Β-carboxy Groups Research Articles

Molybdenum citrate towards the protonation of FeMo-co in nitrogenase

<p indent="0mm">The structure of FeMo-cofactor (FeMo-co) in nitrogenase has been clarified as MoFe₇S₉C[<italic>R</italic>-(H)homocit](Hhis)(cys) (H₄homocit = homocitrate, Hhis = histidine, Hcys = cysteine), where homocitrate chelates to Mo(III) via α-alkoxy/α-hydroxy and α-carboxy groups. Recent model comparisons and theoretical calculations suggested a hydroxy coordinated homocitrate in FeMo-co, which may play an important role in storing and transferring hydrogen source for the reduction of substrate. Herein, two molybdenum citrates [Mo^IV₂O(Hcit)₂(tpy)₂]·3H₂O (<bold>1</bold>) (H₄cit = citrate, tpy = α,α,α-terpyridine) and (H₂tpy)₂[Mo^VI₂O₅(Hcit)₂]·7.5H₂O (<bold>2</bold>) have been obtained via hydrothermal reactions under the reduction of hydrazine hydrochloride in acidic condition. <bold>1 </bold>and <bold>2</bold> were fully characterized by elemental analyses, IR, UV-vis, EPR and ¹³C NMR spectra, bond valence calculations and X-ray single crystal diffractions. Structural analysis indicates that <bold>1</bold> is a binuclear complex, which seven coordination sites are occupied by one μ₂-O atom, three nitrogen atoms of terpyridine, three oxygen atoms from citrate. Citrate chelates to Mo via α-alkoxy, α-carboxy and β-carboxy groups, the uncoordinated β-carboxylic acid group interacts with water molecules through strong hydrogen bonds. <bold>2</bold> is a common binuclear Mo(VI) complex counterbalanced by protonated terpyridine cations. The coordination of citrates in <bold>2</bold> is similar to that of <bold>1</bold>. ¹³C NMR spectrum indicates that <bold>2</bold> partially dissociates in solution. We have also analyzed Mo–O distances of <bold>1</bold> and <bold>2</bold>, as well as previously reported molybdenum α-hydroxycarboxylates. It is found that <italic>trans</italic>-effect of Mo=O group, protonation of α-alkoxy group and oxidation state of Mo are the three major factors that affect the distances between Mo and coordinated atoms from α-hydroxycarboxylates. Mo–O distances are elongated about 0.05−0.13 Å from <italic>trans</italic>-effect, while this effect for α-carboxy group is stronger than that of α-alkoxy group. Mo–O (α-hydroxy) distance is about 0.11 Å longer than that of Mo–O (α-alkoxy) due to protonation effect. Mo–O (α-alkoxy/α-carboxy) distances show negative correlation with oxidation state of Mo. Linear fit gives a Mo^III–O (α-carboxy) distance that is close to those of wild-type FeMo-co and citrate-substituted cofactor of variant Mo-nitrogenase, while the calculated Mo^III–O (α-alkoxy) distance is much shorter. The difference is well in agreement with the protonation effect. This result gives a quantitative conclusion for the protonation mode of homocitrate in FeMo-co. A new protonated model is also deduced for citrate-substituted cofactor of variant Mo-nitrogenase. It seems more structural data of model compounds should be included for statistical analysis, especially those complexes in low valences.

Novel bidentate oxovanadium(IV) glycolate, α-hydroxybutyrate and citrate with terpyridine and their conversions to nitrosyl products

