Carboxyl Atoms Research Articles

Theoretical study of the mechanism of 2F-molecular sieve domain chiral transition of Asn molecules

Abstract Using density functional theory at the level of oniom (b3lyp/6-31g (d, p): uff), we investigated the chiral transition process of the Asparagine molecule in 2F-MOL. Furthermore, a complete chiral transition path reaction potential energy surface was drawn by looking for the extreme value point structures including the transition state and intermediate. The results show that the hydrogen atom on the chiral carbon atom of S-Asparagine molecule can transfer to the other side of the carbon atom via the oxygen atoms of carboxyl atoms as a bridge, to achieve the chiral transition of Asparagine molecule from S-type to R-type. On this path there is four intermediate and five transition states. The biggest reaction energy barrier is 243.9089KJ/mol derived from the fourth transition state TS1-R-Asn@2F-MOL.

Stable carbon isotope diagnostics of mammalian metabolism, a high-resolution isotomics approach using amino acid carboxyl groups.

The carbon isotopic compositions of amino acids are increasingly measured to characterize diets and metabolic response to diets. We report a new high-resolution system to measure the stable carbon isotopic composition of carboxyl atoms within amino acids. The automated system used HPLC to separate amino acids followed by addition of ninhydrin for decarboxylation and transfer of the evolved CO2 to a stable isotope ratio mass spectrometer for δ13CCARBOXYL measurement. The ninhydrin reaction was conducted at acidic pH (1.5) and elevated temperature (160 oC) giving yields close to 100% for most common amino acids. Eight mammalian keratin samples from herbivores (kudu and caribou), omnivores (humans) and carnivores (bowhead and humpback zooplanktivorous whales) were analysed with this new system. The data provide an initial calibration of reference materials to be used in studies of this type and is the first report of carboxyl carbon isotope distributions in mammals. Results showed widespread 13C enrichments in both essential and non-essential amino acid carboxyl groups, likely linked to decarboxylation of amino acids during normal metabolism. Analyses of non-essential amino acid isotope profiles showed (1) consistent and general taxon-level metabolic differences between the herbivore, human and whale samples, (2) marked differences among individual humans, ruminants and whales (3) evidence for gluconeogenesis in the wildlife samples, and (4) extensive 13C enrichment likely associated with fasting in the humpback whale sample. Future mammalian research related to the metabolism of growth, reproduction, aging and disease may benefit from using this technique. Values obtained for internationally available samples USGS42 and USGS43 (Tibetan and Indian human hair) provide a first characterization of reference materials for δ13CCARBOXYL profiles.

Probing the effect of N-alkylation on the molecular recognition abilities of the major groove N7-binding site of purine ligands

The study of the metal binding pattern of N-methyladenines (1-, 3-, 7- or 9-Meade) towards CuII-iminodiacetate-like chelates is addressed on the basis of XRD crystal structures of sixteen novel ternary compounds. Except for three compounds, all others feature an square-based Cu(II) coordination, type 4 + 1, and the efficient cooperation of a CuN7 bond with an intra-molecular N6-H⋯O(coord. carboxylate) interligand interaction as the major metal-binding pattern. The three referred exceptions to this behavior are: (1) the compound [Cu(MIDA)(7Meade)(H2O)]·4H2O, which evidence the CuN3 binding pattern; the (2) [Cu(IDA)(1Meade)(H2O)2]·4H2O, which molecular recognition consist in the CuN9 bond and a (distal aqua)⋯⋯N3(1Meade) intra-molecular interaction, within an octahedral Cu(II) center; and (3) [Cu(IDA)(9Meade)(H2O)2]·3H2O, also with a 4 + 1 + 1 Cu(II) coordination, where the CuN7 bond exists along with an extremely weak N6-H⋯O(coord. carboxylate) interaction (3.33 Å, 140.2°). This former interaction is determined by packing forces that promote the participation of the N6H group in a ‘trifurcated’ H-bond. In conclusion, the cooperation between the CuN7 bond (not possible for 7Meade) and the intra-molecular N6-H⋯O interaction is clearly favored (a) by the H-accepting role of the O-coordinated carboxylate atoms from the iminodiacetate ligands in mer-NO2 conformation and (b) in compounds where the Cu(II) atom exhibits an elongated square-base pyramidal coordination, type 4 + 1.

