The interaction between vitamin B-12 and micelles in aqueous solution

The apparent pK for benzimidazole displacement of a number of cobalamins is markedly affected by the presence of sodium lauryl sulfate micelles. However, micelles of cetyltrimethylammonium bromide or Triton X have little or no effect on the pK. By measuring the apparent pK as a function of sodium lauryl sulfate concentration, the association constants between the micelles and both base on and base off methylcobalamin were calculated. This calculation indicates that the base off form is strongly associated with the micelle while the base on form is not.     

Metal--glutathione interaction in aqueous solution. Nickel(II), cobalt(II) and copper(II) complexes with oxidized glutathione.

The interaction of copper(II), nickel(II) and cobalt(II) ions with oxidized glutathione in aqueous solutions have been examined by spectroscopic methods. Cu(II) is the only ion which interacts with disulphide bridge and forms dimeric species containing the Cu(II)-S-S-Cu(II) unit. Ni(II) and Co(II) bind mainly with the terminal NH2 and COO- groups of glutamic acid, and the complexes formed are of nearly octahedral symmetry. At high pH, in the Co(II)-GSSG solution Co(II) is oxidized to Co(III) with the concomitant reduction of GSSG to GSH. Considerable differences were observed between the oxidized and reduced form of glutathione in the coordination ability towards metal ions.     

The interaction between vitamin B-12 and micelles in aqueous solution.

The apparent pK for benzimidazole displacement of a number of cobalamins is markedly affected by the presence of sodium lauryl sulfate micelles. However, micelles of cetyltrimethylammonium bromide or Triton X have little or no effect on the pK. By measuring the apparent pK as a function of sodium lauryl sulfate concentration, the association constants between the micelles and both base on and base off methylcobalamin were calculated. This calculation indicates that the base off form is strongly associated with the micelle while the base on form is not.     

Coordination chemistry of vitamin C. Part II. Interaction of L-ascorbic acid with Zn(II), Cd(II), Hg(II), and Mn(II) ions in the solid state and in aqueous solution

The reaction of L-ascorbic acid with the zinc group and manganese ions has been investigated in aqueous solution at pH 6-7. The solid salts of the type M (L-ascorbate)2.2H2O, where M = Zn(II), Cd(II) and Mn(II) were isolated and characterized by 13C NMR and Fourier Transform infrared (FT-IR) spectroscopy. Spectroscopic evidence showed that in aqueous solution, the bindings of the Zn(II) and Mn(II) ions are through the ascorbate anion O-3 and O(2)-H groups (chelation), while the Cd(II) ion binding is via the O-3 atom only. In the solid state, the binding of these metal ions would be through two acid anions via O-3, O-2 of the first and O-1, O-3 of the second anion as well as to two H2O molecules, resulting in a six-coordinated metal ion. The Hg(II) ion interaction leads to the oxidation of the ascorbic acid in aqueous solution.     

Kinetic study of the reaction of vitamin C with vitamin E radicals (tocopheroxyls) in solution

New stable vitamin E radicals (7-tert-butyl-5-isopropyltocopheroxyl (4), 5,7-diisopropyltocopheroxyl (5), 7-tert-butyl-5-methyltocopheroxyl (6), and 5,7-diethyltocopheroxyl (7] with two bulky alkyl substituents at ortho positions (C-5 and C-7) have been prepared, and the reaction rates of vitamin C (ascorbic acid (1) and 6-O-stearyl ascorbic acid (2] with these tocopheroxyl radicals in benzene/ethanol/water (2:1:0.1, v/v) solution have been determined spectrophotometrically, using a stopped-flow technique. The second-order rate constants, k2, obtained vary in the order of 10(3), and decrease dramatically in the order 7 greater than 6 greater than 5 greater than 4, as the size of two ortho-alkyl groups in tocopheroxyl increases. The result suggests that the effect of steric hindrance on the reaction rate is considerable. These reaction rates were compared with those of vitamin C with alpha-tocopheroxyl reported by Packer et al. (Nature 278 (1979) 737-738) and Scarpa et al. (Biochim. Biophys. Acta 801 (1984) 215-219).     

