD are key precursors of membrane phospholipids, cholesterol esters, and triglycerides, SCD is pivotal in fatty acid metabolism. Humans have two SCD homologs (SCD1 and SCD5), and mice have four (SCD1-SCD4). SCD1-deficient mice do not become obese or diabetic when fed a high-fat diet because of improved lipid metabolic profiles and insulin sensitivity. Thus, SCD1 is a promising pharmacological target in the treatment of obesity, diabetes, and other metabolic diseases. SCD1 is an integral membrane protein located in the endoplasmic reticulum, and catalyzes the formation of a cis-double bond between the 9th and 10th carbons of stearoyl- or palmitoyl-CoA. The reaction requires molecular oxygen, which is activated by a diiron center, and cytochrome b5, which reduces the diiron center. To better understand the structural basis of these characteristics of SCD function, we crystallized and solved the structure of mouse SCD1 bound to a stearoyl-CoA molecule at 2.6 Å resolution. The structure shows a novel fold comprising four transmembrane helices capped by a cytosolic domain. The acyl chain of the bound stearoyl- CoA is enclosed in a tunnel buried in the cytosolic domain, and the geometry of the tunnel suggests the structural basis for the regioselectivity and stereospecificity of the desaturation reaction. The structure reveals a dimetal center coordinated by a unique configuration of nine conserved histidine residues that implies a potentially novel mechanism for oxygen activation. The structure also illustrates a potential pathway for substrate access and product egress, and a possible route for electron transfer from cytochrome b5 to the diiron center.

Optimizing the electronic properties of photoactive anticancer oxypyridine-bridged dirhodium(II,II) complexes.

A series of partial paddlewheel dirhodium compounds of general formula cis-[Rh2(xhp)2(CH3CN)n][BF4]2 (n = 5 or 6) were synthesized {xhp = 6-R-2-oxypyridine ligands, R = -CH3 (mhp), -F (fhp), -Cl (chp)}. X-ray crystallographic studies indicate the aforementioned compounds contain two cis-oriented bridging xhp ligands, with the remaining sites being coordinated by CH3CN ligands. The lability of the equatorial (eq) CH3CN groups in these complexes in solution is in the order -CH3 > -Cl > -F, in accord with the more electron rich bridging ligands exerting a stronger trans effect. In the case of cis-[Rh2(chp)2(CH3CN)6][BF4]2 (5), light irradiation enhances the production of the aqua adducts in which eq CH3CN is replaced by H2O molecules, whereas the formation of the aqua species for cis-[Rh2(fhp)2(CH3CN)6][BF4]2 (7) is only slightly increased by irradiation. The potential of both compounds to act as photochemotherapy agents was evaluated. A 16.4-fold increase in cytotoxicity against the HeLa cell line was observed for 5 upon 30 min irradiation (λ > 400 nm), in contrast to the nontoxic compound 7, which is in accord with the results from the photochemistry. Furthermore, the cell death mechanism induced by 5 was determined to be apoptosis. These results clearly demonstrate the importance of tuning the ligand field around the dimetal center to maximize the photoreactivity and achieve the best photodynamic action.

A (Ni–SIr)I model for [NiFe]hydrogenase

We report the synthesis and characterization of a μ-hydroxo NiRu complex as a model for the active site of (Ni–SIr)I of [NiFe]hydrogenase. This is the first example of the (Ni–SIr)I model with a bridging hydroxo ligand between dimetal centers and an available coordination site on Ni center cis to the bridging hydroxo ligand. We have determined the structure of the (Ni–SIr)I model complex by X-ray analysis and reported reversible switching between the catalytically inactive (Ni–SIr)I and a catalytically active (Ni–SIr)II models.

Reactions of the unsaturated ditungsten complexes [W2Cp2(μ-PPh2)2(CO)x] (x = 1, 2) with nitric oxide: stereoselective carbonyl displacement and oxygen-transfer reactions of a nitrite ligand.

