N-methylacetohydroxamic Acid Research Articles

Lipophilic chloro-oxo-bis(hydroxamato)vanadium(V) complexes: synthesis methods and structure

New methods for the synthesis of vanadium(V) complexes with hydrophobic alkyl- and aryl-substituted hydroxamic acids as well as hydrophilic ones have been developed. A number of novel coordination compounds of the general formula VOL2Cl (where HL is N-phenylaceto-, N-methylbenzo-, N-methyldecano- or N-methylacetohydroxamic acid) have been synthesized. Complexes of the above formula with alkyl substituents on the ligands have been obtained for the first time. All the compounds obtained have been characterized by UV–Vis, IR, 1H and 51V NMR spectroscopy. Studies using the 51V NMR method showed the presence of two forms of the complexes in solution, which coalesce with increasing temperature. Chloro-oxo-bis(N-methylacetohydroxamato)vanadium(V) has been characterized by X-ray crystallography. The effect of the electron donating properties of the ligand substituents on the structure of the complexes is discussed.

An unexpected new pathway for nitroxide radical production via more reactve nitrogen-centered amidyl radical intermediate during detoxification of the carcinogenic halogenated quinones by N-alkyl hydroxamic acids

We found previously that nitroxide radical of desferrioxamine (DFO•) could be produced from the interaction between the classic iron chelating agent desferrioxamine (DFO, an N-alkyl trihydroxamic acid) and tetrachlorohydroquinone (TCHQ), one of the carconogenic quinoind metabolites of the widely used wood preservative pentachlorophenol. However, the underlying molecular mechanism remains unclear. Here N-methylacetohydroxamic acid (N-MeAHA) was synthesized and used as a simple model compound of DFO for further mechanistic study. As expected, direct ESR studies showed that nitroxide radical of N-MeAHA (Ac-(CH3)NO•) can be produced from N-MeAHA/TCHQ. Interestingly and unexpectedly, when TCHQ was substituted by its oxidation product tetrachloro-1,4-benzoquinone (TCBQ), although Ac-(CH3)NO• could also be produced, no concurrent formation of tetrachlorosemiquinone radical (TCSQ•) and TCHQ was detected, suggesting that Ac-(CH3)NO• did not result from direct oxidation of N-MeAHA by TCSQ• or TCBQ as proposed previously. To our surprise, a new nitrogen-centered amidyl radical was found to be generated from N-MeAHA/TCBQ, which was observed by ESR with the spin-trapping agents and further unequivacally identified as Ac-(CH3)N• by HPLC-MS. The final product of amidyl radical was isolated and identified as its corresponding amine. Analogous radical homolysis mechanism was observed with other halogenated quinoid compounds and N-alkyl hydroxamic acids including DFO. Interestingly, amidyl radicals were found to induce both DNA strand breaks and DNA adduct formation, suggesting that N-alkyl hydroxamic acids may exert their potential side-toxic effects via forming the reactive amidyl radical species. This study represents the first report of an unexpected new pathway for nitroxide radical production via hydrogen abstration reaction of a more reactive amidyl radical intermediate during the detoxification of the carcinogenic polyhalogenated quinones by N-alkyl hydroxamic acids, which provides more direct experimental evidence to better explain not only our previous finding that excess DFO can provide effective but only partial protection against TCHQ (or TCBQ)-induced biological damage, and also the potential side-toxic effects induced by DFO and other N-alkyl hydroxamic acid drugs.

A solution- and gas-phase study of uranyl hydroxamato complexes

This study compares the binding properties towards the uranyl cation of three hydroxamates derived from benzohydroxamic acid (BHAH), N-methylacetohydroxamic acid (NMAH) and 1-hydroxypiperidine-2-one (PIPOH), as models of the binding sites found in natural chelators that could be involved in the mobilization of uranyl in the vicinity of abandoned mines. Solution speciation studies of the UO 2 2+ /BHAH system by affinity capillary electrophoresis allowed to estimate stability constants for both ML [log K110 = 7.4(1)] and ML2 [log K120 = 7.0(1)] species in aqueous media [0.1 M (H,Na)ClO4, 25 °C], which lie in-between those reported by us elsewhere for NMA− and PIPO−. By contrast, gas phase studies demonstrate that one of the U=O bonds can be activated in the NMA− and PIPO− complexes, whereas this is not the case for the BHA− complex.

