Characterization of Mn(II) and Mn(III) binding capability of natural siderophores desferrioxamine B and desferricoprogen as well as model hydroxamic acids

Complexes formed between Mn(II) ion and acetohydroxamic acid (HAha), benzohydroxamic acid (HBha), N-methyl-acetohydroxamic acid (HMeAha), DFB model dihydroxamic acids (H₂(3,4-DIHA), H₂(3,3-DIHA), H₂(2,5-DIHA), H₂(2,5-H,H-DIHA), H₂(2,4-DIHA), H₂(2,3-DIHA)) and two trihydroxamate based natural siderophores, desferrioxamine B (H₄DFB) and desferricoprogen (H₃DFC) have been investigated under anaerobic condition (and some of them also under aerobic condition). The pH-potentiometric results showed the formation of well-defined complexes with moderate stability. Monohydroxamic acids not, but all of the dihydroxamic acids and trihydroxamic acids were able to hinder the hydrolysis of the metal ion up to pH ca. 11. Maximum three hydroxamates were found to coordinate to the Mn(II) ion, but presence of water molecule in the inner-sphere was also indicated by the corresponding relaxivity values even in the tris-chelated complexes. Moreover, prototropic exchange processes were found to increase the relaxation rate of the solvent water proton over the value of [Mn_aqua]²⁺ in the protonated Mn(II)-siderophore complexes at physiological pH. The much higher stability of Mn(III)-hydroxamate (especially tris-chelated) complexes compared to the corresponding Mn(II)-containing species results in a significantly decreased formal potential compared to the Mn(III)_aqua/Mn(II)_aqua system. As a result, air oxygen becomes an oxidizing agent for these manganese(II)-hydroxamate complexes above pH 7.5. The oxidation processes, followed by UV-Vis spectrophotometry, were found to be stoichiometric only in the case of the tris-chelated complexes of siderophores, which predominate above pH 9. ESI-MS provided support about the stoichiometry and cyclic-voltammetry was used to determine the stability constants for the tris-chelated complexes, [Mn(HDFB)]⁺ and [MnDFC].     

Interaction between iron(II) and hydroxamic acids: oxidation of iron(II) to iron(III) by desferrioxamine B under anaerobic conditions

Interaction between iron(II) and acetohydroxamic acid (Aha), alpha-alaninehydroxamic acid (alpha-Alaha), beta-alaninehydroxamic acid (beta-Alaha), hexanedioic acid bis(3-hydroxycarbamoyl-methyl)amide (Dha) or desferrioxamine B (DFB) under anaerobic conditions was studied by pH-metric and UV-Visible spectrophotometric methods. The stability constants of complexes formed with Aha, alpha-Alaha, beta-Alaha and Dha were calculated and turned out to be much lower than those of the corresponding iron(II) complexes. Stability constants of the iron(II)-hydroxamate complexes are compared with those of other divalent 3d-block metal ions and the Irving-Williams series of stabilities was found to be observed. Above pH 4, in the reactions between iron(II) and desferrioxamine B, the oxidation of the metal ion to iron(III) by the ligand was found. The overall reaction that resulted in the formation of the tris-hydroxamato complex [Fe(HDFB)]+ and monoamide derivative of DFB at pH 6 is: 2Fe2+ + 3H4DFB+ = 2[Fe(HDFB)]+ + H3DFB-monoamide+ + H2O + 4H+. Based on these results, the conclusion is that desferrioxamine B can uptake iron in iron(III) form under anaerobic conditions.     

Complexation of desferricoprogen with trivalent Fe, Al, Ga, In and divalent Fe, Ni, Cu, Zn metal ions: effects of the linking chain structure on the metal binding ability of hydroxamate based siderophores

Complexes of the natural siderophore, desferricoprogen (DFC), with several trivalent and divalent metal ions in aqueous solution were studied by pH-potentiometry, UV-Vis spectrophotometry and cyclic voltammetry. DFC was found to be an effective metal binding ligand, which, in addition to Fe(III), forms complexes of high stability with Ga(III), Al(III), In(III), Cu(II), Ni(II) and Zn(II). Fe(II), however, is oxidized by DFC under anaerobic conditions and Fe(III) complexes are formed. By comparing the results with those of desferrioxamine B (DFB), it can be concluded that the conjugated beta-double bond slightly increases the stability of the hydroxamate chelates, consequently increases the stability of mono-chelated complexes of DFC. Any steric effect by the connecting chains arises only in the bis- and tris-chelated complexes. With metal ions possessing a relatively big ionic radius (Cu(II), Ni(II), Zn(II), In(III)) DFC, containing a bit longer chains than DFB, forms slightly more stable complexes. With smaller metal ions the trend is the opposite. Also a notable difference is that stable trinuclear complex, [Cu(3)L(2)], is formed with DFC but not with DFB. Possible bio-relevance of the Fe(II)/Fe(III) results is also discussed in the paper.     

