Showing papers on "Stability constants of complexes published in 1986"

Uric acid-iron ion complexes. A new aspect of the antioxidant functions of uric acid.

Abstract: In order to survive in an oxygen environment, aerobic organisms have developed numerous mechanisms to protect against oxygen radicals and singlet oxygen. One such mechanism, which appears to have attained particular significance during primate evolution, is the direct scavenging of oxygen radicals, singlet oxygen, oxo-haem oxidants and hydroperoxyl radicals by uric acid. In the present paper we demonstrate that another important 'antioxidant' property of uric acid is the ability to form stable co-ordination complexes with iron ions. Formation of urate-Fe3+ complexes dramatically inhibits Fe3+-catalysed ascorbate oxidation, as well as lipid peroxidation in liposomes and rat liver microsomal fraction. In contrast with antioxidant scavenger reactions, the inhibition of ascorbate oxidation and lipid peroxidation provided by urate's ability to bind iron ions does not involve urate oxidation. Association constants (Ka) for urate-iron ion complexes were determined by fluorescence-quenching techniques. The Ka for a 1:1 urate-Fe3+ complex was found to be 2.4 X 10(5), whereas the Ka for a 1:1 urate-Fe2+ complex was determined to be 1.9 X 10(4). Our experiments also revealed that urate can form a 2:1 complex with Fe3+ with an association constant for the second urate molecule (K'a) of approx. 4.5 X 10(5). From these data we estimate an overall stability constant (Ks approximately equal to Ka X K'a) for urate-Fe3+ complexes of approx. 1.1 X 10(11). Polarographic measurements revealed that (upon binding) urate decreases the reduction potential for the Fe2+/Fe3+ half-reaction from -0.77 V to -0.67 V. Thus urate slightly diminishes the oxidizing potential of Fe3+. The present results provide a mechanistic explanation for our previous report that urate protects ascorbate from oxidation in human blood. The almost saturating concentration of urate normally found in human plasma (up to 0.6 mM) represents 5-10 times the plasma ascorbate concentration, and is orders of magnitude higher than the 'free' iron ion concentration. These considerations point to the physiological significance of our findings.

Coordination of lanthanides by two polyamino polycarboxylic macrocycles: formation of highly stable lanthanide complexes

Abstract: The formation constants of a few lanthanide complexes with DOTA (1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid) and TETA (1,4,8,11-tetraazacyclotetradecane-N,N',N'',N'''-tetraacetic acid) have been measured by potentiometric and competition methods. The ligand DOTA forms the most stable lanthanide chelates known so far (log K/sub ML/ = 28.2-29.2) while the stability of the TETA compounds at 80 /sup 0/C (log K/sub ML/ = 14.5-16.5) is comparable to the stability of the EDTA complexes. A competition method with the oxalate anion as a probe had to be used for determining the formation constants of the DOTA lanthanide chelates because of the high stability of these compounds. The relative stability of the DOTA and TETA complexes in accounted for by steric factors with reference to known solution- and solid-state structures. 20 references, 2 tables.

Anion Coreceptor Molecules. Linear Molecular Recognition in the Selective Binding of Dicarboxylate Substrates by Ditopic Polyammonium Macrocycles

Abstract: Three macrocyclic hexaamines 1, 2, and 4, and the acyclic tetraamine 5 and hexaamine 6 have been synthesized. The hexaamines 1, 2, and 4 are ditopic coreceptor molecules containing two triamine subunits which may bind anionic substrates when protonated. The stability constants of the complexes between the protonated forms of the macrocyclic polyamines and terminal dicarboxylates −O2C−(CH2)m- CO2− as well as amino-acid and dipeptide dicarboxylates have been determined by pH-metric measurements. Around neutral pH, 1 and 2 give mainly complexes of the fully protonated species 1·6H+ and 2·6H+, whereas 4 yields predominantly complexes of 4·5H+ and 4·4H+. The stability sequences of the complexes formed indicate preferential binding of the dianionic substrates whose length is compatible with the separation of the triammonium binding subunits in the protonated receptor molecules 1, 2, and 4. This selectivity pattern corresponds to a process of linear molecular recognition based on ditopic binding between the two ammonium subunits of the coreceptor and the terminal carboxylates of the substrate of complementary length. The complexes of the acyclic ligands 5 and 6 are much weaker and much less selective, indicating a marked macrocyclic effect on both stability and selectivity of binding, i.e. on recognition.