A Comparative IRMPD and DFT Study of Fe3+ and UO22+ Complexation with N-Methylacetohydroxamic Acid.

Iron(III) and uranyl complexes of N-methylacetohydroxamic acid (NMAH) have been investigated by mass spectrometry, infrared multiphoton dissociation (IRMPD) spectroscopy, and density functional theory (DFT) calculations. A comparison between IRMPD and theoretical IR spectra enabled one to probe the structures for some selected complexes detected in the gas phase. The results show that coordination of Fe3+ and UO22+ by hydroxamic acid is of a very similar nature. Natural bond orbital analysis suggests that bonding in uranyl complexes possesses a slightly stronger ionic character than that in iron complexes. Collision-induced dissociation (CID), IRMPD, and 18O-labeling experiments unambiguously revealed a rare example of the U═O bond activation concomitant with the elimination of a water molecule from the gaseous [UO2(NMA)(NMAH)2]+ complex. The U═O bond activation is observed only for complexes with one monodentate NMAH molecule forming a hydrogen bond toward one "yl" oxygen atom, as was found by DFT calculations. This reactivity might explain oxygen exchange observed for uranyl complexes.

82 - Nitroxide Radical Production from Hydroxamic Acids and Carcinogenic Halogenated Quinones via an Unexpected Nitrogen-centered Radical Intermediate Pathway

We have found previously that a nitroxide radical could be produced from desferrioxamine (DFO) with tetrachloro-hydroquinone (TCHQ, the major genotoxic metabolite of the widely used wood preservative pentachlorophenol). However, the underlying molecular mechanism remains unclear. In this study, N-methyl acetohydroxamic acid (N-MeAHA) was used as a simple model compound for DFO for further mechanistic study. As expected, the N-MeAHA-nitroxide radical (N-MeAHA-O•) was found to be produced during the reaction between N-MeAHA and TCHQ by direct ESR. Interestingly and unexpectedly, when TCHQ was substituted by tetrachloro-1,4-benzoquinone (TCBQ), N-MeAHA-O• was also observed, but no tetrachlorosemiquinone anion radical (TCSQ•—) or TCHQ was detected, suggesting the formation of N-MeAHA-O• is not a direct redox reaction between N-MeAHA and TCBQ (or TCSQ•—). To detect whether other radical intermediates were formed during the reaction of N-MeAHA and TCBQ, ESR spin-trapping method was used with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as trapping agent. We found, surprisingly, that a new nitrogen-centered radical was produced, which was identified as Ac-(CH3)N• by HPLC-MS and the high resolution FTICR-MS. Taken together, we propose the production of nitroxide radical from N-MeAHA and TCHQ is via an unexpected new pathway via H-abstraction from the N-OH group by the novel nitrogen-centered radical intermediate.

Conformational and structural studies of N-methylacetohydroxamic acid and of its mono- and bis-chelated uranium(VI) complexes