Pb(II)-binding capability of aminohydroxamic acids: primary hydroxamic acid derivatives of alpha-amino acids as possible sequestering agents for Pb(II)

Complexes of aminohydroxamic acids, D,L-alpha-alaninehydroxamic acid (alpha-Alaha), sarcosinehydroxamic acid (Sarha), D,L-N-methyl-alpha-alaninehydroxamic acid (N-Me-alpha-Alaha), beta-alaninehydroxamic (beta-Alaha), L-aspartic acid-beta-hydroxamic acid (Asp-beta-ha), L-glutamic acid-gamma-hydroxamic acid (Glu-gamma-ha) and L-histidinehydroxamic acid (Hisha) with lead(II) in aqueous solution were studied by pH-potentiometric, 1H NMR and electrospray ionization mass spectrometric (ESI MS) methods. The results were compared to those of a simple monohydroxamic acid, acetohydroxamic acid and the effects of the amino group, hydroxamate-N, as well as, additional side chain donors on the co-ordination mode and on the stability of the complexes formed were evaluated. It was found that the amino nitrogen atom situating in beta- or in gamma-position (beta-Alaha, Asp-beta-ha, Glu-gamma-ha) does not co-ordinate to Pb(II), only hydroxamate type chelates are formed before the hydrolytic processes. However, the amino-N in alpha-position (alpha-Alaha, Sarha, Hisha) seems to form a stable 5-membered (N,N)-type chelate together with the deprotonated hydroxamate-N above pH 6. On the other hand, the hydroxamate (O,O)-type chelate also exists. Since steric reasons do not allow the coordination of these two chelates of a molecule to the same Pb(II) ion, polynuclear complexes with mixed co-ordination modes are formed with the alpha-derivatives above pH 6. Simple hydroxamate type complexes are formed with N-Me-alpha-Alaha, where the hydroxamate-N is not able to co-ordinate. The co-ordination of the side chain imidazole of Hisha is not measurable, while a weak interaction of the side chain carboxylates of Asp-beta-ha and especially of Glu-gamma-ha can be suggested.     

Structure-based differences between the metal ion selectivity of two siderophores desferrioxamine B (DFB) and desferricoprogen (DFC): why DFC is much better Pb(II) sequestering agent than DFB?

Complexation of desferrioxamine B (DFB) and desferricoprogen (DFC) with Cd(II) and Pb(II) toxic ions as well as complexation of DFC with Ca(II) and Mg(II) essential metals have been investigated and the results have been compared to those with other metal ions. The two siderophores have moderate Cd(II)-binding ability, but both, and especially DFC, bind Pb(II) in high stability complexes. Surprisingly, significant differences exist between Pb(II)-complexation of DFB and DFC. Namely, a maximum of two hydroxamate groups of a DFB coordinate to a Pb(II) ion, the third one binds to another metal ion with high preference and the formation of a trinuclear species, [Pb₃(DFBH)₂]²⁺, is predominant even at 1:1 metal to ligand ratio in this system. On the contrary, DFC forms mononuclear complex, [ML], with much higher stability and the formation of the trinuclear complex is negligible compared to DFB. The 6s² electron-pair of Pb(II), which remains always inert during complexation with hydroxamic acids and also with DFB, seems to become active in the DFC complexes (due to the effect of the double bonds in β-position to each hydroxamate), what, at least in some extent, allows the coordination of all the three hydroxamates of DFC to the same Pb(II) ion. This way of interaction (unique with a hydroxamate-based compound) results in significant stability increase, and, as a consequence, DFC is much better Pb(II)-chelating agent than DFB. Although DFC forms unexpectedly high stability complexes with Mg(II) compared to Ca(II), but even Mg(II), compared to many other metals, is not an efficient DFC-binding metal. Therefore, any sequestration of this biologically very important metal is not likely from a living organism by DFC.     