A series of monomeric α-hydroxycarboxylato oxovanadium(IV) complexes [VO(H2cit)(tpy)]·H2O (1) (H4cit = citric acid, tpy = 2,2′:6′,2-terpyridine), [VO(glyc)(tpy)]·5.5H2O (2) (H2glyc = glycolic acid) and [VO(α-hbut)(tpy)]·3H2O (3) (α-H2hbut = α-hydroxybutyratic acid) have been obtained from the reactions of vanadyl sulfate with α-hydroxycarboxylates and terpyridine in acidic solutions. These complexes feature bidentate citrate, glycolate or α-hydroxybutyrate respectively. The ligand chelates to vanadium atom through α-hydroxy (in 1) or α-alkoxy (in 2 and 3) and α-carboxy groups, while β-carboxy groups of citrate in 1 are free to participate strong hydrogen bonds with neighboring citrate. With comparable chelation, 1 shares a similar V–Oα-hydroxy distance [2.168(1) Å] with that observed in FeV-cofactor [2.17 Å] [1]. Moreover, nitrosyl vanadium complexes [V(NO)(glyc)(tpy)]·3H2O (4) and [V(NO)(α-hbut)(tpy)]·4H2O (5) were obtained via reductions of synthetic solutions of 2 and 3 with hydroxylamine respectively. The terminal oxygen atoms were substituted by linear nitrosyl groups in 4 and 5. They were fully characterized by UV–vis, IR, EPR spectra, X-ray structural analyses and theoretical bond valence calculations.

Conversions of monomeric, dimeric and tetrameric lanthanum and samarium citrates with ethylenediaminetetraacetates in aqueous solutions

Our continued research on lanthanide citrate with ethylenediaminetetraacetate results in an isolation of a monomeric samarium complex K4[Sm(Hcit)(EDTA)]·10H2O (1) (H4cit = citric acid and H4EDTA = ethylenediaminetetraacetic acid) from the reaction of lanthanide ethylenediaminetetraacetate trihydrates with citric acid at pH ∼6.5, where the three coordinated water molecules of K[Sm(EDTA)(H2O)3]·5H2O (4) were substituted by the α-hydroxy, α-carboxy and β-carboxy groups of citrate. When involving lanthanum element, the reaction of lanthanum ethylenediaminetetraacetate trihydrates with dimeric lanthanum complex K4(NH4)4[La2(Hcit)2(EDTA)2]·17H2O (2) gives a novel tetrameric lanthanum citrate with ethylenediaminetetraacetate K2(NH4)8[[La(EDTA)(H2O)2]2[La2(Hcit)2(EDTA)2]]·22H2O (3). The main structural feature of 2 consists of a dinuclear unit deca-coordinated by citrate and EDTA, while tetramer 3 is a dinuclear unit bridged with two lanthanum ethylenediaminetetraacetates through pendent β-carboxy groups. It is interesting to note that 1–3 contain strong intramolecular hydrogen bonds between α-hydroxy and β-carboxy groups with very good solubilities in water. The relationships between the isolations of mixed EDTA–citrate lanthanides and modified Pechini method are discussed.

Iron molybdenum nitrilotriacetate and iminodiacetate – spectroscopy, structural characterization and CO2 adsorption

The tetranuclear iron molybdate Na6[(MoO2)2O2Fe2(nta)4]·16H2O (1) and its isomorphous complexes form 2D water layer structures in a modular manner, and a decanuclear heterometallic polymer, [(MoO4)2FeII4FeIII4(ida)8]n (2), contains an interesting cubic 3D microporous structure with a 3.3 Å diameter hole and can adsorb a small amount of CO2.

α-Hydroxy coordination of mononuclear vanadyl citrate, malate and S-citramalate with N-heterocycle ligand, implying a new protonation pathway of iron–vanadium cofactor in nitrogenase