The thermodynamics and kinetics of the cis/trans isomerism of N-methylacetohydroxamic acid (NMAH) and its conjugated base (NMA−) have been reinvestigated in aqueous media by 1H NMR spectroscopy. Hindered rotation around the central C–N bond due to electronic delocalization becomes slow enough on the NMR time scale to observe both rotamers in equilibrium in D2O at room temperature. By properly assigning the methyl group resonances, evidence for the prevalence of the E over the Z form is unambiguously provided [K300=[E]/[Z]=2.86(2) and 9.63(5) for NMAH and NMA−, respectively], closing thereby a long-lasting dispute about the most stable conformer. To that end, calculations of the chemical shifts by density functional theory (DFT), which accurately reproduced the experimental data, turned out to be a much more reliable method than the direct computation of the relative energy for each conformer. The Z⇌E interconversion dynamics was probed at 300K in D2O by 2D exchange-correlated spectroscopy (EXSY), affording the associated rate constants [kZE=9.0(2) s−1 and kEZ=3.14(5) s−1 for NMAH, kZE=0.96(3) s−1 and kEZ=0.10(2)s−1 for NMA−] and activation barriers at 300K [ΔG≠ZE=68.0kJmol−1 and ΔG≠EZ=70.6kJmol−1 for NMAH, ΔG≠ZE=73.6kJmol−1 and ΔG≠EZ=79.2kJmol−1 for NMA−]. For the first time, mono- and bis-chelated uranium(VI) complexes of NMA− have been isolated. Crystals of [UO2(NMA)(NO3)(H2O)2] and [UO2(NMA)2(H2O)] have been characterized by X-ray diffractometry, infrared and Raman spectroscopies.

Structure copper(II) complexes with N-methylacetohydroxamic acid in crystal and solution

The structure of the dimeric complex [CuL2]2 (HL is N-methylacetohydroxamic acid) formed by two square-planar CuL2 moieties with the trans coordination of the ligands has been studied by X-ray crystallography. In methylene chloride at 297 K, the complex exists as two species, which are monomeric as probed by electron paramagnetic resonance. At 80 K, the dimeric and monomeric forms are identified. The relative concentrations of paramagnetic forms and their magnetic resonance parameters have been determined from simulation of EPR spectra.

Conformational analysis of a secondary hydroxamic acid in aqueous solution by NOE spectroscopy

Hydroxamic acids are metal-binding compounds used by micro-organisms and possess applications in medicine and industry. Hydroxamic acids favor two conformations, E and Z; metal binding is limited to the Z conformation. The Z conformation may be identifiable by NOE spectroscopy, but analysis is complicated by the potential for long-range coupling as well as for relayed NOEs due to conformational switching. In this report, we re-examine the reported conformational preference of N-methyl acetohydroxamic acid (NMHA) in D(2)O using NOE spectroscopy. We find that the favored conformation of NMHA in aqueous solution is the E conformation, contrary to an earlier report. NOE build-up curves are proposed as a valuable tool to probe conformational behavior in similar systems.

Infrared Spectroscopic Studies of Siderophore-Related Hydroxamic Acid Ligands Adsorbed on Titanium Dioxide

The adhesion of bacteria to metal oxide and other mineral surfaces may involve bacterial siderophores, many of which contain hydroxamic acid (Ha) ligands. The adsorption behavior of the siderophore-related ligands acetohydroxamic acid, N-methylformohydroxamic acid, N-methylacetohydroxamic acid, and 1-hydroxy-2-piperidone on titanium dioxide thin films has been investigated using in situ ATR-IR spectroscopy with variation of concentration and pH. All the ligands were found to adsorb strongly on the TiO2 surface as hydroxamate ions and form bidentate surface complexes. Vibrational modes involving C=O stretching and N-O stretching of these ligands were assigned by comparing observed IR spectra with those calculated by the density functional method at the B3LYP/6-31+G(d) level. Calculated spectra of the complex [Ti(Ha)(OH)4]-, used to model the TiO2 surface, were compared with observed spectra of adsorbed hydroxamic acids. These results suggest that hydroxamic acid ligands in siderophores would be expected to bind to metal (oxide) and mineral surfaces during bacterial adhesion processes.

