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Mononuclear Mo and W enzymes require a unique ligand known as molybdopterin (MPT). This ligand binds the metal through a dithiolene chelate, and the dithiolene bridges a reduced pyranopterin group. Pyran scission and formation have been proposed as a reaction of the MPT ligand that may occur within the enzymes to adjust reactivity at the Mo atom. We address this issue by investigating oxo-Mo(IV) model complexes containing dithiolenes substituted by pterin or quinoxaline and a hydroxyalkyl poised to form a pyran ring. While the pterin-dithiolene model complex exhibits a low energy, reversible pyran cyclization, here we report that pyran cyclization does not spontaneously occur in the quinoxalyl-dithiolene model. However, protonating the quinoxalyl-dithiolene model induces pyran cyclization forming an unstable, pyrano-quinoxalyl-dithiolene complex which subsequently dehydrates and rearranges to a pyrrolo-quinoxlyl-dithiolene complex that was previously characterized. The protonated pyrano-quinoxalyl-dithiolene complex was characterized by absorption spectroscopy and cyclic voltammetry, and these results suggest pyran cyclization leads to a significant change in the Mo electronic structure exhibited as a strong intraligand charge transfer (ILCT) transition and 370 mV positive shift of the Mo(V/IV) reduction potential. The influence of protonation on quinoxaline reactivity supports the hypothesis that the local protein environment in the second coordination sphere of molybdenum cofactor (Moco) could control pyran cyclization. The results also demonstrate that the remarkable chemical reactivity of the pterin-dithiolene ligand is subtly distinct and not reproduced by the simpler quinoxaline analog that is often used to replace pterin in synthetic Moco models.

An overview of the pyranopterin dithiolene (MPT) component of the molybdenum cofactor (Moco) and how MPT may contribute to enzymatic catalysis is presented. The chapter begins with a brief review of MPT and Moco biosynthesis and continues to explore the nature of what is arguably the most electronically complex ligand in biology. To explore this complexity, we have dissected MPT into its relevant molecular components. These include the redox-active ene-1,2-dithiolate (dithiolene) and pterin moieties, which are bridged by a pyran that may be found in ring-opened or ring-closed configurations. The various redox possibilities of MPT bound to Mo are presented, along with the electronic structure of the redox components. MPTs are found to display a remarkable conformational variance in pyranopterin Mo enzymes. This is discussed in terms of a relationship to enzyme function and the potential for the observed non-planer distortions to reflect different MPT oxidation and tautomeric states. The chapter ends with a series of case studies featuring model compounds that highlight how biomimetic small molecule studies have contributed to furthering our understanding of the roles this remarkable ligand plays in the catalytic cycles of the enzymes.

The two Arxula adeninivorans-based bioassays (A-YGS and A-YGFS) described here provide sensitive and reliable screening for glucocorticoids in aquatic environments. The biocomponents of A-YGS and A-YGFS were constructed to carry the human glucocorticoid receptor (hGRα) and the phytase gene (phyK, derived from Klebsiella sp. ASR1) or a fluorescence dsRED gene (derived from the Discosoma sp.), as reporter genes. The responses of A-YGS and A-YGFS were measured photometrically and spectrofluorometrically respectively. The half effective concentration (EC50) and limit of detection (LoD) values for dexamethasone obtained with A-YGS and A-YGFS were 0.81 and 0.29 μM, and 9.42 and 0.47 μM, respectively. Furthermore, both bioassays exhibited different binding specificities for several natural and synthetic glucocorticoids: A-YGS – corticosterone > cyproterone acetate > mifepristone > dexamethasone > cortisol > methylprednisolone > prednisolone and A-YGFS – cortisol >corticosterone >dexamethasone >prednisolone >betamethasone >methylprednisolone. As proof of principle, A-YGS was used to assay total glucocorticoids in river water, wastewater influent and effluent taken from a wastewater treatment plant in Germany. Glucocorticoids were not detected in both A-YGS and LC–MS/MS in seven of the eight samples, however a dexamethasone equivalent (DEQ) was detected in an influent wastewater sample using A-YGS (0.055 μM), while LC–MS/MS analysis showed that hydrocortisone and cortisone were present at 0.063 and 0.083 nM, respectively.