Unlike the most of α-alkoxy coordination in α-hydroxycarboxylates to vanadium, novel α-hydroxy coordination to vanadium(IV) has been observed for a series of chiral and achiral monomeric α-hydroxycarboxylato vanadyl complexes [VO(H2cit)(bpy)]·2H2O (1), [VO(Hmal)(bpy)]·H2O (2), [VO(H2cit)(phen)]·1.5H2O (3), [VO(Hmal)(phen)]·H2O (4), and [ΔVO(S-Hcitmal)(bpy)]·2H2O (5), [VO(H2cit)(phen)]2·6.5H2O (6), which were isolated from the reactions of vanadyl sulfate with α-hydroxycarboxylates and N-heterocycle ligands in acidic solution. The complexes feature a tridentate citrate, malate or citramalate that chelates to vanadium atom through their α-hydroxy, α-carboxy and β-carboxy groups; while the other β-carboxylic acidic group of citrate is free to participate strong hydrogen bonds with lattice water molecule. The neutral α-hydroxy group also forms strong intermolecular hydrogen bonds with water molecule and the negatively-charged α-carboxy group in the environment. The inclusion of a hydrogen ion in α-alkoxy group results in the formation of a series of neutral complexes with one less positive charge. There are two different configurations of citrate with respect to the trans-position of axial oxo group, where the complex with trans-hydroxy configuration seems more stable with less hindrance. The average bond distances of VOhydroxy and VOα-carboxy are 2.196 and 2.003Å respectively, which are comparable to the VO distance (2.15Å) of homocitrate in FeV-cofactor of V-nitrogenase. A new structural model is suggested for R-homocitrato iron vanadium cofactor as VFe7S9C(R-Hhomocit) (H4homocit=homocitric acid) with one more proton in homocitrate ligand.

Solid and solution study of tetranuclear zinc citrates with N-donor chelates

Tetranuclear zinc(II) citrates with N-heterocycle ligand, [Zn4(Hcit)2(phen)4(H2O)4]·2NO3·10H2O (1) and [Zn4(Hcit)2(bpy)4(H2O)6]·2NO3·12H2O (2) (H4cit = citric acid, phen = 1,10-phenanthroline, bpy = 2,2′-bipyridine), were synthesized from aqueous solution and characterized by Infrared, Powder X-ray diffraction, fluorescence spectra, and X-ray structural analyses. Complexes 1 and 2 are centrosymmetric tetranuclear species with two crystallographically independent Zn(II) ions, which display different coordination geometries, while the citrate ligand shows similar coordination modes and bridges three different Zn ions. In 1, the Zn1 is five-coordinate by three oxygens from two citrates and two nitrogens from phen. There exist weak interactions between Zn1 and O4a atom of β-carboxy group of citrate (Zn1⋯O4a = 2.895 Å). Zn2 is six-coordinate by two oxygens from one citrate, two nitrogens from phen, and two water molecules. Zn1 and Zn2 are bridged by one citrate to form a dinuclear unit, which is further extended to a tetranuclear unit. In 2, two citrates bridge two pairs of symmetry-related Zn(II) ions into a tetranuclear unit. The structural analyses of 1 and 2 indicate that different tetranuclear species are caused by arrangement of citrates with different steric hindrance of N-chelates. 1H and 13C NMR spectra indicate that 2 remains coordinated in D2O. Moreover, both 1 and 2 show strong fluorescent emission in dilute solution and solid state.

Substitution of gadolinium ethylenediaminetetraacetate with phosphites: towards gadolinium deposit in nephrogenic systemic fibrosis

In neutral media, reactions of gadolinium ethylenediaminetetraacetates with phosphorous acid result in the formation of the mixed-ligand polymeric complex K3n[Gd(EDTA)(HPO3)]n·7nH2O () and dimeric complex Na6[Gd2(EDTA)2(HPO3)2]·2.5NaCl·21H2O () (H4EDTA = ethylenediaminetetraacetic acid) in warm solution. Further substitution with citric acid gives the monomeric gadolinium citrate with EDTA (NH4)2Na[Gd(EDTA)(H2cit)]·4H2O (). The compounds were characterized by elemental analysis, single crystal X-ray diffraction, FT-IR, ESI-MS and thermogravimetric analysis. Structural analysis indicates that three coordinated water molecules in the gadolinium ethylenediaminetetraacetate trihydrates are replaced by phosphite ions (HPO3(2-)) in the compounds and . Gadolinium atoms are octa-coordinated by EDTA and the phosphite ion, the latter links adjacent Gd-EDTA units to generate an infinite one-dimensional chain in compound and a dimeric octatomic ring in . In complex , coordinated water molecules were substituted by the α-hydroxy, α-carboxy and β-carboxy groups of citrate. Citrate is favourable for inhibiting the formation of Gd-EDTA phosphite. All the complexes are very easily soluble in water. The solution behavior of the isostructural lanthanum complexes was probed with (13)C and (31)P NMR spectra in D2O for comparison. ESI-MS analysis and recrystallization proved that complexes and dissociate to the monomeric unit of Gd-EDTA and free HPO3(2-) in aqueous solution. Substitutions of gadolinium ethylenediaminetetraacetates to and are attributed to be the cause of nephrogenic systemic fibrosis in some way.