Computational insights about the dynamic behavior for the inclusion process of deprotonated and neutral aspirin in β-cyclodextrin

Molecular dynamics was used to study the inclusion of neutral and deprotonated aspirin into the β-cyclodextrin (β-CD) cavity. The molecular dynamic simulation allows following the time dependent behavior of the formation of the inclusion complex. For both complexes, we find a reasonable and a realistic pattern of the complexation. The calculations show a single pathway consisting of a no reversible binding process leading to the complexation of aspirin. Whereas for deprotonated aspirin it has been observed a reversible binding, in which one way leads to the binding form, and the reverse way to the unbinding form. Throughout the simulation, the penetration of aspirin (ASA) or deprotonated aspirin (ASA−) inside the cavity occurs only with a phenyl ring entering first through the wider or narrower rim. The determination of free energy using unbiased and biased simulations of the corresponding inclusion processes gives more favorable inclusion process of aspirin than deprotonated aspirin. The inclusion of the guest molecule is found deeply embedded within ASA:β-CD complex whereas it is partial in ASA−:β-CD complex. Also, the orientation A of both complexes is found more favorable of ca. 1.9 kcal/mol, and of ca. 0.8 kcal/mol, respectively for neutral and deprotonated complex. Aspirin molecule establish one H-bond between the hydrogen carboxylic atom and one oxygen atom of primary hydroxyl group of β-CD; this H-bond is detected during about 20% of the simulation period. In addition, we found that water molecules in the first solvation layer are implied with hydrogen carboxylic atom and the keto oxygen atoms within H-bonds. While, water molecules of the second solvation layer is in interact with the O1 and O2 oxygen atoms of aspirin. Accordingly, based on the obtained results we can consider that the hydrophobic/hydrophilic interactions are the most important driving forces of the complexation assisted by stabilizing H-bonds.

Ab Initio QM/MM Modeling of the Rate-Limiting Proton Transfer Step in the Deamination of Tryptamine by Aromatic Amine Dehydrogenase.

Aromatic amine dehydrogenase (AADH) and related enzymes are at the heart of debates on the roles of quantum tunneling and protein dynamics in catalysis. The reaction of tryptamine in AADH involves significant quantum tunneling in the rate-limiting proton transfer step, shown by large H/D primary kinetic isotope effects (KIEs), with unusual temperature dependence. We apply correlated ab initio combined quantum mechanics/molecular mechanics (QM/MM) methods, at levels up to local coupled cluster theory (LCCSD(T)/(aug)-cc-pVTZ), to calculate accurate potential energy surfaces for this reaction, which are necessary for quantitative analysis of tunneling contributions and reaction dynamics. Different levels of QM/MM treatment are tested. Multiple pathways are calculated with fully flexible transition state optimization by the climbing-image nudged elastic band method at the density functional QM/MM level. The average LCCSD(T) potential energy barriers to proton transfer are 16.7 and 14.0 kcal/mol for proton transfer to the two carboxylate atoms of the catalytic base, Asp128β. The results show that two similar, but distinct pathways are energetically accessible. These two pathways have different barriers, exothermicity and curvature, and should be considered in analyses of the temperature dependence of reaction and KIEs in AADH and other enzymes. These results provide a benchmark for this prototypical enzyme reaction and will be useful for developing empirical models, and analyzing experimental data, to distinguish between different conceptual models of enzyme catalysis.

Cocaethylene, the in vivo product of cocaine and ethanol, is a narcotic more potent than its precursors.

The molecular conformation and supramolecular architecture of cocaethylene [systematic name: ethyl (1R,2R,3S,5S)-3-benzoyloxy-8-methyl-8-azabicyclo[3.2.1]octane-2-carboxylate], C18H23NO4, have been determined for the first time. Cocaethylene is a narcotic produced in vivo when cocaine and ethanol are administered concomitantly. The intra- and intermolecular features of cocaethylene and its less potent narcotic precursor cocaine are very similar. The only molecular difference is in the conformation of the methyl group of the ethoxycarbonyl group. Similar to cocaine, the carboxylate atoms and the α-C atom are coplanar in cocaethylene, but the methyl C atom of the ethyl group is bent by ca 90° away from this plane in the narcotic reported here. The main supramolecular motif is a one-dimensional chain stabilized by weak C-H...O contacts.