Factors affecting the metal ion-hydroxamate interactions II: effect of the length of the connecting chain on the Fe(III), Mo(VI) and V(V) complexation of some new desferrioxamine B (DFB) model dihydroxamic acids

Three new dihydroxamic acids (HO(CH₃)NCO-(CH₂)₂-CO-NH-(CH₂)_x-CON(CH₃)OH where the x values are 4; 3 and 2, and the compounds are abbreviated as 2,4-DIHA, 2,3-DIHA and 2,2-DIHA), containing the peptide group in a certain position to one of the two functional groups and in different distances to the other one, were synthesized and their complexation with Fe(III), Mo(VI) and V(V) was studied by pH-potentiometric, spectrophotometric and in some cases by CV methods to evaluate the redox behaviour of the Fe(III) complexes and assess their potential biological activity as siderophore models. All these compounds are structural models for the natural siderophore, desferrioxamine B (DFB). The results were compared to those of the complexes of 2,5-DIHA having the same connecting chain structure and length as DFB has, and the effects of the length of the connecting chain on the co-ordination mode and on the stability of the complexes formed were evaluated.Very similar stability of the mono-chelated complexes formed with all these dihydroxamic acids was found. All the results obtained suggest that one dihydroxamic acid (even the 2,2-DIHA) is able to complete the four coordination sites of a MoO₂ ²⁺ core forming simple mononuclear complexes. Favoured monomeric structures of the bis-chelated complexes of these dihydroxamic acids are also suggested with V(V) having the smallest ionic radius among the three metal ions studied. In the case of iron(III), however, clear indication was obtained for the slightly different complexation behaviour of 2,2-DIHA. Namely, the formation of the mononuclear bis-chelated complex with this shortest ligand seems to have sufficient strain to induce the formation of bimetallic species such as [Fe(2,2-DIHA)₂Fe)]²⁺.     

Factors that influence siderophoremediated iron bioavailability: catalysis of interligand iron (III) transfer from ferrioxamine B to EDTA by hydroxamic acids

Deferriferrioxamine B (H3DFB) is a linear trihydroxamic acid siderophore with molecular formula NH2(CH2)5[N(OH)C(O)(CH2)2C(O)NH(CH2)5]2N(OH)C(O)CH3 that forms a kinetically and thermodynamically stable complex with iron(III), ferrioxamine B. Under the conditions of our study (pH = 4.30, 25 degrees C), ferrioxamine B, Fe(HDFB)+, is hexacoordinated and the terminal amine group is protonated. Addition of simple hydroxamic acids, R1C(O)N(OH)R2 (R1 = CH3, R2 = H; R1 = C6H5, R2 = H; R1 = R2 = CH3), to an aqueous solution of ferrioxamine B at pH = 4.30, 25.0 degrees C, I = 2.0, results in the formation of ternary complexes Fe(H2DFB)A+ and Fe(H3DFB)A2+, and tris complexes FeA3, where A- represents the bidendate hydroxamate anion R1C(O)N(O)R2-. The addition of a molar excess of ethylenediaminetetraacetic acid (EDTA) to an aqueous solution of ferrioxamine B at pH 4.30 results in a slow exchange of iron(III) to eventually completely form Fe(EDTA)- and H4DFB+. The addition of a hydroxamic acid, HA, catalyzes the rate of this iron exchange reaction: (formula; see text) A four parallel path mechanism is proposed for reaction (1) in which catalysis occurs via transient formation of the ternary and tris complexes Fe(H2DFB) A+, Fe(H3DFB)A2+, and FeA3. Rate and equilibrium constants for the various reaction paths to products were obtained and the influence of hydroxamic acid structure on catalytic efficiency is discussed. The importance of a low energy pathway for iron dissociation from a siderophore complex in influencing microbial iron bio-availability is discussed. The system represented by reaction (1) is proposed as a possible model for in vivo catalyzed release of iron from its siderophore complex at the cell wall or interior, where EDTA represents the intracellular storage depot or membrane-bound carrier and HA represents a low molecular weight hydroxamate-based metabolite capable of catalyzing interligand iron exchange.     