Crystalline and solution chemistry of tetrameric and dimeric molybdenum(VI) citrato complexes

Reaction of citrate with molybdenum(VI) salts under controlled conditions of reactant ratios and pH values in aqueous solutions affords a family of molybdenum citrates. It includes tetranuclear complexes K2(NH4)2[(MoO2)4O3(Hcit)2]·5H2O (1), (NH4)5[(MoO2)4O3(Hcit)(cit)]·3H2O (2) and K2(NH4)4[(MoO2)4O3(cit)2]·7H2O (3), dinuclear complexes K2[(MoO2)2O(H2cit)2]·4H2O (5), (NH4)3[(MoO2)2O(H2cit)(Hcit)] (6), (NH4)14{[(MoO2)2O(Hcit)2][(MoO2)2O(Hcit)(cit)]2}·14H2O (8) and (NH4)6[(MoO2)2O(cit)2]·3.5H2O (9), as well as a mononuclear complex (NH4)4[MoO3(cit)]·2H2O (10) (H4cit=citric acid), where 2, 5, 6, and 8 are new type of citrato complexes. X-ray single-crystal structural analyses reveal that the molybdenum atom in these complexes is hexa-coordinated in an approximately octahedral geometry. Each citrate ligand uses its α-alkoxy, α-carboxy and one β-carboxy groups to act as a tridentate ligand while the other β-carboxy group is left free. Also, structural analyses show that some of the complexes possess strong hydrogen bonds that lead to one and two dimensional polymeric associations. All of these complexes have been investigated by 13C and 1H NMR spectroscopy. The NMR spectra demonstrate that these complexes except 1 and 8 obviously undergo decomposition in solution. Furthermore, four series of solutions with the molar ratio Mo:cit=2:1, 1:1, 1:2, and 1:4 in the pH range 0.1–9.0 were investigated by 13C NMR spectroscopy. The solution NMR studies reveal that the state of equilibriums among different species [4,2]n−, [2,2]n−, [2,1]n−, [1,2]n− and/or [1,1]n− being established under each set of specified conditions.

Hydrogen bonding-controlled assemblies of polymeric and octanuclear tungsten citrates

Water-soluble polymeric tungsten citrate (Him)10n(NH4)2n[(WO2)2O(Hcit)2]3n·10nH2O (1) and octanuclear tungsten citrate (Him)8(NH4)13{[(WO2)2O(Hcit)(cit)H(cit)(Hcit)O(WO2)2][(WO2)2O(cit)2]2}·14H2O (2) (H4cit=citric acid, im=imidazole), were obtained from aqueous solution. They were characterized by elemental analyses, NMR and IR spectra, thermogravimetric (TG) analyses and X-ray structural analyses. The basic units of the two complexes contain a dimeric tungsten citrate in different protonated forms, where tungsten atom is six-coordinated in an approximately octahedral geometry. Each citrate ligand uses its α-alkoxy, α-carboxy and one β-carboxy group to act as a tridentate ligand, while the other β-carboxy or carboxylic acid group is uncoordinated. The presences of the protonated/deprotonated terminal carboxylates and their participation in hydrogen-bonding interactions play an important role in the overall structures. In complex 1, a hexanuclear tungsten citrate species [(WO2)2O(Hcit)2]312− is linked by strong H-bonding between β-carboxylic acid group and terminal oxygen [2.587(6)Å], which extends into one-dimensional polymeric chain. In complex 2, there exist two types of strong hydrogen bonds between β-carboxy and carboxylic acid groups [2.467(8), 2.516(6)Å], forming an octanuclear species {[(WO2)2O(Hcit)(cit)H(cit)(Hcit)O(WO2)2][(WO2)2O(cit)2]2}21−. This is supported by the absence of IR band around 1700cm−1 for protonated carboxylic acid. 13C NMR spectra show the coordination of α-alkoxy and α-carboxy groups of citrate ligand in both complexes.