A p-coumaroyl esterase from Rhizoctonia solani was used to decrease 5-O-chlorogenic acid (5-CQA) content in coffee powder. HPLC-UV showed a decline of up to 98% of 5-CQA after the enzyme treatment. Effects on aroma were determined by means of aroma extract dilution analysis. Flavour dilution factors of treated and control extract differed in four volatile compounds only. Effect on aroma and taste was evaluated by sensory tests. No significant differences were perceived, and no off-flavour nor off-taste was noted. As chlorogenic acids are suspected to cause stomach irritating effects in sensitive people, the enzyme treatment offers a technically feasible approach to improve the quality of coffee beverages by reducing 5-CQA concentration without significantly affecting the aroma and taste profile.

The feruloyl esterase (FAE) gene EST1 from the basidiomycete Pleurotus sapidus was heterologously expressed in Escherichia coli and Pichia pastoris. Catalytically active recombinant Est1 was secreted using P. pastoris as a host. For expression in P. pastoris, the expression vector pPIC9K was applied. The EST1 gene was cloned with an N-terminal α-mating factor pre-pro sequence and expressed under the control of a methanol inducible alcohol oxidase 1 promotor. Est1 was purified to homogeneity using ion exchange and hydrophobic interaction chromatography. The recombinant Est1 showed optima at pH 5.0 and 50 °C, and released ferulic acid from saccharide esters and from the natural substrate destarched wheat bran. Substrate specificity profile and descriptor-based analysis demonstrated unique properties, showing that Est1 did not fit into the current FAE classification model. Transferuloylation synthesis of feruloyl-saccharide esters was proven for mono- and disaccharides.

A type D ferulic acid esterase (FAE) was identified in the culture supernatant of Streptomyces werraensis, purified, sequenced, and heterologously produced in E. coli BL21(DE3)Star by co-expressing chaperones groES-groEL (69 U L−1). The unique enzyme with a mass of about 48 kDa showed no similarity to other FAEs, and only moderate homology (78.5%) to a Streptomycete β-xylosidase. The purified reSwFAED exhibited a temperature optimum of 40 °C, a pH optimum in the range from pH seven to eight and a clear preference for bulky natural substrates, such as 5-O-trans-feruloyl-l-arabinofuranose (FA) and β-d-xylopyranosyl-(1→2)-5-O-trans-feruloyl-l-arabinofuranose (FAX), compared to the synthetic standard substrate methyl ferulate. Treatment of wheat dough with as little as 0.03 U or 0.3 U kg−1 reSwFAED activity resulted in a significant increase of the bun volume (8.0 or 9.7%, resp.) after baking when combined with polysaccharide-degrading enzymes from Aspergillus. For the first time, the long-standing, but rarely proven positive effect of a FAE in baking was confirmed.

A decarboxylase (IfPAD) from the ascomycete Isaria farinosa converted ferulic acid to 4-vinylguaiacol (4-VG), a volatile which imparts the distinct "smoke flavor" of pyrolized wood. The activity was enhanced by adding (E)-ferulic acid to the culture medium and peaked with 3.6 U g-1 mycelium (1 μmol 4-VG min-1). The coding sequence of 543 bp was translated into a 25 kDa protein with a homology of 91 % to putative phenolic acid decarboxylases of its teleomorph, Cordyceps militaris, and Beauveria bassiana, the anamorph of Cordyceps bassiana. Cold shock expression in Escherichia coli yielded 411 U g-1 wet mass. Substrate conversion required a hydroxyl substituent para to a trans-unsaturated C3-side chain of the aromatic ring. Km and kcat/Km values were determined to 0.3 mM and 78.4 mM-1s-1 for p-coumaric acid and 1.9 mM and 45.1 mM-1s-1 for (E)-ferulic acid, respectively. The native enzyme and its recombinant counterpart showed pH-optima at pH 6.0 and pH 5.5, and low temperature optima of 19 °C and 14 °C, respectively. IfPAD produced 4-VG from destarched wheat bran and sugar beet fiber, confirming activity on complex plant biomass. This is the first report on the biochemical characterization of a phenolic acid decarboxylase from a filamentous ascomycete.

The syntheses and X-ray structures of two molybdenum pyranopterin dithiolene complexes in biologically relevant Mo(4+) and Mo(5+) states are reported. Crystallography reveals that these complexes possess a pyran ring formed through a spontaneous cyclization reaction of a dithiolene side-chain hydroxyl group at a C═N bond of the pterin. NMR data on the Mo(4+) complex suggest that a reversible pyran ring cyclization occurs in solution. These results provide experimental evidence that the pyranopterin dithiolene ligand in molybdenum and tungsten enzymes could participate in catalysis through dynamic processes modulated by the protein.