Suberoylanilide hydroxamic acid, a potent histone deacetylase inhibitor; its X-ray crystal structure and solid state and solution studies of its Zn(II), Ni(II), Cu(II) and Fe(III) complexes

Reaction of the potent hydroxamate-based histone deacetylase (HDAC) inhibitor, suberoylanilide hydroxamic acid (SAHA), with hydrated metal salts of Fe(III), Cu(II), Ni(II) and Zn(II) yielded a tris-hydroxamato complex in the case of Fe(III) and bis-hydroxamato complexes in the case of Cu(II), Ni(II) and Zn(II) both in the solid state and in solution. Reaction of the secondary hydroxamic acid, N-Me-SAHA, also yielded a tris-hydroxamato complex in the case of Fe(III) and bis-hydroxamato complexes in the case of Cu(II), Ni(II) and Zn(II) in solution. These metal complexes have the hydroxamato moiety coordinated in an O,O’-bidentate fashion. Stability constants of the metal complexes formed with SAHA and N-Me-SAHA in a DMSO/H₂O 70/30%(v/v) mixture are described. A novel crystal structure of SAHA together with a novel synthesis for N-Me-SAHA are also reported.     

Effect of Organic Acids on Reactions Catalyzed by Manganese Peroxidase from Phanerochaete chrysosporium

The effect of several organic acids on the oxidation of Mn(II) catalyzed by manganese peroxidase was studied. Reactivities of manganese peroxidase and chemically prepared Mn(III) organic acid complexes towards phenolic compounds were compared. If lactate appears to be the best complexant for manganese peroxidase activity, chemically prepared Mn(III)—lactate complex is a less effective oxidant towards phenolic compounds than other Mn(III)—complexes. Our results agree with the hypothesis that certain organic acids are involved in the catalytic cycle of manganese peroxidase. Malonate and lactate seem to be the most attractive complexants for practical applications of manganese peroxidase and were used in enzymatic treatment of hardwood kraft pulp. Bleaching of kraft pulp was studied and after alkaline extraction, a significant decrease of kappa number was measured. The bleaching was enhanced in lactate buffer.     

A convenient assay for siderochrome hydrolytic enzymes

Conditions are described under which ethylenediaminetetraacetate (EDTA) rapidly dissociates ferric ion from chelation with monohydroxamic acids and dihydroxamic acids, but under which the metal is not dissociated from natural trihydroxamic acids (siderochromes). Based upon this observation, enzymes which hydrolyze siderochromes to monohydroxamic acid subunits can be conveniently assayed spectrophotometrically. The characteristic absorption at 425–440 nm of the siderochrome decreases linearly during hydrolysis. The application of this assay to an enzyme isolated from an unidentified species of penicillium is described.     

Solution and solid-state studies of complexation of transition-metal cations and Al(III) by aroylhydrazones derived from nicotinic acid hydrazide

Reactions of first series transition-metal cations, Cd(II) and Al(III) with two aroylhydrazones derived from nicotinic acid hydrazide and salicylaldehyde or o-vanillin were studied at 25 °C in buffered dioxane/water 1/1 mixture (pH 5.8) by means of spectrophotometric and spectrofluorimetric titrations. The addition of Mn(II) or Cd(II) ions in hydrazone solutions had no effect on their absorption spectra whereas the addition of Ni(II) and Cr(III) immediately caused precipitation. The reaction of Zn(II) with salicylaldehyde derivative was found to be photosensitive. Relatively high conditional stability constants of 1:1 complexes of Cu(II), Zn(II) and Al(III) with both ligands were determined. Solid complexes of Cu(II), Ni(II) and Zn(II) with aroylhydrazones studied were isolated and characterized by elemental and thermogravimetric analyses, magnetic susceptibility measurements (in the case of Cu(II) and Ni(II)) and IR spectrometry.     

Equilibrium and structural studies on metal complexes of carbohyrates and their derivatives

A summary is presented of the studies of our group on metal complexes of carbohydrates (aldoses, ketoses, mono-, di- and polysaccharides) and their derivatives (aldonic, alduronic acids, polyalcohols, amino sugars, amino acid sugar adducts, AMP, ATP, etc.). The results are reported of equilibrium, electrochemical, solution and solid-state structural studies of complexes of transition metals [Cu(II), Fe(III), Ni(II), Zn(II), Co(II), Ag(I), Mn in different oxidation states and organotin(IV)]. The structural parameters (coordination number, bond distance, and Debye-Waller factor) obtained by extended X-ray absorption fine structure spectroscopic (EXAFS) spectroscopy are discussed in detail. The general rules concerning the formation and structure of such complexes are emphasized.     