The molecular simulation of the miscibility, mechanical properties and physical cross-linking behavior of the poly(vinyl alcohol)/poly(acrylic acid) composited membranes

The miscibility and mechanical properties of poly vinyl alcohol (PVA) and poly acrylic acid (PAA)-composited membranes were studied with molecular simulation. The Flory–Huggins parameters (δ) were calculated to prove the good miscibility of PVA and PAA. The radial distribution functions of hydroxyl and carboxyl atoms and the average number of H-bonds were observed to indicate the degree of physical cross-linking between PVA and PAA. The influences of intermolecular physical cross-linking on the glass transition temperature and mechanical properties were estimated. The results revealed that the PVA/PAA membrane with a composition of 2:3 has the best plastic properties, which exhibits a good application value. All of the simulated results showed good agreement with the experimental data. It indicates that the method presented in this work has a promising application prospect.

PH Dependent Synthesis of Two Manganese Compounds based on 5‐(2‐Pyrimidyl)tetrazole‐1‐acetic Acid

AbstractReactions of Hpymtza [Hpymtza = 5‐(2‐pyrimidyl)tetrazole‐1‐acetic acid] with MnCl2·4H2O under different pH conditions, afforded the complexes [Mn(pymtza)2(H2O)4] (1) and [Mn2(pymtza)2Cl2(EtOH)]·H2O (2). The compounds were structurally characterized by elemental analysis, IR spectroscopy and single‐crystal X‐ray diffraction. Compound 1 shows a mononuclear structure, whereas complex 2 has a 1D chain structure. In compound 1, the pymtza ligand only acts in a monodentate manner to coordinate to one central MnII atom by one carboxylate atom, In 2, pymtza acts as tetradentate ligand to connect three MnII ions. Compounds 1 and 2 display 3D networks by hydrogen bonding interactions. Furthermore, the luminescence properties of Hpymtza as well as compounds 1 and 2 were investigated at room temperature in the solid state.

基于柔性配体5-[N-乙酸根(4-吡啶基)]四唑(a4-pytz)与Fe(II)和Sm(III)两个配合物的合成与表征

5-[N-乙酸根(4-吡啶基)]四唑钾盐(简写为Ka4-pytz)与(NH4)2Fe(SO4)2和SmCl3·6H2O在水-甲醇体系中反应，得到两个新的配合物，[Fe(a4-pytz)2·2H2O]·2H2O (1)和[Sm4(a4-pytz)8(H2O)18]Cl3·10H2O (2)。通过元素分析，红外光谱分析及X-射线单晶衍射对两个配合物进行结构表征。单晶衍射结果显示了配合物1和2都具有一维链状结构。但不同的是，配合物1的一维链是通过a4-pytz作为双齿配体，经四唑环上的氮原子和羧基的一个氧原子与Fe(II)相连而成的；而配合物2的结构形成源于羧基的桥联作用，氮原子未配位。通过氢键作用，配合物1和2形成了三维结构。室温下研究了配合物1和2的荧光性质。 Two new coordination compounds, namely, [Fe(a4-pytz)2·2H2O]·2H2O (1) and [Sm4(a4-pytz)8 (H2O)18]Cl3·10H2O (2) were prepared by reactions of K4-pytz[Ka4-pytz=5-(N-acetato(4-pyridyl)) tetrazole potassium salt] with (NH4)2Fe(SO4)2 or SmCl3·6H2O in a mixture of distilled water and methanol solution. These compounds were characterized by elemental analysis, IR and single crystal X-ray diffraction. The diffraction reveals that both 1 and 2 show 1D chains. In compound 1, a4-pytz acts as a bidentate ligand via its tetrazole-N atom and one of the carboxylate-O atoms while a4-pytz acts as tridentate bridging ligand via the two carboxylate atoms in 2. Compounds 1 and 2 show the 3D networks through hydrogen bonding interactions. Furthermore, the luminescent properties were also investigated at room temperature in the solid state.

Crystal structure of nonaaqua[bis(μ2-pyridine-2,4,6-tricarboxylato- κ3O,N,O:(O')dilutetium(III)]trihydrate, Lu2(H2O)9(ptc)2·3H2O, C16H28Lu2N2O24

Abstract C16H28Lu2N2O24, triclinic, P1̄ (no. 2), a = 7.602(2) Å, b = 10.417(2) Å, c = 17.374(4) Å, α = 92.32(3)°, β = 92.08(3)°, γ = 95.74(3)°, V = 1366.7 Å3, Z = 2, Rgt(F) = 0.0489, wRref(F2) = 0.1229, T = 293 K.