Isolations and characterization of highly water-soluble dimeric lanthanide citrate and malate with ethylenediaminetetraacetate

Highly water-soluble lanthanum and cerium citrates or malates with ethylenediaminetetraacetate (NH(4))(8)[Ln(2)(Hcit)(2)(EDTA)(2)]·9H(2)O [Ln = La, 1; Ce, 2], K(8)[La(2)(Hcit)(2)(EDTA)(2)]·16H(2)O (3) and K(6)[Ln(2)(Hmal)(2)(EDTA)(2)]·14H(2)O [Ln = La, 4; Ce, 5] (H(4)cit = citric acid, H(3)mal = malic acid, and H(4)EDTA = ethylenediaminetetracetic acid) were prepared from the reactions of lanthanide ethylenediaminetetraacetate trihydrates with citric or malic acid at pH 5.0-6.5. These compounds were characterized by elemental analyses, IR, TG-DTG, solution (13)C{(1)H} NMR, solid state (13)C NMR spectra and X-ray structural analyses. The main structural feature of the compounds consists of a dinuclear unit deca-coordinated by EDTA and citrate or malate. The α-hydroxy and α-carboxy groups of citrate and malate chelate in five-membered ring with one lanthanide ion, while one of the β-carboxy group coordinates with the other lanthanide ion, forming a dimeric structure. The other pendent β-carboxy groups in 1-3 form very strong intramolecular hydrogen bond with α-hydroxy groups [O1O7 2.594(4), 2.587(8) and 2.57(1) Å for 1-3 respectively]. (13)C NMR spectra of the lanthanum compounds show obvious downfield shifts based on solid and solution NMR measurements, indicating the coordinations of mixed-ligand in lanthanum complexes, while highfield shifts are observed in cerium complexes.

Enhancement of Laccase Activity through the Construction and Breakdown of a Hydrogen Bond at the Type I Copper Center in Escherichia coli CueO and the Deletion Mutant Δα5−7 CueO

CueO is a multicopper oxidase involved in a copper efflux system of Escherichia coli and has high cuprous oxidase activity but little or no oxidizing activity toward various organic substances. However, its activity toward oxidization of organic substrates was found to be considerably increased by the removal of the methionine-rich helical segment that covers the substrate-binding site (Δα5-7 CueO) [Kataoka, K., et al. (2007) J. Mol. Biol. 373, 141]. In the study presented here, mutations at Pro444 to construct a second NH-S hydrogen bond between the backbone amide and coordinating Cys500 thiolate of the type I copper are shown to result in positive shifts in the redox potential of this copper center and enhanced oxidase activity in CueO. Analogous enhancement of the activity of Δα5-7 CueO has been identified only in the Pro444Gly mutant because Pro444 mutants limit the incorporation of copper ions into the trinuclear copper center. The activities of both CueO and Δα5-7 CueO were also enhanced by mutations to break down the hydrogen bond between the imidazole group of His443 that is coordinated to the type I copper and the β-carboxy group of Asp439 that is located in the outer sphere of the type I copper center. A synergetic effect of the positive shift in the redox potential of the type I copper center and the increase in enzyme activity has been achieved by the double mutation of Pro444 and Asp439 of CueO. Absorption, circular dichroism, and resonance Raman spectra indicate that the characteristics of the Cu(II)-S(Cys) bond were only minimally perturbed by mutations involving formation or disruption of a hydrogen bond from the coordinating groups to the type I copper. This study provides widely applicable strategies for tuning the activities of multicopper oxidases.