The synthesis, characterization, binding to calf thymus DNA, and plasmid DNA photocleavage studies of two ruthenium(II) pteridinylphenanthroline complexes are reported where the new pteridinylphenantholine ligands in these complexes are additions to a larger family designed to resemble DNA bases. [Ru(bpy)(2)(L-keto)](PF(6))(2)1 is synthesized from ligand substitution of Ru(bpy)(2)Cl(2) by 4-keto-pteridino[6,7-f]phenanthroline (L-keto). Increasing the reaction temperature during synthesis of 1 causes a ring scission of the L-keto ligand within the pyrimidine ring yielding a second Ru complex, [Ru(bpy)(2)(L-aap)](PF(6))(2)2 where L-aap is 2-amino-3-amidopyrazino[5,6-f]phenanthroline. The ring cleavage reaction is accompanied by the loss of one carbon in the pyrimidine ring. Complexes 1 and 2 are characterized by (1)H NMR, UV/visible absorption and FT-IR spectroscopies and by cyclic voltammetry, and these results are presented in comparison to the previously reported related complexes [Ru(bpy)(2)(L-allox)](PF(6))(2), [Ru(bpy)(2)(L-amino)](PF(6))(2), and [Ru(bpy)(2)(dppz)](PF(6))(2). In addition, 2 has been structurally characterized by X-ray diffraction. Both 1 and 2 are good intercalators of calf thymus DNA as determined by viscometry and binding constants obtained from absorption titrations. Only the ring-cleaved complex 2 exhibits a high degree of pBR322 plasmid photocleavage in contrast to the other pteridinyl-phenanthroline complexes, which exhibit no plasmid DNA photocleavage. Complex 1, however, decomposes in buffer forming the photocleaver 2, demonstrating that sample age and reactivity can affect observed photocleavage. Complex 2 appears to photocleave DNA through a singlet oxygen mechanism.

Extracellular esterase activity was detected in submerged cultures of Rhizoctonia solani grown in the presence of sugar beet pectin or Tween 80. Putative type B feruloyl esterase (FAE) coding sequences found in the genome data of the basidiomycete were heterologously expressed in Pichia pastoris. Recombinant enzyme production on the 5-L bioreactor scale (Rs pCAE: 3245UL-1) exceeded the productivity of the wild type strain by a factor of 800. Based on substrate specificity profiling, the purified recombinant Rs pCAE was classified as a p-coumaroyl esterase (pCAE) with a pronounced chlorogenic acid esterase side activity. The Rs pCAE was also active on methyl cinnamate, caffeate and ferulate and on feruloylated saccharides. The unprecedented substrate profile of Rs pCAE together with the lack of sequence similarity to known FAEs or pCAEs suggested that the Rs pCAE represents a new type of enzyme. Hydroxycinnamic acids were released from agro-industrial side-streams, such as destarched wheat bran (DSWB), sugar beet pectin (SBP) and coffee pulp (CP). Overnight incubation of coffee pulp with the Rs pCAE resulted in the efficient release of p-coumaric (100%), caffeic (100%) and ferulic acid (85%) indicating possible applications for the valorization of food processing wastes and for the enhanced degradation of lignified biomass.

Agro-industrial side-streams are abundant and renewable resources of hydroxycinnamic acids with potential applications as antioxidants and preservatives in the food, health, cosmetic, and pharmaceutical industries. Feruloyl esterases (FAEs) from Schizophyllum commune were functionally expressed in Pichia pastoris with extracellular activities of 6000UL(-1). The recombinant enzymes, ScFaeD1 and ScFaeD2, released ferulic acid from destarched wheat bran and sugar beet pectin. Overnight incubation of coffee pulp released caffeic (>60%), ferulic (>80%) and p-coumaric acid (100%) indicating applicability for the valorization of food processing wastes and enhanced biomass degradation. Based on substrate specificity profiling and the release of diferulates from destarched wheat bran, the recombinant FAEs were characterized as type D FAEs. ScFaeD1 and ScFaeD2 preferably hydrolyzed feruloylated saccharides with ferulic acid esterified to the O-5 position of arabinose residues and showed an unprecedented ability to hydrolyze benzoic acid esters.