Crystal structure, configurational and density functional theory analysis of nickel(II) complexes with pentadentate 1,3-pd3a-type ligands

The O–O–N–N–O-type pentadentate ligands H31,3-pd3a and H31,3-pd2ap (H31,3-pd3a stands for 1,3-propanediamine-N,N,N′-triacetic acid; H31,3-pd2ap stands for 1,3-propanediamine-N,N′-diacetic-N-3-propionic acid) and the corresponding novel octahedral nickel(II) complexes have been prepared and characterized. H31,3-pd3a and H31,3-pd2ap ligands coordinate to nickel(II) ion via five donor atoms (three deprotonated carboxylate atoms and two amine nitrogens) affording octahedral geometry in case of all investigated Ni(II) complexes. A sixth place within octahedra has been occupied by the molecule of water. A six coordinate, octahedral geometry has been established crystallographically for the K[Ni(1,3-pd3a)(H2O)]·3H2O complex. Structural data correlating similar chelate Ni(II) complexes have been used for an extensive strain analysis. This is discussed in relation to information obtained for similar complexes. The infra-red and electronic absorption spectra of the complexes are interpreted and compared with related complexes of known geometries. Density functional theory (DFT) has been used to model the most stable geometry isomer and Natural Energy Decomposition Analysis (NEDA) to reveal the energetic relationship of these compounds. The results from density functional studies have been compared with X-ray data. NEDA has been done for the [LNi]− and [H2O] units.

Nickel(ii) in chelate N2O2 environment. DFT approach and in-depth molecular orbital and configurational analysis

The O-N-N-O-type tetradentate ligands H2S,S-eddp (H2S,S-eddp stands for S,S-ethylenediamine-N,N'-di-2-propionic acid) and H2edap (H2edap stands for ethylenediamine-N-acetic-N'-3-propionic acid) and the corresponding novel octahedral nickel(II) complexes have been prepared and characterized. N2O2 ligands coordinate to the nickel(II) ion via four donor atoms (two deprotonated carboxylate atoms and two amine nitrogens) affording octahedral geometry in the case of all investigated Ni(II) complexes. A six coordinate, octahedral geometry has been verified crystallographically for the s-cis-[Ni(S,S-eddp)(H2O)2] complex. Structural data correlating similarly chelated Ni(II) complexes have been used to carry out an extensive configuration analysis. Molecular mechanics and Density Functional Theory (DFT) have been used to model the most stable geometric isomer, yielding, at the same time, significant structural and spectroscopic (TDDFT) data. The results from density functional studies have been compared to X-ray data. Natural Bond Orbital (NBO) and Natural Energetic Decomposition Analysis (NEDA) have been done for the [Ni(edda-type)(H2O)(2-n)] and nH2O fragments. Molecular orbital analysis (MPA) is given as well. The infra-red and electronic absorption spectra of the complexes are discussed in comparison to the related complexes of known geometries.

Anilinium-3-carboxylate 3-carboxyanilinium nitrate

The title compound, C7H8NO2 +·NO3 −·C7H7NO2, exists in the form of a protonated dimer of two anilinium-3-carboxyl­ate mol­ecules related by an inversion center, and a nitrate anion located on a twofold rotation axis. The bridging H atom occupies, with equal probability, the two sites associated with the carboxyl atoms. In addition to the strong O—H⋯O hydrogen bond, in the crystal, the various units are linked via N—H⋯O and C—H⋯O hydrogen bonds forming a three-dimensional structure.

Crystal structure, configurational and DFT-NEDA analysis of nickel(II) complexes with pentadentate ed3a-type ligands