Formation, structures, and reactivity of cis-hydroxy-, cis-methoxy-, and cis-mercaptoiridium hydrides. Oxidative addition of water to Ir(I)

Abstract: Although mononuclear hydrido hydroxy complexes may play an important role in water activation, including the water gas shift reaction, olefin and nitrile hydration, exchange reactions, and photodissociation of water, only few of them are known. The early-transition-metal complexes of this type tend to dimerize, whereas the late-transition-metal complexes tend to be unstable in the absence of excess water, probably because of an unfavorable formation constant. None of these complexes has been crystallographically characterized. They report here the isolation, properties, and crystallographic characterization, revealing some unusual features, of a stable, mononuclear hydrido hydroxy complex formed by facile water oxidative addition to Ir(I). A rare example of a mononuclear hydrido methoxy complex derived from it is also described. For comparison a hydrido mercapto complex is also presented.

Studies of copper(I) olefin complexes. Formation constants of copper olefin complexes with 2,2′-bipyridine, 1,10-phenanthroline, and their derivatives

Abstract: Preparation de 25 nouveaux complexes [Cu(biL)(olefine)] + (biL=derive de bipy et phenanthroline; olefine=ethylene et derives). Constantes de stabilite

Metal anthracycline complexes as a new class of anthracycline derivatives. Pd(II)-adriamycin and Pd(II)-daunorubicin complexes: physicochemical characteristics and antitumor activity.

Abstract: Pd(II) complexes of two anthracyclines, adriamycin and daunorubicin, have been studied. Using potentiometric absorption, fluorescence, and circular dichroism measurements, we have shown that adriamycin can form two complexes with Pd(II). The first complex (I) involves two molecules of drug per Pd(II) ion; one of the molecules is chelated to Pd(II) through the carbonyl oxygen on C12 and the phenolate oxygen on C11, and the other one is bound to Pd(II) through the nitrogen of the amino sugar. This complexation induces a stacking of the two molecules of drug. In the second complex (II), two Pd(II) ions are bound to two molecules of drug (A1 and A2). One Pd(II) is bound to the oxygen on the carbons C11 and C12 of molecule A1 and the amino sugar of molecule A2 whereas the second Pd(II) ion is bound to the oxygen on C11 and C12 of molecule A2 and the amino sugar of molecule A1. The same complexes are formed between Pd(II) and daunorubicin. The stability constant for complex II is beta = (1.3 +/- 0.5) X 10(22). Interaction with DNA has been studied, showing that almost no modification of the complex occurred. This complex displays antitumor activity against P-388 leukemia that compares with that of the free drug. Complex II, unlike adriamycin, does not catalyze the flow of electrons from NADH to molecular oxygen through NADH dehydrogenase.

Aluminum(III)-Citrate Complexes: a Potentiometric and 13C N.M.R.-Study

Abstract: The equilibrium reactions of citrate ion with protons and aluminium(III) have been studied by potentiometric titration in aqueous solution, I 0.10 M KCl, 25°C. The protonation constants for citrate, log Ki, corrected for K+-citrate ion pairing, were determined as 5.90±0.02, 4.35±0.01 and 2.91±0.02, i = 1-3 respectively. The model which gave the best fit to experimental data for the aluminium-citrate equilibrium system (TL/TM > 5.3) included AlHL + ( logβ 11.02±0.02), AlL (8.35±0.23), AlL23- (13.40±0.35), Al(HL)L2- (17.36±0.11), AlL2H-14-[ pK (AlL23-)6.07±0.24] and Al(LH-1)25- [ pK (AlL2H-14-) 7.09±0.08]. 13C n.m.r.data established that ligand exchange in the aluminium(III) complexes is slow on the n.m.r . time scale. The spectra were consistent with deprotonation of the coordinated C(2)-OH group commencing between pH 5 and 6, corresponding to formation of the species designated AlL2H-14- and Al(LH-1)25-. The broad structure of the- CH2-, -COOH (central) and -COOH (terminal) resonances for coordinated citrate was indicative of coordination isomers and/or the presence of both mono and bis complexes.