The molybdenum cofactor at the catalytic heart of mononuclear molybdoenzymes comprises a molybdenum ion coordinated by one or two dithiolene ligands containing an N-heterocyclic structure known as pterin. Understanding the details of the unusual combination of molybdenum with pterin and dithiolene is the impetus behind employing model chemistry to investigate the cofactor’s broad redox capabilities. This chapter highlights the major efforts to synthesize pterin-containing models and study their chemical properties. The history of identification of the cofactor’s pyranopterin dithiolene ligand and the details of pterin redox chemistry are reviewed, followed by an account of the synthesis and analysis of pterin-inspired chemical models. The implications of these models’ chemical reactivity and redox features that provide a fundamental basis for understanding the molybdenum cofactor are included. In addition, we highlight the potential directions of the field.

The molybdenum cofactor is composed of a molybdenum coordinated by one or two rather complicated ligands known as either molybdopterin or pyranopterin. Pterin is one of a large family of bicyclic N-heterocycles called pteridines. Such molecules are widely found in Nature, having various forms to perform a variety of biological functions. This article describes the basic nomenclature of pterin, their biological roles, structure, chemical synthesis and redox reactivity. In addition, the biosynthesis of pterins and current models of the molybdenum cofactor are discussed.

The interactions of five bis(bipyridyl) Ru(II) complexes of pteridinyl-phenanthroline ligands with calf thymus DNA have been studied. The pteridinyl extensions were selected to provide hydrogen-bonding patterns complementary to the purine and pyrimidine bases of DNA and RNA. The study includes three new complexes [Ru(bpy)(2)(L-pterin)](2+), [Ru(bpy)(2)(L-amino)](2+), and [Ru(bpy)(2)(L-diamino)](2+) (bpy is 2,2'-bipyridine and L-pterin, L-amino, and L-diamino are phenanthroline fused to pterin, 4-aminopteridine, and 2,4-diaminopteridine), two previously reported complexes [Ru(bpy)(2)(L-allox)](2+) and [Ru(bpy)(2)(L-Me(2)allox)](2+) (L-allox and L-Me(2)allox are phenanthroline fused to alloxazine and 1,3-dimethyalloxazine), the well-known DNA intercalator [Ru(bpy)(2)(dppz)](2+) (dppz is dipyridophenazine), and the negative control [Ru(bpy)(3)](2+). Reported are the syntheses of the three new Ru-pteridinyl complexes and the results of calf thymus DNA binding experiments as probed by absorption and fluorescence spectroscopy, viscometry, and thermal denaturation titrations. All Ru-pteridine complexes bind to DNA via an intercalative mode of comparable strength. Two of these four complexes--[Ru(bpy)(2)(L-pterin)](2+) and [Ru(bpy)(2)(L-allox)](2+)--exhibit biphasic DNA melting curves interpreted as reflecting exceptionally stable surface binding. Three new complexes--[Ru(bpy)(2)(L-diamino)](2+), [Ru(bpy)(2)(L-amino)](2) and [Ru(bpy)(2)(L-pterin)](2+)--behave as DNA molecular "light switches."

This study describes the development of a bioassay to detect the presence of progesterone and progesterone-like molecules in wastewater samples. The basis of the bioassay is the integration of the human progesterone receptor gene into the yeast Arxula adeninivorans for the constitutive synthesis of the receptor. After incubation, binding of the analyte to the receptor induces the production of a reporter protein. Two reporter proteins were compared for detection parameters such as half-maximal activity (EC50), limit of detection (LoD) and limit of quantification (LoQ). When the extracellular phytase K was used, an EC50 value of 155 ng L(-1) and a LoD of 27 ng L(-1) progesterone were obtained after 4 h incubation, while use of the fluorescent dsRED as the reporter protein, resulted in an EC50 of 320 ng L(-1) and a LoD of 65 ng L(-1) after 20 h incubation. Use of phytase K as the reporter protein offers decreased incubation time and increased sensitivity; however the dsRED reporter system is less labor-intensive. Additionally, the affinity of known agonists and antagonists of the human progesterone receptor was determined. The utility of this bioassay was confirmed by measuring total progesterone equivalent concentration of samples from a wastewater treatment plant. The A. adeninivorans-based transactivation assay was able to measure concentrations of about 311 ng L(-1) in the influent stream but could not detect progesterone activity in effluent. One key feature of the assay is the robustness of A. adeninivorans, which allows sample measurement without any sample preparation.