Synthesis, spectroscopic, and structural characterization of two malate zinc coordination polymers with imidazole or 2,2′-bipyridine

Zinc(II) coordination polymers, [Zn(Hmal)(im)(H2O)] n · 2nH2O (1) and [Zn(Hmal)(bpy)] n · 3nH2O (2) (H3mal = malic acid, im = imidazole, bpy = 2,2′-bipyridine), were synthesized from aqueous solution and characterized by elemental analyses, infrared and fluorescence spectra, thermogravimetric analyses, and single-crystal X-ray structural analyses. In 1, zinc is coordinated by imidazole, water, and tridentate malate in octahedral geometry. The β-carboxy group of malate further bridges with the other zinc forming 1-D polymeric chains. A pair of 1-D chains self-assemble to generate a double chain by strong hydrogen bonds between imidazole and malate. Furthermore, neighboring pairs of double chains are extended to form the final 3-D framework through intermolecular hydrogen bonds. In 2, the malates link Zn in a bidentate–monodentate fashion to form spiral-shaped chains that extend into a 3-D supramolecular structure by π–π stacking interactions and intermolecular hydrogen bonds. Complex 1 exhibits strong fluorescence at room temperature.

PH and molar ratio dependent formation of monomeric, dimeric and tetrameric cobalt malate complexes

The reactions of cobalt(II) chloride with racemic malic acid (H 3mal = C 4H 6O 5) result in the isolation of monomeric, dimeric and tetrameric cobalt malato complexes: (NH 4) 2[Co( R-Hmal)( S-Hmal)] · 2H 2O ( 1), [Co 2( R-Hmal)( S-Hmal)(H 2O) 4] n · 2 nH 2O ( 2), K 4[Co 4(OH) 2( R-mal) 2( S-mal) 2(H 2O) 4] · 10H 2O ( 3) and trans-[Co( R-H 2mal)( S-H 2mal)(H 2O) 2] · 2H 2O ( 4). The formations of the malato complexes are dependent on the pH value, the molar ratio of the solutions, the reaction temperature and the counterions. In the water-soluble compound 1, the Co II ion is octahedrally coordinated by two tridentate malates via their α-hydroxy, α-carboxy and β-carboxy groups. The malate ligands in 2 coordinate with the cobalt ion via their α-hydroxy and α-carboxy groups, while the β-carboxy group acts as a bridging ligand for the other two cobalt ions, forming a novel dimeric unit [Co 2( R-Hmal)( S-Hmal)(H 2O) 4], which further connects into a layered structure through links from the oxygen atoms of the β-carboxy groups. Complex 3 is a tetranuclear mixed-valence species. Both of the Co II ions exist in trans-[Co( R-mal)( S-mal)(H 2O) 2] units, which are linked by a Co III 2(OH) 2 unit with bridging α-alkoxy and β-carboxy groups. Compound 4 is the main product of reaction between cobalt chloride and excess malate under weakly acidic conditions.

PH Dependent formations of dinuclear molybdenum(V) and incomplete cubane molybdenum(IV) complexes with nitrilotriacetate

Nitrilotriacetato oxomolybdenum(V) dinuclear complex (NH 4) 4[ cis-Mo 2O 4(nta) 2] · 7H 2O ( 1) (H 3nta = nitrilotriacetic acid) and its incomplete cubane-type trinuclear molybdenum(IV) complex [Mo 3O(OH) 3(Hnta) 3] · Cl · 3H 2O ( 2) are isolated in a weak acidic and an acidic solutions, respectively (pH 4.5 and 1.0). The nitrilotriacetate ligand in the anion 1 or the cation 2 displays tridentate mode through the nitrogen atom and the oxygen atoms of β-carboxy groups. Unusual protonations of bridged oxygen atoms in 2 have not effect on the skeleton of molybdenum(IV) cap Mo 3O unit. There are obvious dissociations of molybdenum(V) complexes based on 13C NMR observations in solution.