Complexes of the histamine H2-antagonist cimetidine with divalent and monovalent copper ions

Abstract: La cimetidine forme des complexes 1:1 et 1:2 avec l'ion Cu(II) en solutions aqueuses Constantes de stabilite Role important de Cu dans les activites pharmacologiques de la cimetidine

Copper(II) complexes of diastereoisomeric dipeptides in aqueous solution. Effect of side-chain groups on the thermodynamic stereoselectivity

Abstract: Preuve de la stereoselectivite thermodynamique dans la formation des complexes deprotones avec formation favorisee thermodynamique, des complexes des diastereoisomeres LL

Raman spectroscopic investigations on the chromium(VI) equilibria part 2—species present, influence of ionic strength and CrO42− ? Cr2O72− equilibrium constant

Abstract: In addition to CrO42− and Cr2O72−, species such as H2CrO4, HCrO4− and HCr2O72− have been considered to exist in Cr(VI) equilibria in aqueous solution. The Raman spectra of dilute solutions of Cr(VI) at different pH have confirmed the non-existence of the last three species in a Cr(VI) concentration range between 0.1 M and the lower detection limit of concentration (ca 0.003 M). The proposed equilibrium, 2CrO42− +2H+⇌Cr2O72− +H2O, seems to be the only one existing between pH 1 and 11. A quantitative Raman study of the influence of ionic strength on the corresponding apparent formation constant β′22 led us to the determination of the equilibrium constant at zero ionic strength, namely log β22=14.85±0.04 at 25°C. In more acidic aqueous Cr(VI) solutions, up to 12M in HNO3, preliminary investigations corroborated the presence of the trichromate and tetrachromate species.

Thermodynamics of formation of inclusion compounds in water. α-Cyclodextrin–alcohol adducts at 298.15 K

Abstract: The interaction in water of ethanol, n-propanol, n-butanol, isobutanol, s-butanol and t-butanol with α-cyclodextrin (hexacycloamylose, αCD) has been studied calorimetrically at 298.15 K. The results indicate that, with the exception of the t-butanol, these alcohols form inclusion complexes with this dextrin. The calorimetric method employed allows the determination of the thermodynamic parameters characterizing the binding process, namely the enthalpy, ΔHB°, the apparent equilibrium constant,K′B, and then the free energy, ΔGB°′, and entropy, TΔSB°′. The complexes that result are weak, as shown by the low values of the constants. The absolute values of ΔHB° and K′B increase with increasing length of the alkyl chain. Branching of the alkyl chain lowers the value of K′B. Evidence is given to show that t-butanol does not form a complex with αCD, probably because of steric hindrance: for this system the calorimetric data are treated according to an approach derived from the McMillan–Mayer treatment of solutions.

Small macrocyclic ligands with mixed nitrogen- and oxygen-donor atoms

Abstract: Stabilite des complexes de Cu, Ni, Zn, Cd, Pb, Ca et Hg divalents des macrocycles oxygenes et/ou azotes. Etude de l'effet macrocyclique

INTERACTION OF β-CYCLODEXTRIN WITH BILE SALTS IN AQUEOUS SOLUTIONS

Abstract: β-Cyclodextrin (β-CD) forms inclusion complexes with bile salts (sodium cholate, sodium deoxycholate, sodium glycocholate and sodium taurocholate) in aqueous solutions. In the presence of bile salts, the guest molecules of β-CD complexes are excluded from the cavity of β-CD and the free molecules increase with the concentration of bile salt up to cmc. Above cmc they are partitioned between the aqueous and micellar phases. Below cmc the exchange reaction proceeds depending on the formation constants of the guest molecule with β-CD and the concentration of bile salt. Above cmc, the free molecules in aqueous phase decrease with increasing concentration of bile salt because of the partitioning to the micellar phase. These results may be related to the absorption of β-CD complexes administered orally and also the metabolism of cholesterol when the complexes are administered orally for a long period of time.