The dicarbonyl complex trans-[W2Cp2(μ-PPh2)2(CO)2] (Cp = η(5)-C5H5) reacted rapidly with NO (5% in N2) at 273 K to give selectively cis-[W2Cp2(μ-PPh2)2(NO)2]. In contrast, the analogous reactions of monocarbonyl [W2Cp2(μ-PPh2)2(μ-CO)] yielded either trans-[W2Cp2(μ-PPh2)2(NO)2] or the nitrito complex [W2Cp2(μ-PPh2)2(ONO)(CO)(NO)] (W-W = 2.9797(4) Å), depending on experimental conditions, with the latter presumably arising from reaction with trace amounts of oxygen in the medium. The stereoselectivity of the above reactions can be rationalized by assuming the participation of 33-electron [W2Cp2(μ-PPh2)2(CO)(NO)] intermediates which rapidly add a second molecule of NO via η(2)-C5H5 intermediates to eventually yield the corresponding dinitrosyls with inversion of the stereochemistry at the dimetal center, as supported by density functional theory (DFT) calculations. The nitrito complex was thermally unstable and evolved through oxygen transfer either to the carbonyl ligand, to yield the above dinitrosyls with release of CO2, or to the phosphide ligand, to give the phosphinito derivative cis-[W2Cp2(μ-OPPh2)(μ-PPh2)(NO)2], depending on experimental conditions. According to DFT calculations, the first process would involve transient dissociation/recombination of the nitrite ligand followed by coupling to carbonyl to give an intermediate with a chelate W{C,N-C(O)ON(O)} ring. Indeed, the nitrite ligand could be easily removed upon reaction of the nitrito complex with Na(BAr'4), but immediate decomposition also took place to render the electron-precise dicarbonyl [W2Cp2(μ-PPh2)2(CO)2(NO)]BAr'4 (W-W = 2.9663(3) Å) as the unique product (Ar' = 3,5-C6H3(CF3)2). Attempts to decarbonylate the latter complex photochemically yielded instead the oxo derivatives cis- and trans-[W2Cp2(μ-PPh2)2(O)(NO)]BAr'4 as the only isolable products (W-W = 2.980(2) and 3.0077(3) Å, respectively).

P–C and C–C Coupling Processes in the Reactions of the Phosphinidene-Bridged Complex [Fe2(η5-C5H5)2(μ-PCy)(μ-CO)(CO)2] with Alkynes

The title complex reacts with excess HC≡C(p-tol) at room temperature to give selectively the complex [Fe2Cp{μ–η5:κ1-C5H4PCyCHCH(p-tol)}(CO)3] (Fe–Fe = 2.6930(7) Å), which displays a bridging (cyclopentadienylidene)(alkenyl)phosphine ligand resulting from an unprecedented coupling of the phosphinidene ligand to both the alkyne and one of the cyclopentadienyl (Cp) groups. At 348 K, however, a mixture is obtained of the above compound and two isomers of the phosphide-acyl complex [Fe2Cp2{μ–κ1:κ1,η1-CyPCHC(p-tol)C(O)}(μ-CO)(CO)] differing in the relative orientation of the Cp rings with respect to the Fe2P plane (Fe–Fe = 2.6069(8) Å in the cis isomer). The latter complex is decarbonylated photochemically to give the dicarbonyl derivative [Fe2Cp2{μ–η3:η1-PCyCHC(p-tol)}(μ-CO)(CO)], which displays a rare four-electron donor phosphaallyl ligand bridging the dimetal center. The stoichiometric reaction with dimethyl acetylenedicarboxylate proceeded analogously to yield the complexes [Fe2Cp{μ–η5:κ1-C5H4PCyC(CO2Me)CH(CO2Me)}(CO)3] and [Fe2Cp2{μ–κ1:κ1,η1-CyP(CO2Me)C(CO2Me)C(O)}(μ-CO)(CO)]. However, the reaction with 2 equiv of alkyne at 273 K gave selectively the dicarbonyl complex [Fe2Cp2{μ–κ1:η2-PCy(CCO2Me)4}(μ-CO)2] with a four-electron-donor phosphole ligand bridging the metal atoms derived from the coupling of the phosphinidene ligand with two alkyne molecules. The reactions with methyl propiolate turned out to be particularly complex since several compounds were formed in significant amounts depending on the reaction conditions. The (cyclopentadienylidene)(alkenyl)phosphine complex [Fe2Cp{μ–η5:κ1-C5H4PCyCHCH(CO2Me)}(CO)3] was selectively formed in the reaction with 2 equiv of alkyne in toluene at 348 K, whereas the slow addition of a dichloromethane solution of the title complex to a solution of 2 equiv of alkyne in the same solvent at 273 K led to the exclusive formation of the (alkenyl)(alkynyl)phosphine complex [Fe2Cp2(μ-CO)2(CO){κ1-PCy(C2H2CO2Me)(C2CO2Me)}] (Fe–Fe = 2.5246(8) Å in the cis isomer). All these products seem to follow from of a common zwitterionic intermediate resulting from the nucleophilic attack of the pyramidal phosphinidene ligand of the title complex to the less protected α carbon of the alkyne, which then evolves differently depending on the alkyne and experimental conditions.