Metal-binding ability of histidine-containing peptidehydroxamic acids: Imidazole versus hydroxamate coordination

Three new peptidehydroxamic acids (l-alanyl-l-histidinehydroxamic acid, l-Ala-l-HisNHOH, l-alanyl-l-alanyl-l-histidinehydroxamic acid, l-Ala-l-Ala-l-HisNHOH and l-histidyl-l-alaninehydroxamic acid, l-His-l-AlaNHOH) were synthesized and their complexation with Cu(II), Ni(II) and Zn(II) were studied by pH-potentiometric, UV-Vis, CD, ¹H NMR, EPR and ESI-MS methods. Each of the studied peptide derivatives involves one side-chain imidazole unit and the effect of this group on the metal binding of the hydroxamic moiety is evaluated in the paper. The obtained results are compared to those of the complexes of some histidine-containing di- or tripeptides and also to those of hydroxamic derivatives of aliphatic peptides.A competition between the hydroxamate and imidazole functions occurs in all systems, but the extent differs from metal to metal, from ligand to ligand and depends very much on the pH. The imidazole was found to play the most determinant role in the Cu(II) complexes, somewhat less in the Ni(II)-containing ones, while (except the case of l-Ala-l-HisNHOH) negligible role was found in the Zn(II)-complexes. Common feature of the Ni(II)- and especially Cu(II)-containing systems is that if an imidazole-N is displaced by a hydroxamate, imidazole-bridged di- and polynuclear complexes are formed.     

A novel barley yellow stripe 1-like transporter (HvYSL2) localized to the root endodermis transports metal-phytosiderophore complexes

Recent advances in our understanding of how graminaceous plants take up insoluble forms of iron from the rhizosphere and mobilize them in plant tissues are primarily based on the identification of various transporters that are specific to metal-phytosiderophore (PS) complexes containing mugineic acid and deoxymugineic acid. Barley (Hordeum vulgare L.) yellow stripe 1 (HvYS1) is a metal-PS transporter that preferentially transports Fe(III)-PS compared with other metal complexes. Here, we report the cloning and characterization of HvYSL2, a novel metal-PS transporter encoding gene. HvYSL2 is composed of 702 amino acids with 14 transmembrane domains, which are conserved among this class of transporters, and exhibits 67.3% identity to HvYS1. Electrophysiological experiments with Xenopus laevis oocytes revealed that HvYSL2 transports PS complexes with Fe(III), Zn(II), Ni(II), Cu(II), Mn(II) or Co(II); this constitutes a broader range of substrate preference than HvYS1. Real-time PCR analysis revealed that HvYSL2 mRNA is expressed in shoots and also in roots, where it is induced under iron-deficient conditions. Moreover, immunohistochemistry in roots revealed that HvYSL2 is localized to the endodermis, whereas HvYS1 is expressed primarily in the epidermis. These data suggest that HvYSL2 is spatially distinct from HvYS1 and plays a unique role in delivering a broad range of essential metals in barley.     

Greater binding affinity of trivalent antimony to a CCCH zinc finger domain compared to a CCHC domain of kinetoplastid proteins

It has been reported recently that Sb(III) competes with Zn(II) for its binding to the CCHC zinc finger domain of the HIV-1 NCp7 protein, suggesting that zinc finger proteins may be molecular targets for antimony-based drugs. Here, the interaction of Sb(III) with a CCCH zinc finger domain, which is considered to play a crucial role in the biology of kinetoplastid protozoa, has been characterized for the first time. The binding characteristics of Sb(III) were compared between a CCCH-type peptide derived from a kinetoplastid protein and two different CCHC-type zinc finger peptides. The formation of 1 : 1 Zn-peptide and Sb-peptide complexes from the different peptides was demonstrated using circular dichroism, UV absorption, fluorescence spectroscopies and ESI-MS. Titration of the Zn-peptide complexes with SbCl(3) was performed at pH 6 and 7, exploiting the intrinsic fluorescence of the peptides. The differential spectral characteristics of the peptides allowed for competition experiments between the different peptides for binding of Zn(II). The present study establishes that Sb(III) more effectively displaces Zn(II) from the CCCH peptide than CCHC ones, as a result of both the greater stability of the Sb-CCCH complex (compared to Sb-CCHC complexes) and the lower stability of the Zn-CCCH complex (compared to Zn-CCHC complexes). Comparison of the binding characteristics of Sb(III) or Zn(II) between the CCHC-type peptides with different amino acid sequences supports the model that not only the conserved zinc finger motif, but also the sequence of non-conserved amino acids determines the binding affinity of Sb(III) and Zn(II). These data suggest that the interaction of Sb(III) with CCCH-type zinc finger proteins may modulate, or even mediate, the pharmacological action of antimonial drugs.     