New insights into the solution equilibrium of molybdenum(vi)–hydroxamate systems: 1H and 17O NMR spectroscopic study of Mo(vi)–desferrioxamine B and Mo(vi)–monohydroxamic acid systems

To complement our previous pH-potentiometric and spectrophotometric investigations, in the present work 17O NMR studies on Mo(VI)–desferrioxamine B (DFB) and Mo(VI)–acetohydroxamic acid (Aha) systems, and 1H NMR on Mo(VI)–Aha, Mo(VI)–benzohydroxamic acid (Bha) and –N-methylacetohydroxamic acid (MeAha) have been performed. Complete equilibrium models for all the studied systems are presented in this paper. Formation of a hydrogen bond could be suggested between the hydroxamate-NH of the coordinated primary monohydroxamic acids and oxo ligands of molybdenum under acidic conditions and, at ca. neutral pH, deprotonation of that NH was found. This is the first time that this process in a Mo(VI)–hydroxamic acid system has been detected. The hydroximato chelate formed in this manner is unusually stable, and able to compete with hydrolytic processes up to basic pH, which results in the surprising fact that the interaction between Mo(VI) and the small primary molecules, Aha and Bha exists up to much higher pH than between Mo(VI) and the known powerful tris-chelator natural compound, DFB.

Kinetics and mechanism of iron(III) dissociation from the dihydroxamate siderophores alcaligin and rhodotorulic acid.

The kinetics and mechanism of siderophore ligand dissociation from their fully chelated Fe(III) complexes is described for the highly preorganized cyclic tetradentate alcaligin and random linear tetradentate rhodotorulic acid in aqueous solution at 25 degrees C (Fe2L3 + 6H+ reversible 2 Fe3+ aq + 3 H2L). At siderophore:Fe(III) ratios where Fe(III) is hexacoordinated, kinetic data for the H(+)-driven ligand dissociation from the Fe2L3 species is consistent with a singly ligand bridged structure for both the alcaligin and rhodotorulic acid complexes. Proton-driven ligand dissociation is found to proceed via parallel reaction paths for rhodotorulic acid, in contrast with the single path previously observed for the linear trihydroxamate siderophore ferrioxamine B. Parallel paths are also available for ligand dissociation from Fe2(alcaligin)3, although the efficiency of one path is greatly diminished and dissociation of the bis coordinated complex Fe(alcaligin)(OH2)2+ is extremely slow (k = 10(-5) M-1 s-1) due to the high degree of preorganization in the alcaligin siderophore. Mechanistic interpretations were further confirmed by investigating the kinetics of ligand dissociation from the ternary complexes Fe(alcaligin)(L) in aqueous acid where L = N-methylacetohydroxamic acid and glycine hydroxamic acid. The existence of multiple ligand dissociation paths is discussed in the context of siderophore mediated microbial iron transport.

Kinetics and Mechanism of the Complexation of La3+ and Cu2+ Ions with N-Methylacetohydroxamic Acid and Desferrioxamine B1

The kinetics and mechanism of the complexation of La3+ and Cu2+ ions with desferrioxamine B (H4dfb+) and N-methylacetohydroxamic acid (NMHA) in aqueous medium were studied by stopped-flow and 1H NMR methods. The equilibrium constants for reactions (Mn+ + HA ⇄ MA(n-1)+ + H+) of NMHA with Cu2+ and La3+ were determined by the combined pH−spectral titration method at 25 ± 0.1 °C as (1.58 ± 0.02) × 10-1 and (3.5 ± 0.2) × 10-4, respectively. In aqueous solution of 2 M ionic strength, the kinetic parameters for complexation of La3+ and Cu2+ with NMHA were determined as k(25 °C) = 3.0 ± 0.3 s-1, ΔH⧧ = 76 ± 3 kJ mol-1, ΔS⧧ = 19 ± 7 J K-1 mol-1, ΔV⧧ = +5.3 ± 0.5 cm3/mol and k(25 °C) = 3.4 ± 0.2 s-1, ΔH⧧ = 69 ± 1 kJ mol-1, ΔS⧧ = −3 ± 3 J K-1 mol-1, ΔV⧧ = +5.0 ± 0.5 cm3/mol, whereas with H4dfb+ they were determined as k(25 °C) = 2.9 ± 0.3 s-1, ΔH⧧ = 76 ± 1 kJ mol-1, ΔS⧧ = 34 ± 4 J K-1 mol-1, ΔV⧧ = +5.2 ± 0.5 cm3/mol; and k(25 °C) = 3.0 ± 0.4 s-1, ΔH⧧ = 72 ± 1 kJ mol-1, ΔS⧧ = 5 ± 2 J K-1 mol-1, ΔV⧧ = +3.4 ± 0.2 cm3/mol, respectively. The rotation about the C−N hydroxamate bonds in NMHA and H4dfb+ is characterized by k(25 °C) = 11 ± 2 s-1, ΔH⧧ = 76 ± 5 kJ mol-1, ΔS⧧ = 31 ± 16 J K-1 mol-1, ΔrV(cis⇄trans) = +1.5 ± 0.8 cm3/mol, ktrans→cis(25 °C) = 2.8 ± 0.5 s-1, ΔV⧧trans→cis = +12 ± 4 cm3/mol and by k(25 °C) = 9 ± 1 s-1, ΔH⧧ = 69 ± 6 kJ mol-1, ΔS⧧ = 6 ± 18 J K-1 mol-1, ΔrV(cis⇄trans) = +0.6 ± 0.3 cm3/mol, ktrans→cis(25 °C) = 2.6 ± 0.3 s-1, ΔV⧧trans→cis = +5 ± 2 cm3/mol, respectively. The results suggest that the slow rotation around the hydroxamate C−N bond is the rate-determining step for the complexation reactions.