The conserved pterin dithiolene ligand that coordinates molybdenum (Mo) in the cofactor (Moco) of mononuclear Mo enzymes can exist in both a tricyclic pyranopterin dithiolene form and as a bicyclic pterin-dithiolene form as observed in protein crystal structures of several bacterial molybdoenzymes. Interconversion between the tricyclic and bicyclic forms via pyran scission and cyclization has been hypothesized to play a role in the catalytic mechanism of Moco. Therefore, understanding the interconversion between the tricyclic and bicyclic forms, a type of ring-chain tautomerism, is an important aspect of study to understand its role in catalysis. In this study, equilibrium constants (K(eq)) as well as enthalpy, entropy, and free energy values are obtained for pyran ring tautomerism exhibited by two Moco model complexes, namely, (Et4N)[Tp*Mo(O)(S2BMOPP)] (1) and (Et4N)[Tp*Mo(O)(S2PEOPP)] (2), as a solvent-dependent equilibrium process. Keq values obtained from (1)H NMR data in seven deuterated solvents show a correlation between solvent polarity and tautomer form, where solvents with higher polarity parameters favor the pyran form.

An extracellular chlorogenic acid esterase from Ustilago maydis (UmChlE) was purified to homogeneity by using three separation steps, including anion-exchange chromatography on a Q Sepharose FF column, preparative isoelectric focusing (IEF), and, finally, a combination of affinity chromatography and hydrophobic interaction chromatography on polyamide. SDS-PAGE analysis suggested a monomeric protein of ∼71 kDa. The purified enzyme showed maximal activity at pH 7.5 and at 37°C and was active over a wide pH range (3.5 to 9.5). Previously described chlorogenic acid esterases exhibited a comparable affinity for chlorogenic acid, but the enzyme from Ustilago was also active on typical feruloyl esterase substrates. Kinetic constants for chlorogenic acid, methyl p -coumarate, methyl caffeate, and methyl ferulate were as follows: K m values of 19.6 μM, 64.1 μM, 72.5 μM, and 101.8 μM, respectively, and k cat / K m values of 25.83 mM −1 s −1 , 7.63 mM −1 s −1 , 3.83 mM −1 s −1 and 3.75 mM −1 s −1 , respectively. UmChlE released ferulic, p -coumaric, and caffeic acids from natural substrates such as destarched wheat bran (DSWB) and coffee pulp (CP), confirming activity on complex plant biomass. The full-length gene encoding UmChlE consisted of 1,758 bp, corresponding to a protein of 585 amino acids, and was functionally produced in Pichia pastoris GS115. Sequence alignments with annotated chlorogenic acid and feruloyl esterases underlined the uniqueness of this enzyme.

Over the past two decades, a plethora of crystal structures of molybdenum enzymes has appeared in the literature providing a clearer picture of the enzymatic active sites and increasing the challenge to chemists to develop accurate models for those sites. In this minireview we discuss the most recent model studies aimed to reproduce detailed features of the pterin–dithiolene ligand, both as the uncoordinated form and as a chelate coordinated to molybdenum.

A series of molybdenum−pterin complexes produced from reactions of molybdenum and pterin reagents in various oxidation states has been investigated by X-ray photoelectron spectroscopy (XPS). Prior difficulties in making oxidation state assignments for the metal center and coordinated pterin can be resolved through comparison of Mo 3d binding energies (BE) for these new complexes with the BEs of standard molybdenum complexes. XPS analysis of molybdenum−pterin complexes produced from reactions of Mo(VI) reagents with tetrahydropterins show binding energies that are shifted by 1.5−1.8 eV to lower energies as compared to the BEs observed for the oxo-Mo(VI) reagents. The opposite shift in BE values is observed for complexes prepared from Mo(IV) chloride and fully oxidized pterins where BEs shift to higher values with respect to those for the starting Mo(IV) reagents. Remarkably, the BEs obtained for Mo−pterin complexes originating from Mo(VI)−tetrahydropterin reactions are nearly identical with those from Mo(I...