Pyridine-2,6-bis(thiocarboxylic acid) Produced by <Emphasis Type="BoldItalic">Pseudomonas stutzeri</Emphasis> KC Reduces Chromium(VI) and Precipitates Mercury,Cadmium, Lead and Arsenic

Interactions of the Pseudomonas stutzeri KC siderophore pyridine-2,6-bis(thiocarboxylic acid) (pdtc) with chromium(VI), mercury(II), cadmium(II), lead(II), and arsenic(III) are described. Pdtc was found to reduce Cr(VI) to Cr(III) in both bacterial cultures and in abiotic reactions with chemically synthesized pdtc. Cr(III) subsequently formed complexes with pdtc and pdtc hydrolysis products, and their presence was confirmed using electrospray ionization-mass spectrometry (ESI-MS). Cr(III):pdtc complexes were found to slowly release Cr(III) as chromium sulfide and possibly Cr(III) oxides. Pdtc also formed poorly soluble complexes with Hg, Cd, Pb, and As(III). Hydrolysis of those complexes led to the formation of their respective metal sulfides as confirmed by energy dispersive X-ray spectroscopy (EDS) elemental analysis. The pdtc-producing strain P. stutzeri KC showed higher tolerance to most of these metals as compared to a pdtc-negative mutant. A novel role of pdtc is postulated as its involvement in providing an extracellular pool of thiols that are used for redox processes in detoxification of the bacterial extracellular environment. These redox processes can be mediated by transition metal:pdtc complexes.     

Chemical properties of water-soluble porphyrins. 4. The reaction of a 'picket-fence-like' iron (III) complex with the superoxide oxygen couple

Solution properties of the iron-(III) 'picket-fence-like' porphyrin, Fe(III)-alpha,alpha,alpha, beta-tetra-ortho (N-methyl-isonicotinamidophenyl) porphyrin, (Fe(III)PFP) were investigated. These were acid/base properties of the aquo complex with pKa of 3.9 and its aggregation (formation of dimer with K = 1 X 10(-10) dm3 mol-1), complex formation with cyanide ions and 1-methyl imidazole (1-MeIm), spectral properties of the three iron complexes in their ferric and ferrous form and the one-electron reduction potential of these complexes. Knowing these properties, the reaction of the ferric complexes, aquo, dicyano and bis (1-MeIm), with the superoxide radical and other reducing radicals were studied using the pulse radiolysis technique. The second-order reaction rate constant of O2- with the iron (III) aquo complex which governs the catalytic efficiency of the metalloporphyrin upon the disproportionation of the superoxide radical was 7.6 X 10(7) dm3 mol-1 s-1, two orders of magnitude faster when compared to the reaction of each of the other complexes. The reduction by other radicals with all iron (III) complexes had similar second-order rate constants (10(9) to 10(10) dm3 mol-1 s-1). The reduction reaction in all cases produced Fe(II)PEP and no intermediate was found. The oxidation reaction of Fe(II)PEP by O2- was one order of magnitude faster when compared to the reduction of Fe(III)PFP by the same radical. Since the reactivity of O2- toward the three iron (III) porphyrin complexes follows their reduction potentials, it is suggesting the formation of a peroxo Fe(II) porphyrin as an intermediate. The reactions of the Fe(II)PFP complexes with dioxygen were also studied. The aquo complex was found to be first order in O2 and second order in Fe(II)PFP, suggesting the formation of a peroxo Fe(II) porphyrin as an intermediate. The intermediate formation was corroborated by evidence of the rapid CO binding reaction to the aquo complex of Fe(II)PFP. The two other complexes reacted very slowly with O2 as well as with CO.     