Study of copper(II) ternary complexes with phosphocreatine and some polyamines in aqueous solution

Ternary systems of Cu(II) with phosphocreatine (PCr) and the polyamines (PAs), ethylenediamine (en), 1,3-diaminopropane (tn), putrescine (Put), spermidine (Spd), and spermine (Spm), were investigated in aqueous solution through potentiometry, ultraviolet-visible, EPR and Raman spectroscopy. The binary complex CuPCr was also studied by Raman spectroscopy, and the calculation of the minimum stabilization energy was done assuming this molecule in aqueous solution. The stability constants of the CuPCrPA ternary complexes were determined by potentiometry (T = 25 °C, I = 0.1 mol L^− 1, KNO₃). The stability order determined was CuPCrSpm > CuPCrSpd > CuPCren > CuPCrtn > CuPCrPut, the same order of the corresponding binary complexes of Cu(II) with these polyamines. The evaluation of intramolecular PA-PCr interactions in protonated and deprotonated species of ternary complexes was carried out using the equation Δlog K = log β_{CuPCrPAHq + p} − (log β_CuPAHq + log β_CuPCrHp). All of the CuPCrPA ternary complexes have a square planar structure and are bonded to PCr through the nitrogen atom of the guanidine group and the oxygen atom of the phosphate group, and to the PAs through two nitrogen atoms of the amine groups. The structure of the complex CuPCrSpm is planar with distortion towards tetrahedral. Calculation of the minimum stabilization energy for the CuPCr and CuPCrenH complexes confirmed the proposed coordination mode.     

Thermodynamic and spectroscopic study for the interaction of dimethyltin(IV) with L-cysteine in aqueous solution

Thermodynamic and spectroscopic properties of the species formed by dimethyltin(IV) cation with L-cysteine (cys) were studied by potentiometric, calorimetric, UV and NMR investigations in aqueous solution. The resulting speciation model showed the formation of five complex species: (CH(3))(2)Sn(cys)H(+), (CH(3))(2)Sn(cys)(0), (CH(3))(2)Sn(cys)OH(-), (CH(3))(2)Sn(cys)(2)H(-), (CH(3))(2)Sn(cys)(2)(2-). The stability and the formation percentages, for the mononuclear mixed species in particular, are very high, in a wide pH range. Thermodynamic parameters indicate that the enthalpy values are exothermic and the enthalpic contribution to the stability is higher than entropic one. Individual UV spectra of cys and dimethyltin(IV)-cys species were calculated. Spectroscopic results of UV and (1)H NMR investigations fully confirm the speciation model. The structures calculated from NMR investigations show that all the species have an eq-(CH(3))(2)-tbp structure.     

Specificity in the interaction between poly(L-glutamate) and iron (III) complex ions in aqueous solution

The binding process between sodium poly(L -glutamate) and trans-2,2′,2″,2?-tetrapyridyl-Fe(III) complex ions in aqueous solution at pH around 7 has been studied by means of equilibrium dialysis and optical measurements. The binding isotherm indicates the occurrence of a cooperative process, whereby bound molecules facilitate the association of additional molecules. According to circular dichroism (CD) data, this effect is coupled with that which sees a conformational change in the charged polypeptide upon progessive binding of complex counterions. All these features are discussed in the light of the structural characteristics of the interacting species. A stereochemical model of the association “complex” is proposed.     

Thermodynamics of the interaction of RbCl with some monosaccharides (D-glucose, D-galactose, D-xylose, and D-arabinose) in aqueous solutions at 298.15K

The Gibbs energy interaction parameters of RbCl with some monosaccharides (D-glucose, D-galactose, D-xylose, and D-arabinose) in water, g(ES), were obtained from electromotive force (emf) measurements of the electrochemical cell without liquid junction and containing two ion-selective electrodes (ISE): K-ISEmid R:RbCl(m(E))mid R:ISE-Cl and K-ISEmid R:RbCl(m(E)),saccharide (m(S))mid R:ISE-Cl, at 298.15K. The enthalpy interaction parameters of RbCl with these monosaccharides in water, h(ES), are determined according to the McMillan-Mayer theory from the measurements of the enthalpies of mixing of aqueous RbCl solutions with aqueous monosaccharide solutions, as well as the enthalpies of dilution of RbCl and monosaccharide solutions in pure water at 298.15K by a calorimetric method. Furthermore, the entropy interaction parameters, s(ES), can be evaluated through g(ES) and h(ES). The results suggest that the electrostatic interactions of these monosaccharides with RbCl in water are predominant compared with structural interactions, and these parameters are controlled primarily by the stereochemical structure of the monosaccharides in water.     