The coordination of alloxazine and pterins to molybdenum(IV) is demonstrated in this study. The synthesis of MoOCl3(pteridineH), where pteridineH is the protonated form of 1,3,7,8-tetramethylalloxazine (tmaz), 2-pivaloyl-6,7-dimethylpterin (piv-dmp), and 6,7-dimethylpterin (dmp), proceeds readily starting from Mo(IV)Cl4(acetonitrile)2 and the pteridine ligand in chloroform or methanol. X-ray crystal structures of MoOCl3(tmazH) (1) and MoOCl3(piv-dmpH) (2) show that Mo chelates each pteridine at the carbonyl oxygen and pyrazine nitrogen and that the pteridine ligand is protonated at the other nitrogen in the pyrazine ring. A third X-ray structure for MoOCl3(H3dmp) (4) is included in this work since its determination permits the comparison of metrical parameters for the oxidized and reduced forms of a pterin in identical molybdenum coordination environments. The major difference observed in the structures of 2 as compared to 4 is the Mo−N5 bond length which is significantly shorter in compound 4 containing ...

The synthesis and structure of a new molybdenum complex coordinated by a reduced pterin is reported. [MoOCl(detc)(H3dmp)]Cl (1) (where detc is diethyldithiocarbamate and H3dmp is 6,7-dimethyl-6,7,8-trihydropterin) is prepared from MoOCl2(detc)2 and H4dmp (6,7-dimethyl-5,6,7,8-tetrahydropterin). The X-ray structure determination of [MoOCl(detc)(H3dmp)]Cl·MeOH reveals an octahedral complex where the reduced pterin ligand coordinates through the carbonyl oxygen and pyrazine ring nitrogen atoms. An extensive hydrogen bonding network in the crystal lattice connects adjacent complexes, the chloride counterion, and the molecule of methanol. This hydrogen bonding persists in solution where it is identified by characteristic absorptions in the electronic spectrum. Dimethyl sulfoxide (DMSO) oxidizes [MoOCl(detc)(H3dmp)]Cl, producing 1 equiv of dimethylpterin and 1/2 equiv of oxidized dithiocarbamate, tetraethylthiuramdisulfide (TETDS), a reaction that constitutes a net five-electron oxidation of the molybdenum comp...

An extracellular feruloyl esterase (PeFaeA) from the culture supernatant of Pleurotus eryngii was purified to homogeneity using cation exchange, hydrophobic interaction, and size exclusion chromatography. The length of the complete coding sequence of PeFaeA was determined to 1668 bp corresponding to a protein of 555 amino acids. The catalytic triad of Ser-Glu-His demonstrated the uniqueness of the enzyme compared to previously published FAEs. The purified PeFaeA was a monomer with an estimated molecular mass of 67 kDa. Maximum feruloyl esterase (FAE) activity was observed at pH 5.0 and 50 °C, respectively. Metal ions (5 mM), except Hg(2+), had no significant influence on the enzyme activity. Substrate specificity profiling characterized the enzyme as a type A FAE preferring bulky natural substrates, such as feruloylated saccharides, rather than small synthetic ones. Km and kcat of the purified enzyme for methyl ferulate were 0.15 mM and 0.85 s(-1). In the presence of 3 M NaCl activity of the enzyme increased by 28 %. PeFaeA alone released only little ferulic acid from destarched wheat bran (DSWB), whereas after addition of Trichoderma viride xylanase the concentration increased more than 20 fold.

The combination of a pterin and a transition metal in many enzymes is the motivation for exploring the chemistry of pteridine complexes in detail. Unlike other biological ligands for essential transition metals, pterin is unique in displaying multi-electron redox reactivity, an ability that resembles the redox capabilities of transition metals. It is perhaps because these two partners, metal and pterin, have this chemical similarity that their compounds defy traditional categorization by formal oxidation number. The result challenges the chemist to formulate fresh interpretations of these deceptively ordinary complexes. This review concerns reports of metal-pterin complexes that appeared from the early 1980s through 1996. In a few cases older literature is briefly mentioned to build a context for the newer work. The review comprises four sections. Section 1 introduces the pteridine family and its important contributions to biochemistry. Section 2 is devoted to studies of molybdenum (6+) complexes reacted with reduced pterins. Section 3 describes redox interactions between reduced pterins and the first row metals copper and iron. Finally, Section 4 turns to a discussion of the electronic interactions in flavin complexes of various metals. An Epilogue closes the review.