Syntheses, Spectroscopies and Structures of Molybdenum(VI) Complexes with Homocitrate

Initial investigations into the possible role of homocitric acid in iron molybdenum cofactor (FeMo-co) of nitrogenase lead us to isolate and characterize two tetrameric molybdate(VI) species. The complexes K2(NH4)2[(MoO2)4O3(R,S-Hhomocit)2].6H2O (1) and K5[(MoO2)4O3(R,S-Hhomocit)2]Cl.5H2O (2) (homocitric acid = H4homocit, C7H10O7) are prepared from the reactions of acyclic homocitric acid and molybdates, which represent the first synthetic structural examples of molybdenum homocitrate complexes. The homocitrate ligand trapped by tetranuclear molybdate coordinates to the molybdenum(VI) atom through alpha-alkoxy and alpha-, beta-carboxy groups. The physical properties, structural parameters, and their possible biological relevances are discussed.

Speciation of water-soluble titanium citrate: Synthesis, structural, spectroscopic properties and biological relevance

Titanium(IV) citrate complexes with different anions Na 3[Ti(H 2cit) 2(Hcit)] · 9H 2O ( 1), K 4[Ti(H 2cit)(Hcit) 2] · 4H 2O ( 2), K 5[Ti(Hcit) 3] · 4H 2O ( 3) and Na 7[TiH(cit) 3] · 18H 2O ( 4) (H 4cit = citric acid) were isolated in pure forms from the solutions of titanate and citrate at various pH values. X-ray structural analyses revealed the presence of a monomeric tricitrato titanium unit in the four complexes. Each Ti(IV) ion is coordinated octahedrally by the three citrate ligands in different protonated forms. The citrate ligand chelates bidentately to the titanium ion through its negatively charged α-alkoxy and α-carboxy groups. This is consistent with the large downfield 13C NMR shifts for the carbon atoms bearing the α-alkoxy and α-carboxy groups. The very strong hydrogen-bonds existing in the protonated and deprotonated β-carboxy groups may be the key factor for the stabilization of the titanium citrate complexes. When the pH value is lower than 7.0, 13C NMR spectra of 1:3 Ti:citrate solutions are similar to those of the titanium citrate complexes isolated at the corresponding pH values. The dissociation of free citrate increases with the rise of pH value. However, 13C NMR spectra of 1:3 Ti:citrate solutions indicate that there may exist different citrate titanium species when the pH value is higher than 7.0.

A Palladium‐Catalyzed Glycosylation Reaction: The de novo Synthesis of Natural and Unnatural Glycosides.

AbstractFor Abstract see ChemInform Abstract in Full Text.

On-resin solid-phase synthesis of asparagine N-linked glycopeptides: use of N-(2-acetoxy-4-methoxybenzyl)(AcHmb) aspartyl amide-bond protection to prevent unwanted aspartimide formation

The N-(2-acetoxy-4-methoxybenzyl)(AcHmb) backbone amide-protecting group has been applied in an on-resin solid-phase synthesis of asparagine N-linked glycopeptides. Backbone protection of the -Asp(OAllyl)-Ala-aspartyl amide bond suppressed the formation of aspartimide during both the initial chain assembly and the subsequent activation and glycosylation of the aspartyl β-carboxy group. A 1.1–1.25 mole excess of glycosylamine coupled quantitatively, with minimal side reaction, to the activated aspartyl β-carboxy group of the fully assembled backbone-protected peptide-resin. A parallel synthesis without backbone amide protection produced substantial aspartimide peptide, particularly during the initial chain assembly.

The structure of syringomycins A 1, E and G

By a combination of 1D and 2D 1H- and 13C-NMR, FAB-MS, and chemical and enzymatic reactions carried out at the milligram level, it has been demonstrated that syringomycin E, the major phytotoxic antibiotic produced by Pseudomonas syringae pv. syringae, is a new lipodepsipeptide. Its amino acid sequence is Ser-Ser-Dab-Dab-Arg-Phe-Dhb-4(Cl)Thr-3(OH)Asp with the β-carboxy group of the C-terminal residue closing a macrocyclic ring on the OH group of the N-terminal Ser, which in turn is N-acylated by 3-hydroxydodecanoic acid. Syringomycins A 1 and G, two other metabolites of the same bacterium, differ from syringomycin E only in their fatty acid moieties corresponding, respectively, to 3-hydroxydecanoic and 3-hydroxytetradecanoic acid.