Assembly of a Photoactivatable Cofactor Triad within a Designed Protein

Natural proteins hide the essential requirements for their function beneath layers of unnecessary complexity. To test our understanding of structure-function relationships and to develop proteins with novel properties, we engineer functional model proteins (maquettes) from scratch. Previously, we designed and demonstrated a 14.7 kDa 4-α-helix bundle that catalyzes light-activated electron transfer from zinc protoporphyrin IX to heme. However, without the use of a sacrificial electron-donor in solution, the electron and hole quickly recombine. In order to stabilize the charge-separated state for useful chemistry, we designed an extended 22.4 kDa version of this protein that incorporates a third cofactor, a di-metal center, to serve as an electron donor. A tyrosine residue was inserted between the zinc porphyrin and the di-metal site, similar to the redox-active YZ of photosystem II. The design of this protein was guided by analysis of natural cytochromes and carboxylate-bridged diiron and dimanganese proteins as well as previous designed proteins from the Dutton, DeGrado, Lombardi and Haehnel groups. The distances between cofactors were selected by applying the Moser-Dutton ruler to maximize electron transfer efficiency. Each cofactor binding site included multiple helical glycine residues to impart local flexibility to the protein backbone, minimize steric clashing between amino acid side chains and cofactors, and prevent cofactors from creating long-range structural distortions that could result in anticooperative binding. Ultraviolet/visible spectroscopy showed that all three cofactors bind stoichiometrically with dissociation constants less than 1 μM at pH 7.0, suggesting that modularity of these designed maquettes is extremely useful in constructing functional manmade enzymes. Ultimately, we would like to incorporate these functional maquettes in vivo to make them a part of natural redox cycles and to do useful catalysis.

Synthesis and Decarbonylation Reactions of Diiron Cyclopentadienyl Complexes with Bent-Phosphinidene Bridges