Interactions of D-cellobiose with p-toluenesulfonic acid in aqueous solution: a (13)C NMR study

The effects of adding D(2)SO(4), and p-toluenesulfonic acid-d to D-cellobiose dissolved in D(2)O were investigated at 23°C by plotting (13)C NMR chemical shift changes (Δδ) against the acid to D-cellobiose molar ratio. (13)C Chemical shifts of all 18 carbon signals from α and β anomers of D-cellobiose showed gradual decreases due to increasing acidity in aqueous D(2)SO(4) medium. The C-1 of the α anomer showed a slightly higher response to increasing D(+) concentration in the surrounding. In the aqueous p-toluenesulfonic acid-d medium, C-6' and C-4' carbons of both α, and β anomeric forms of D-cellobiose are significantly affected by increasing the sulfonic acid concentrations, and this may be due to a 1:1 interaction of p-toluenesulfonic acid-d with the C-6', C-4' region of the cellobiose molecule.     

Reaction of a model siderophore with Ni(II), Co(II) and Zn(II) in aqueous solution: kinetics and spectroscopy

A spectroscopic study was performed showing that the [Fe(III)(L(2-))(2)](1-) (L(2-)=dopacatecholate) complex reacts with Ni(II), Co(II) and Zn(II) in an aqueous solution containing S(2)O(3)(2-) resulting in the soluble [M(L(1-))(3)](1-) (L(1-)=dopasemiquinone; M=Ni(II), Co(II) or Zn(II) complex species. The Raman and IR spectra of the [CTA][M(L(1-))(3)] complexes, CTA=hexadecyltrimethylammonium cation, in the solid state were obtained. The kinetic constants for the metal substitution reactions were determined at four different temperatures, providing values for DeltaH(not equal), DeltaS(not equal) and DeltaG(not equal). The reactions were slow (k=10(-11) Ms(-1)) and endothermic. The system investigated can be considered as a simplified model to explain some aspects of siderophore chemistry.     

Kinetics of the formation and hydrolysis reactions of some thiomolybdate(VI) anions in aqueous solution

The kinetics of the formation of the thiomolybdate ions MoOS₃²⁻ and MoS₄²⁻ were determined spectroscopically from the addition of excess sulphide to MoO₂S₂²⁻ in pH buffered media (6–8) at 30 °C. The reverse (hydrolysis) reactions of MoO₂S₂²⁻ and MoOS₃²⁻ were measured under the same conditions. The reaction rates measured are shown below:

Values of the rate-constants (s⁻¹) obtained at pH 7.0 were k₁₀ 2.4 × 10⁻³, k₂₁ 1.5 × 10⁻⁵, k₃₀ 2.1 × 10⁻⁵, k₂₃ 6.0 × 10⁻⁴, and k₃₄ 1.9 × 10⁻⁵; where the results are comparable they are in good agreement with those obtained by earlier workers, although different conditions were used. However, in this work it was found that certain reactions had to be mathematically treated as two consecutively occurring reactions. There is also a difference in interpretation of the mechanism of the hydrolysis reactions of the tri- and tetrathio ions. In general the lability towards further S replacement of O atoms, and the reverse reaction, decreased with increased S substitution. All reaction rates increased with increasing H⁺ ion concentration, mostly this was a linear relationship over the limited pH range examined. The effect of the H⁺ ion is interpreted in terms of protonation of the oxythiomolybdate ions at an O atom leading to increased lability.     

Self-association of some anthocyanins in neutral aqueous solution

A good contrast in optical properties caused by self-association was found between malvin and cyanin quinonoidal bases. Circular dichroism measurements of the pigments in neutral aqueous solutions show that molecules of the malvin quinonoidal base self-associate quickly and the conformational orientation of the aggregates is opposite to those formed by cyanin quinonoidal bases. Hypsochromism and hypochromism in the visible absorption also occurred on the formation of malvin aggregates. CD comparisons of malvin, hirsutin and 4′-O-methylmalvin suggest that a methoxyl group in the B-ring of the anthocyanidin strongly suppresses self-association. It is proposed that the driving forces for self-association are mainly hydrophobic interactions among the aromatic nuclei stacked parallel to each other which are surrounded by the hydrophilic glucose moieties in a suitable spatial arrangement. Furthermore, the glucose moiety at the 5-position rather than that at the 3-position plays an important role in the self-association of these anthocyanidin 3,5-diglucosides. Addition of sodium chloride promotes self-association and the greater stability of the anthocyanins in solution.     