Tannic acid inhibits in vitro iron-dependent free radical formation

The antioxidant activity of tannic acid (TA), a plant polyphenol claimed to possess antimutagenic and anticarcinogenic activities, was studied by monitoring (i) 2-deoxyribose degradation (a technique for OH detection), (ii) ascorbate oxidation, (iii) ascorbate radical formation (determined by EPR analysis) and (iv) oxygen uptake induced by the system, which comprised Fe(III) complexes (EDTA, nitrilotriacetic acid (NTA) or citrate as co-chelators), ascorbate and oxygen. TA removes Fe(III) from the co-chelators (in the case of EDTA, this removal is slower than with NTA or citrate), forming an iron-TA complex less capable of oxidizing ascorbate into ascorbate radical or mediating 2-deoxyribose degradation. The effectiveness of TA against 2-deoxyribose degradation, ascorbate oxidation and ascorbate radical formation was substantially higher in the presence of iron-NTA (or iron-citrate) than with iron-EDTA, which is consistent with the known formation constants of the iron complexes with the co-chelators. Oxygen uptake and 2-deoxyribose degradation induced by Fe(II) autoxidation were also inhibited by TA. These results indicate that TA inhibits OH formation induced by Fe(III)/ascorbate/O(2) mainly by arresting Fe(III)-induced ascorbate oxidation and Fe(II) autoxidation (which generates Fe(II) and H(2)O(2), respectively), thus limiting the production of Fenton reagents and OH formation. We also hypothesize that the Fe(II) complex with TA exhibits an OH trapping activity, which explains the effect of TA on the Fenton reaction.     

Organic Constituents and Complexation of Nickel(II), Iron(III), Cadmium(II), and plutonium(IV) in Soybean Xylem Exudates

The xylem exudates of soybean (Glycine max cv Williams), provided with fixed N, were characterized as to their organic constituents and in vivo and in vitro complexation of plutonium, iron, cadmium, and nickel. Ion exchange fractionation of whole exudates into their compound classes (organic acid, neutral, amino acid, and polyphosphate), followed by thinlayer electrophoresis, permitted evaluation of the types of ligands which stabilize each element. The polyvalent elements plutonium(IV) and iron(III) are found primarily as organic acid complexes, while the divalent elements nickel(II) and cadmium(II) are associated primarily with components of the amino acid/peptide fraction. For plutonium and cadmium, it was not possible to fully duplicate complexes formed in vivo by back reaction with whole exudates or individual class fractions, indicating the possible importance of plant induction processes, reaction kinetics, and/or the formation of mixed ligand complexes. The number and distribution of specific iron- and nickel-containing complexes varies with plant age and appears to be related to the relative concentration of organic acids and amino acids/peptides being produced and transported in the xylem as the plant matures.     

Separation of the poly(glycerophosphate) lipoteichoic acids of Enterococcus faecalis Kiel 27738, Enterococcus hirae ATCC 9790 and Leuconostoc mesenteroides DSM 20343 into molecular species by affinity chromatography on concanavalin A

This study shows for the first time microheterogeneity of 1,3-linked poly(glycerophosphate) lipoteichoic acids. The lipoteichoic acids investigated were those of Enterococcus faecalis Kiel 27738 (I), Enterococcus hirae (Streptococcus faecium) ATCC 9790 (II), and Leuconostoc mesenteroides DMS 20343 (III). Lipoteichoic acids II and III are partially substituted by mono-, di-, tri-, and tetra-alpha-D-glucopyranosyl residues with (1----2) interglycosidic linkages. Lipoteichoic acid I is substituted with alpha-kojibiosyl residues only. Lipoteichoic acids I and III additionally carry D-alanine ester. Lipoteichoic acids were separated on columns of concanavalin-A-Sepharose according to their increasing number of glycosyl substituents per chain. It was evident that all molecular species are usually glycosylated and that alanine ester and glycosyl residues occur on the same chains. The chain lengths of lipoteichoic acid I and II vary between 9-40 glycerophosphate residues, whereas those of lipoteichoic acid III appear to be uniform (33 +/- 2 residues). Molecular species differ in the extent of glycosylation but their content of alanyl residues is fairly constant. All lipoteichoic acids contain a small fraction (5-15%) different in composition from the bulk and most likely reflecting an early stage of biosynthesis. Two procedures for chain length determination of poly(glycerophosphate) lipoteichoic acids are described.