Syntheses and physical properties of pterin and pteridine complexes for first-row transition metals are described. Characterization used single-crystal x-ray diffraction, spectroscopic, and microanalytical methods. These data all indicate that pterin and pteridine ligands chelate via oxygen and nitrogen atoms. Three copper complexes have been structurally determined. Cu(tppb)(pterin) [tppb − =tris(3-phenylpyrazolyl)hydroborate] crystallizes in the triclinic space group P1

The preparation and characterization of new model complexes for the molybdenum cofactor are reported. The new models are distinctive for the inclusion of pterin-substituted dithiolene chelates and have the formulation Tp ∗ MoX(pterin-R-dithiolene) (Tp ∗ = tris(3,5,-dimethylpyrazolyl)borate), X = O, S, R = aryl. Syntheses of Mo(4+) and (5+) complexes of two pterin-dithiolene derivatives as both oxo and sulfido compounds, and improved syntheses for pterinyl alkynes and [Et 4 N][Tp ∗ Mo IV (S)S 4 ] reagents are described. Characterization methods include electrospray ionization mass spectrometry, electrochemistry, infrared spectroscopy, electron paramagnetic resonance and magnetic circular dichroism. Cyclic voltammetry reveals that the Mo(5+/4+) reduction potential is intermediate between that for dithiolenes with electron-withdrawing substituents and simple dithiolates chelates. Electron paramagnetic resonance and magnetic circular dichroism of Mo(5+) complexes where X = O, R = aryl indicates that the molybdenum environment in the new models is electronically similar to that in Tp ∗ MoO(benzenedithiolate).

The combination of a pterin and a transition metal in many enzymes is the motivation for exploring the chemistry of pteridine complexes in detail. Unlike other biological ligands for essential transition metals, pterin is unique in displaying multi-electron redox reactivity, an ability that resembles the redox capabilities of transition metals. It is perhaps because these two partners, metal and pterin, have this chemical similarity that their compounds defy traditional categorization by formal oxidation number. The result challenges the chemist to formulate fresh interpretations of these deceptively ordinary complexes. This review concerns reports of metal-pterin complexes that appeared from the early 1980s through 1996. In a few cases older literature is briefly mentioned to build a context for the newer work. The review comprises four sections. Section 1 introduces the pteridine family and its important contributions to biochemistry. Section 2 is devoted to studies of molybdenum (6+) complexes reacted with reduced pterins. Section 3 describes redox interactions between reduced pterins and the first row metals copper and iron. Finally, Section 4 turns to a discussion of the electronic interactions in flavin complexes of various metals. An Epilogue closes the review.

This work provides the first extensive study of the redox reactivity of the pyranopterin system that is a component of the catalytic site of all molybdenum and tungsten enzymes possessing molybdopterin. The pyranopterin system possesses certain characteristics typical of tetrahydropterins, such as a reduced pyrazine ring; however, it behaves as a dihydropterin in redox reactions with oxidants. Titrations using ferricyanide and dichloroindophenol (DCIP) prove a 2e−/2H+ stoichiometry for pyranopterin oxidations. Oxidations of pyranopterin by Fe(CN)6 3− or DCIP are slower than tetrahydropterin oxidation under a variety of conditions, but are considerably faster than observed for oxidations of dihydropterin. The rate of pyranopterin oxidation by DCIP was studied in a variety of media. In aqueous buffered solution the pyranopterin oxidation rate has minimal pH dependence, whereas the rate of tetrahydropterin oxidation decreases 100-fold over the pH range 7.4–8.5. Although pyranopterin reacts as a dihydropterin with oxidants, it resists further reduction to a tetrahydropterin. No reduction was achieved by catalytic hydrogenation, even after several days. The reducing ability of the commonly used biological reductants dithionite and methyl viologen radical cation was investigated, but experiments showed no evidence of pyranopterin reduction by any of these reducing agents. This study illustrates the dual personalities of pyranopterin and underscores the unique place that the pyranopterin system holds in the spectrum of pterin redox reactions. The work presented here has important implications for understanding the biosynthesis and reaction chemistry of the pyranopterin cofactor in molybdenum and tungsten enzymes.

A new monoanionic dithiolene ligand is found in Tp*MoO(S(2)BMOQO). A combination of X-ray crystallography, electronic absorption spectroscopy, resonance Raman spectroscopy, and bonding calculations reveal that the monoanionic dithiolene ligand possesses considerable thiolate-thione character resulting from an admixture of an intraligand charge transfer excited state into the ground state wave function. The unusual dithiolene exhibits a highly versatile donor-acceptor character that dramatically affects the Mo(IV/V) redox couple and points to a potentially noninnocent role of the pterin fragment in pyranopterin Mo enzymes.