Microscopic acid–base equilibria of a synthetic hydroxamate siderophore analog, piperazine-1,4-bis(N-methylacetohydroxamic acid)

The protonation behavior of the cyclic diaminodihydroxamate ligand, piperazine-1,4-bis(N-methylacetohydroxamic acid) (H2L1), has been studied at both the macroscopic and the microscopic level. Potentiometric and 1H NMR techniques have been used for the study of this ligand as well as several model compounds: N-methylchloroacetohydroxamic acid, glycinehydroxamic acid and piperidino(N-methylacetohydroxamic acid). Molecular modeling calculations have also been performed to predict the most stable conformations and to estimate relevant contributions to the overall protonation process. The results of the protonation microconstants show that the N-donors in H2L1 are much less basic than the O-donors. The protonated amine moieties release most of their protons in the acid region while the deprotonation of the hydroxamate moieties starts only above pH 5. The theoretical modeling calculations show the effect of electrostatic interactions and internal hydrogen bonds on the interactivity of the basic sites throughout the protonation process.

Dihydroxamic acid complexes of iron (III): ligand p Ka and coordinated water hydrolysis constants

A series of dihydroxamic acid ligands of the formula [RN(OH)C(O)] 2(CH 2) n , ( n = 2, 4, 6, 7, 8; R = CH 3, H) has been studied in 2.0 M aqueous sodium perchlorate at 25.0 °C. These ligands may be considered as synthetic analogs to the siderophore rhodotorulic acid. Acid dissociation constants (p K a) have been determined for the ligands and for N-methylacetohydroxamic acid (NMHA). The p K a1 and p K a2 values are: n = 2, R = CH 3 (8.72, 9.37); n = 4, R = CH 3 (8.79, 9.37); n = 6, R = CH 3; n = 7, R = CH 3 (8.95, 9.47); n = 8, R = CH 3 (8.93, 9.45); n = 8, R = H (9.05, 9.58). Equilibrium constants for the hydrolysis of coordinated water (log K) have been estimated for the 1:1 feeric complexes of the ligands n = 2, 4, 8; R = CH 3. The N = 8 ligand forms a monomeric complex with Fe(III) while the n = 2 and 4 ligands form dimeric complexes. For hydrolysis of the n = 8 monomeric complex, log K 1 = −6.36 and log K 2 = −9.84. Analysis of the spectrophotometric data for the dimeric complexes indicates deprotonation of all four coordinated waters. The successive hydrolysis constants, log K 1–4, for the dimeric complexes are as follows: n = 2 (−6.37, −5.77, −10.73, −11.8); n = 4 (−5.54, −5.07, −11.57, −10.17). The log K 2 values for the dimers are unexpectedly high, higher in fact than log K 1, inconsistent with the formation of simple ternary hydroxo complexes. A scheme is proposed for the hydrolysis of the ferric dihydroxamate dimers, which includes the possible formation of μ-hydroxo and μ-oxo bridges.