Mechanisms of nitric oxide release from nitrovasodilators in aqueous solution: reaction of the nitroprusside ion ([Fe(CN)5NO]2-) with L-ascorbic acid.

Nitric Oxide (NO) is a very diverse endogenous molecule and is responsible for a number of mammalian physiological activities. Sodium nitroprusside (SNP) is a long known hypotensive agent due to its ability to produce NO in situ. In this study, we have carried out a detailed investigation on the kinetics and mechanism of the decomposition of SNP by L-ascorbic acid to afford NO in aqueous medium. Spectrophotometric and electrochemical methods were employed in these studies. Experiments were performed under an argon atmosphere and reaction rates were strongly pH dependent and increases with pH. NO release increases up to about pH 7 after which it decreases at higher pHs. The two ionized forms of ascorbate present in aqueous solution reduce SNP in the order A2->HA->H2A (H2A=L-ascorbic acid) to release NO. The outer-sphere reduction of SNP by ascorbate involved three clear stages with NO being released in the last stage. Our kinetic results also show catalysis by group 1 alkali metal ions to increase down the group from Li+ to Cs+. A detail study presented here, strongly illustrate the possibility of the biological antioxidant, L-ascorbic acid, to play a vital role in the in situ metabolism of nitrosocompounds such as SNP to produce NO.     

Stabilities,solubility, and kinetics and mechanism for formation and hydrolysis of some palladium(II)and platinum(II) iodo complexes in aqueous solution

Complex formation between Pd(II), Pt(II) and iodide has been studied at 25 °C for an aqueous 1.00 M perchloric acid medium. Measurements of the solubility of PdI₂(s) in aqueous mercury(II) perchlorate and of AgI(s) and PdI₂(s) in aqueous solutions of Pd²⁺(aq) and Ag⁺(aq) gave the solubility product of PdI₂(s) as K_s^o=(7±3) × 10⁻³² M³, which is much smaller than previous literature values.The stability constants β₁=[MI(H₂O)₃⁺]/([M(H₂O)₄²⁺][I⁻]) for the two systems were obtained as the ratio between rate constants for the forward and reverse reactions of (i).

The following values of k₁ (s⁻¹ M⁻¹), k₋₁ (s⁻¹) and β₁ (M⁻¹) were obtained at 25 °C: (1.14±0.11) × 10⁶, (0.92±0.18), (12±4) × 10⁵ for MPd, and (7.7±0.4), (8.0±0.7) × 10⁻⁵, (9.6±1.3) × 10⁴ for MPt. Combination with previous literature data gives the following values of log(β₁ (M⁻¹)) to log(β₄ (M⁻⁴)): 6.08, ∼22, 25.8 and 28.3 for MPd, and 4.98, ∼25, ∼28, and ∼30 for MPt. The present results show that the large overall stability constants β₄ observed for the M²⁺I⁻ systems are most likely due to a very large stability of the second complex MI₂(H₂O)₂, which is probably a cis-isomer. A distinct plateau in the formation curve for mean ligand number 2 is obtained both for MPd and Pt. The other iodo complexes are not especially stable compared to those of chloride and bromide.ΔH^≠ (kJ mol⁻¹) and ΔS^≠ (JK⁻¹ mol⁻¹) for the forward reaction of (i), MPd, are (17.3±1.7) and (−71±5), and for the reverse reaction of (i) MPd, (45±3) and (−95±6), respectively. The kinetics are compatible with associative activation (I_a). The contribution from bond-breaking in the formation of the transition state seems to be less important for Pd than for Pt.     

The interaction of calcium ions with glycocholate micelles in aqueous solution.

The formation of soluble complexes of Ca2+ ions and glycocholate has been demonstrated. The dissociation constant is 26 nmol/litre and a maximum of 2 Ca2+ ions are bound to each glycocholate micelle. The formation of this complex is shown to be reversible. Binding is increased by the introduction of phosphatidylcholine into the micelle; it is decreased by a decrease in pH and by increased counter-ion concentration. The biological significance of these effects is discussed.