The proacrosin gene is transcribed in diploid spermatogenic cells and translated in haploid round spermatids. In order to evaluate sequences which are involved in proacrosin gene transcription, DNA-protein interactions were analyzed in 1.2 kb of the 5'flanking region of the rat gene. 13 protein binding sites were identified by DNase I footprinting using nuclear extracts from rat testis and brain, respectively. Five footprints (F1, F3, F7, TS2, TS3) which suggest an interaction with testis specific nuclear factors were further examined by gel retardation assays. Three testis specific binding sites (F1, F7, TS2, located 472bp, 697bp and 1004bp upstream of ATG, respectively) could be identified with both methods. The binding site F1 contains a motif which is similar to a testis specific footprint found in mouse protamine 1 gene. The nucleotide sequence of F7 contains the recognition motif of an isoform of the transcription factor GATA1, which is expressed in testis. Furthermore F1 and F7 are located in that part of the 5'flanking region of the proacrosin gene, which can direct proacrosin gene expression in germ cells of male transgenic mice.

A method is described to aid the instruction of undergraduate and graduate students in inert atmosphere techniques. A highly oxygen-sensitive organometallic compound, a titanium metallocene, changes color form blue to yellow when exposed to dioxygen contaminant, thereby providing an easily visualized monitor for students learning to manipulate the special glassware and operations typical of inert atmosphere reactions.

Mo (CO) (PhC 2 H) (P Et 3 ) 2 Br 2 cristallise dans le systeme monoclinique, groupe d'espace P2 1/n et sa structure est affinee jusqu'a R=0,048

The complex reaction of CS/sub 2/ with Mo(CO)/sub 2/(S/sub 2/CNEt/sub 2/)/sub 2/ in the presence of 2 equiv of triphenylphosphine forms the dinuclear complex Mo/sub 2/(S/sub 2/CNEt/sub 2/)/sub 3/(..mu..-CSC(S)S)(..mu..-S/sub 3/C/sub 2/NEt/sub 2/) (1) in 95% yield. The identity of 1 was determined by a single-crystal X-ray study. Two molecules of 1 crystallize in space group P1 with a = 15.749 (6) A, b = 16.013 (7) A, c = 9.740 (6) A, ..cap alpha.. = 107.26 (3)/sup 0/, ..beta.. = 105.92 (4)/sup 0/, and ..gamma.. = 71.14 (2)/sup 0/. The similarity of the structure of 1 to that of Mo/sub 2/(..mu..-S)(..mu..-EtC identical with CEt)(S/sub 2/CNMe/sub 2/)/sub 3/(SCNMe/sub 2/)(4) encourages development of a qualitative molecular orbital scheme for 1 from the analysis presented for the dinuclear alkyne derivative 4. Both of these dinuclear compounds have formal Mo(III) centers in roughly conlateral-biotahedral geometries. Addition of Me/sup +/ using the (Me/sub 3/O)(BF/sub 4/) reagent yields a cation that may be subsequently reduced with use of K(HB(O-i-Pr)/sub 3/). Cyclic voltammetry and visible spectroscopy data are compared for these dinuclear species. 30 references, 4 figures, 6 tables.

Several size fractions of natural loess from Long Island, New York including sand and ventifacts were studied with the scanning electron microscope to determine if distinguishing surface features could be found. Long Island loess compares very closely to descriptions of a number of European loess deposits except for the inclusion of carbonate not found on Long Island. Ventifact and sand grain surfaces were experimentally prepared using natural loess as an abrasive agent, and compared well with those surfaces found in the natural materials. Given favourable circumstances, it is possible via scanning electron microscopy to distinguish surface features created by aeolian action in loess deposits and associated materials.

Caracterisation au moyen de la spectrometrie IR des complexes de cobalt (II). Etude radiocristallographique avec determination de la structure moleculaire

Ausgehend von den Hexacarbonylen (I) wird uber die Carbonyliodide (III) und (IV) wie aufgezeigt eine verbesserte Darstellungsmethode fur die Mo(II)- und W(II)-dithiocarbamate (VI) demonstriert.

Structure cristalline du complexe forme avec le butyne-2 comme coordinat. Cristallisation dans le groupe PZ1/n avec a=12,372, b=12,145 et c=14,184 A, β=101,50°. Affinement jusqu'a R=0,056. Coordination octaedrique deformee autour des atomes Mo. Mise en evidence par RMN de deux rearrangements intramoleculaires distincts