Siderophore analogues. Synthesis and chelating properties of a new macrocyclic trishydroxamate ligand

A new iron(III)-specific ligand, 1,5,9-triazacyclododecane-N,N′,N″-tris(N-methylacetohydroxamic acid) H3L, containing three hydroxamic acid groups as pendant arms on a macrocyclic triamine backbone, has been synthesized and characterized. Its acid–base and chelating properties with iron(III) and copper(II) ions have been studied by potentiometric and spectrophotometric techniques. The mechanism of electron transfer as well as the kinetics of dissociation and stability constants of reduced species, which are thought to be important in the biological activity of this siderophore analogue, have been studied by voltammetric methods. This ligand has proved to be biologically active and its properties were compared with ferrichrome and ferrioxamine B.

A new iron(III) ion sequestering ligand: synthesis, solution chemistry and electrochemistry

A new cyclic diaminodihydroxamic ligand, piperazine-1,4-bis(N-methylacetohydroxamic acid)(H2L2), presenting a reasonable analogy with the naturally occurring siderophore rhodotorulic acid (H2L1), has been prepared through an easy two-step process. This ligand, which has proved to be biologically active, forms a very stable complex [Fe2L23]. The ligand and its iron and copper complexes have been characterized by potentiometric and spectrophotometric techniques. The mechanism of electron transfer in the complexes has been studied by voltammetric methods, and the kinetics of dissociation of the iron(II) complex, which might be of crucial importance in the biological activity of this siderophore analogue, was also investigated.

Enzyme models. The attachment of N-methylhydroxamic acid to cyclohexaamylose. Its reactivity and specificity with phenyl esters

The N-methylacetohydroxamic acid group has been introduced into cyclohexaamylose by the following sequence of reactions: (1) carboxymethylation of cyclohexaamylose by iodoacetic acid, (2) methylation of carboxymethylcyclohexaamylose with diazomethane, and (3) reaction of the carboxymethylcyclohexaamylose methyl ester with N-methylhydroxylamine to form the N-methylacetohydroxamic acid-substituted cyclohexaamylose. By employing purification procedures involving ionexchange chromatography, the synthesis yielded a mono-substituted cyclohexaamylose- N-methylacetohydroxamic acid with selective modification of the C-2, C-3 hydroxyl group side of the cyclohexaamylose ring. p-Nitrophenyl acetate and 2-hydroxy-5-nitro-α-toluenesulfonic acid sultone react 20- and 70-fold faster with cyclohexaamylose- N-methylacetohydroxamic acid than with N-methylmethoxyacetohydroxamic acid. Cyclohexaamylose- N-methylacetohydroxamic acid also displays a marked kinetic stereospecificity for p-nitroover m-nitrophenyl acetate (whereas cyclohexaamylose itself exhibits the reverse stereospecificity). These reactions were shown to be competitively inhibited by cyclohexanol. This evidence indicates that cyclohexaamylose- N-methylacetohydroxamic acid binds the substrate in a reversible complexation step prior to nucleophilic attack and thus is an enzyme model.

Clefts of the face in animal experiments

Acetohydroxamic and N-methylacetohydroxamic acid induce clefts of lip, alveolus and palate or clefts of mandible in rats in the order of 1%, in addition to other malformations, after application of critical doses on the 12th day of pregnancy. The number of survivors together with the other malformations, but not the formation of the clefts of the primary palate, depends on the administered dose. The conditions for the induction of cleft lip, alveolus and palate in animal experiments are analysed and discussed.

N-methylacetohydroxamic acid catalyzed ester hydrolysis

N-methylacetohydroxamic acid catalyzed ester hydrolysis