The O–O–N–N–O-type pentadentate ligands H3ed3a, H31,2-pd3a, H3eda2p and H3ed3p (H3ed3a stands for ethylenediamine-N,N,N′-triacetic acid; H31,2-pd3a stands for 1,2-propanediamine-N,N,N′-triacetic acid; H3eda2p stands for ethylenediamine-N-acetato-N,N′-di-3-propionic acid and H3ed3p stands for ethylenediamine-N,N,N′-tri-3-propionic acid) and the corresponding novel octahedral nickel(II) complexes have been prepared and characterized. H3ed3a, H31,2-pd3a, H3eda2p and H3ed3p ligands coordinate to nickel(II) ion via five donor atoms (three deprotonated carboxylate atoms and two amine nitrogens) affording octahedral geometry in case of all investigated Ni(II) complexes. A sixth place within octahedra has been occupied by the molecule of water. A six coordinate, octahedral geometry has been established crystallographicaly for the [Ni(H2O)6][Ni(ed3a)(H2O)]2·2H2O complex. Structural data correlating similar chelate Ni(II) complexes have been used to carry out an extensive configuration analysis. This is discussed in relation to information obtained for similar complexes. The infra-red and electronic absorption spectra of the complexes are discussed in comparison with related complexes of known geometries. Molecular mechanics and density functional theory (DFT) have been used to model the most stable geometric isomer yielding, at the same time, significant structural data. The results from density functional studies have been compared with X-ray data. Natural Energetic Decomposition Analysis (NEDA) has been done for the LNi⋯OH2 unit.

Characterization of humic substances in bio-treated municipal solid waste landfill leachate

Considerable organic matter remains in municipal solid waste landfill leachate after biological treatments. Humic substances (HSs) dominate the organic matter in bio-treated landfill leachate. In this study, the HSs from landfill leachate treated by membrane bioreactor (MBR-HSs) were analyzed via elemental analysis, ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, and charge polarized magic-angle spinning-13C-nuclear magnetic resonance. The characteristic absorption in the UV wavelength range indicated the presence of high C=C and C=O double bonds within the MBR-HSs. Compared with commercial HSs, MBR-HSs had lower carbon content [48.14% for fulvic acids (FA) and 49.52% for humic acids (HA)], higher nitrogen content (4.31% for FA and 6.16% for HA), lower aromatic structure content, and higher carbohydrate and carboxylic atoms of carbon content. FA predominantly had an aliphatic structure, and HA had less condensed or substituted aromatic ring structures than natural HA. The aromatic carbon content of MBR-HSs was lower than that of humus-derived HSs but higher than that of waste-derived HSs, indicating that MBR-HSs appeared to be more similar to humus-derived HSs than waste-derived HA.

Crystallographic evidence for noncoplanar catalytic aspartic acids in plasmepsin II resides in the Protein Data Bank

The carboxylate atoms of the two catalytic aspartic acid residues in aspartic proteases are nearly coplanar and in the uncomplexed form share an in-plane nucleophilic water molecule that is central to the mechanism of these enzymes. This note reports that while reviewing the electron-density maps derived from the deposited data for uncomplexed plasmepsin II from Plasmodium falciparum [Asojo et al. (2003), J. Mol. Biol. 327, 173-181; PDB code 1lf4], it was discovered that the aspartic acid residues in this structure should in fact be distinctly noncoplanar. The crystallographic model from the deposited coordinates has been re-refined against the 1.9 A resolution published diffraction data to an R(cryst) of 21.2% and an R(free) of 22.2%. The catalytic water molecule is present, but the plane of the carboxylate group of Asp214 is rotated by 66 degrees from its original position.

Synthesis, characterization, and magnetic properties of new binuclear Cu IICu II bis(oxamato) complexes

Two new neutral, binuclear Cu IICu II bis(oxamato) complexes with the formula [Cu 2(opba)(pmdta)(MeOH)] · 1/2MeOH · dmf ( 3) and [Cu 2(nabo)(pmdta)(MeOH)] ( 4), with opba = o-phenylene-bis(oxamato), nabo = 2,3-naphthalene-bis(oxamato), pmdta = N, N, N′, N″, N″-pentamethyldiethylenetriamine and dmf = dimethylformamide have been synthesized and their crystal structures have been determined. The structure of 3 consists of dimeric [Cu 2(opba)(pmdta)(MeOH)] entities, joined together by mutual intermolecular Cu⋯O contacts of the Cu 2+ ion of one [Cu(opba)] 2− complex fragment and one carboxylate atom of the oxamato group of a second [Cu(opba)] 2− complex fragment. The structure of 4 consists of neutral binuclear complexes joined together by hydrogen bonds and π–π interactions, giving rise to an unique supramolecular 1D-chain. The magnetic properties of 3 and 4 were studied by susceptibility measurements versus temperature. For the intramolecular J parameter, identical values of (−114 ± 2) cm −1 ( 3) and (−112 ± 2) cm −1 ( 4) were obtained.