Potential lanthanide ion selective reagents. 3. Metal complex formation with 1,7-diaza-4,10,13-trioxacyclopentadecane-N,N'-diacetic acid

Abstract: Stability constants for the ligand 1,7-diaza-4,10,13-trioxacyclopentadecane-N,N'-diacetic acid (dapda or K21DA) with the lanthanides and several other metal ions have been determined at 25 /sup 0/C in aqueous 0.1 M (CH/sub 3/)/sub 4/NCl medium by a potentiometric method. The results obtained are compared to those obtained for a similar ligand of large cavity size, 1,20-diaza-4,7,13,16-tetraoxacyclooctadecane-N,N'-diacetic acid (dacda or K22DA), which has been previously studied and reported. The stability of dapda is found to reach a peak at Eu(III) with the lanthanide series and is rationalized in terms of the matching of the ligand properties with metal ion characteristics. The transition-metal ions Ni(II), Cu(II), and Zn(II) all form stronger dapda (as compared to dacda) complexes due to a better match of the ligand cavity size and metal ion radius. 18 references, 3 figures, 1 table.

Competitive NMR study of the complexation of several cations with 18-crown-6, 1,10-diaza-18-crown-6, and cryptand-2,2,2 in nonaqueous solutions

Abstract: Determination des constantes de stabilite des complexes des metaux alcalins, Tl + et Ba 2+ dans acetonitrile, acetone et carbonate de propylene

Binding of carbon dioxide to cobalt and nickel tetra-aza macrocycles

Abstract: Cyclic voltammetry of cobalt and nickel complexes of the macrocycle 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetra-azacyclotetradeca-4,11-diene in 0.10M tetraethylammonium perchlorate in dimethyl sulphoxide in the presence of carbon dioxide shows a strong positive shift of the CoII/I reduction potential, leading to an equilibrium constant of 7 × 104 for CO2 binding, whereas the nickel complex shows a very low affinity for CO2 binding in either the NiII or NiI oxidation states.

Metal carbonyl clusters: thermodynamics of their formation and stability. Some novel results on cobalt, rhodium, ruthenium, and mixed clusters

Abstract: A series of reversible cluster formation and degradation reactions with neutral metal carbonyls and carbonyl hydrides was studied by high-pressure IR-spectroscopy, under CO and/or H2 pressure, in alkane solutions. Fundamental reversible reactions were analyzed for cobalt, rhodium, ruthenium, and mixed Co-Fe, Co-Ru and Co-Rh carbonyl systems. Log K vs l/T equations, their graphical illustrations and thermodynamic parameters are given for the homometallic Co, Rh and Ru systems. In the combined system of Co and Ru carbonyls and hydridocarbonyls proper conditions were found where mixed hydridocarbonyl clusters are obtained in stable equilibria with the homonuclear compounds. Of the COx(HRu)4_x(CO)12 species the one with x=3 is strongly favored thermodynamically. In the mixed Co-Fe system no thermodynamically stable compound could be found: HCo3Fe(CO)1 decomposes irreversibly, under CO as well as H2 or N, to homometallic species. Of particular interest is the coordinatively unsaturated mixed species CoRh(CO)7, which represents a key link between the homoand hetero-metallic equilibria of these metals. The connection of our data with thermochemical results from other laboratories, and their application to bond enthapy calculations is presented. INTRODUCTION The chemistry of transition metal carbonyl clusters has attracted considerable interest since the mid-l960's. There is a growing amount of scientific knowledge on the formation and degradation reactions of the cluster carbonyls, however, mainly in terms of preparative routes. In contrast to the dramatic increase of efforts on this field, only a few studies are known which treat in a quantitative way the reversible reactions of type (1) or (2) with the aim of obtaining thermodynamical data for them. Mx(CO)y + a Co n Mx/n(CO){a+y}/n (1) Mx(CO)y + b H2 fl H2b,flMx/fl(CO){y.w}/fl + W CO (2) Besides the p se importance of the knowledge of the reaction enthalpies and entropies for such processes the direct determination of these thermodynamic parameters via measurements of the temperature variation of the equilibrium constants, represents a complementary source of thermochemical data, which adds to the direct calorimetric results. I wish to emphasize two important advantages of our method: (1) the uncertainties of the equilibrium measurements are generally small since the reaction enthalpies themselves are much smaller than the enthalpies of formation of the carbonyls. These relationships are illustrated for the system of the cobalt carbonyls in Fig. 1.

Hexacyclen complexes of sulfate anion

Abstract: Etude des proprietes de complexation du dianion sulfate; donnees thermodynamiques; determination des stœchiometries et des constantes de formation a l'aide de titrages potentiometriques et conductimetriques