The phosphinidene complexes [Fe2Cp2(μ-PR)(μ-CO)(CO)2] (Cp = η5-C5H5; R=Cy, Ph, Mes, Mes*; Mes = 2,4,6-C6H2Me3, Mes* = 2,4,6-C6H2tBu3) were readily prepared in high yields through a two-step procedure starting from the corresponding phosphine complexes [Fe2Cp2(μ-CO)2(CO)(PH2R)]. When R = Cy, Ph, Mes, the first step required the oxidation of the phosphine complex with 1 equiv of [FeCp2]BF4, this being followed by spontaneous dehydrogenation to yield the cationic phosphide complexes [Fe2Cp2(μ-PHR)(μ-CO)(CO)2]BF4, while the formation of the related PMes*H-bridged complex required a double oxidation of the corresponding neutral precursor in the presence of a deprotonating agent. In all cases the final step involved the deprotonation of the above cations with strong bases such as M(OH) (M = Na, K). Attempts to decarbonylate the above phosphinidene complexes by either thermolysis or photolysis in solution gave results strongly dependent on the nature of R. Thus, the photolysis of the cyclohexylphosphinidene complex gave a mixture of the clusters [Fe4Cp4(μ3-PCy)2(μ3-CO)(μ-CO)(CO)] (Fe−Fe distances 2.6115(7) and 2.5332(8) Å) and [Fe3Cp3(μ3-PCy)(μ-PCyH)(μ-CO)2], while the thermal decarbonylation of the mesitylphosphinidene complex in refluxing toluene gave the trinuclear derivative [Fe3Cp3(μ-PMes)(μ-PMesH)(μ-CO)2(CO)], having a trigonal phosphinidene ligand bridging two iron atoms (Fe−P = 2.161(8), 2.165(8) Å). The formation of all these compounds presumably involves the participation of the corresponding intermediate complex [Fe2Cp2(μ-PR)(μ-CO)2], a species likely to contain a trigonal (four-electron-donor) phosphinidene ligand, not detected. In contrast, the supermesitylphosphinidene complex undergoes a C(tBu)−H oxidative addition to the P atom under mild thermal conditions to give the phosphine derivative [Fe2Cp2(μ-CO)2(CO){PH(CH2CMe2)C6H2tBu2}], while its photolysis gave a mixture of the above phosphine complex and the phosphide−hydride derivatives cis- and trans-[Fe2Cp2(μ-H){μ-P(CH2CMe2)C6H2tBu2}(CO)2], the latter resulting from the oxidative addition of a P−H bond to the dimetal center.

Nitrosyl derivatives of the unsaturated dihydrides [Mn 2(μ-H) 2(CO) 6(μ-L 2)] (L 2 = Ph 2PCH 2PPh 2, (EtO) 2POP(OEt) 2)

The title dimanganese complexes react with NO (5% in N 2) at room temperature to give as major products the corresponding hexanitrosyl derivatives [Mn 2(NO) 6(μ-L 2)] in moderate yields, and they react rapidly with NO 2 to give the corresponding hydride derivatives [Mn 2(μ-H)(μ-NO 2)(CO) 6(μ-L 2)], these having a nitrite ligand bridging the dimetal centre through the N and O atoms. The dppm-bridged dihydride also reacts selectively at 273 K with (PPN)NO 2 to give first the nitro derivative (PPN)[Mn 2(μ-H)(H)(NO 2)(CO) 6(μ-dppm)], which then transforms into the nitrosyl complex (PPN)[Mn 2(μ-CO)(CO) 5(NO)(μ-dppm)] at room temperature or above (dppm = Ph 2PCH 2PPh 2; PPN + = [N(PPh 3) 2] +). The latter anion reacts with (NH 4)PF 6 to give the hydride-bridged nitrosyl complex [Mn 2(μ-H)(μ-NO)(CO) 6(μ-dppm)] and with [AuCl(PPh 3)] to give the trinuclear cluster [AuMn 2(μ-NO)(CO) 6(μ-dppm)(PPh 3)] (Mn–Au = ca. 2.68 Å; Mn–Mn = 2.879(2) Å). Both products are derived from the addition of the added electrophile at the intermetallic bond and rearrangement of the nitrosyl ligand into a bridging position. In contrast, methylation of the anion with CF 3SO 3Me takes place at the nitrosyl ligand to yield the unstable methoxylimide derivative [Mn 2(μ-NOMe)(CO) 6(μ-dppm)]. Analogous reactions at the nitrosyl ligand take place upon the addition of HBF 4·OEt 2 to the nitrosyl-bridged hydrides [Mn 2(μ-H)(μ-NO)(CO) n(μ-dppm) m] (n = 6, m = 1; n = 4, m = 2) to give the corresponding hydroxylimide derivatives [Mn 2(μ-H)(μ-NOH)(CO) n(μ-dppm) m]BF 4, which were also thermally unstable and could not be isolated nor fully characterized.