Mononuclear and dinuclear peroxotungsten complexes with co-ordinated dipeptides as potent inhibitors of alkaline phosphatase activity

New molecular peroxotungstate(VI) complexes with dipeptides as ancillary ligands of the type, [WO(O2)2(dipeptide)(H2O)].3H2O, dipeptide = glycyl-glycine or glycyl-leucine, have been synthesized and characterized by elemental analysis, spectral and physico-chemical methods including thermal analysis. The complexes contain side-on bound peroxo groups and a peptide zwitterion bonded to the metal centre unidentately through an O(carboxylate) atom. Investigations on certain biologically important key properties of these compounds and a set of dimeric compounds in analogous co-ligand environment, Na2[W2O3(O2)4(dipeptide)2].3H2O, dipeptide = glycyl-glycine and glycyl-leucine, reported previously by us revealed interesting features of the compounds. Each of the compounds despite having a 7 co-ordinated metal centre exerts a strong inhibitory effect on alkaline phosphatase activity with a potency higher than that of the free dipeptide, tungstate or peroxotungstate. The compounds exhibit remarkable stability in solutions of acidic as well as physiological pH and are weaker as substrate to the enzyme catalase, compared to H2O2. The mononuclear and dinuclear peroxotungsten compounds are efficient oxidants of reduced glutathione (GSH), a reaction in which only one of the peroxo groups of a diperoxotungsten moiety of the complexes was found to be active.

Heme Coordination by Staphylococcus aureus IsdE

Staphylococcus aureus is a Gram-positive bacterial pathogen and a leading cause of hospital acquired infections. Because the free iron concentration in the human body is too low to support growth, S. aureus must acquire iron from host sources. Heme iron is the most prevalent iron reservoir in the human body and a predominant source of iron for S. aureus. The iron-regulated surface determinant (Isd) system removes heme from host heme proteins and transfers it to IsdE, the cognate substrate-binding lipoprotein of an ATP-binding cassette transporter, for import and subsequent degradation. Herein, we report the crystal structure of the soluble portion of the IsdE lipoprotein in complex with heme. The structure reveals a bi-lobed topology formed by an N- and C-terminal domain bridged by a single alpha-helix. The structure places IsdE as a member of the helical backbone metal receptor superfamily. A six-coordinate heme molecule is bound in the groove established at the domain interface, and the heme iron is coordinated in a novel fashion for heme transporters by Met(78) and His(229). Both heme propionate groups are secured by H-bonds to IsdE main chain and side chain groups. Of these residues, His(229) is essential for IsdE-mediated heme uptake by S. aureus when growth on heme as a sole iron source is measured. Multiple sequence alignments of homologues from several other Gram-positive bacteria, including the human pathogens pyogenes, Bacillus anthracis, and Listeria monocytogenes, suggest that these other systems function equivalently to S. aureus IsdE with respect to heme binding and transport.

Indirect photometry and multinuclear 13C and 183W NMR study of tungstate complexes of sugar acids : Stabilities and structures

The tungstate complexes of the sugar acids, meso-tartaric and l-gulonic acids, have been studied in aqueous solutions. Indirect photometry has been used to determine the stability constants and the stoichiometry of these colourless complexes. Multinuclear 13C and 183W NMR spectroscopies have been used to specify the structures of the complexes and the sites of chelation of the ligand. For meso-tartaric acid, complexes of the lactic type with a mononuclear and with a dinuclear tungsten core have been identified. In these type lactic complexes, only the hydroxyl groups of the carboxyl function and the atom carbon in α position belong to the site of chelation of the tungstate complexes. Both α-hydroxyacid moieties of the meso-tartaric acid are involved in the chelation. For l-gulonic acid, lactic type complexes with a mononuclear and a dinuclear tungsten core have been observed at acidic pH. Upon increasing the pH, the lactic complexes disappear: first the mononuclear, then the dinuclear complexes. In pH range 4–8, for gulonic acid, dual lactic, threo (tetradentate) type complexes occur, the dual lactic, erythro and lactic, threo complexes exist. At pH 10, for gulonic acid, a single complex has been identified in which the ligand is pentadentate. In this complex, all carbon atoms of the ligand are involved in the site of chelation except the carboxyl atom.