On the use of the method of continuous variations

Gil, Victor M. S.; Oliveira, Nuno M.C.

doi:10.1021/ed067p473

Cited by 241 publications

(167 citation statements)

References 11 publications

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“…3. The maximum of the curve at x ϭ 0.5 for 2 clearly illustrates the formation of a 1:1 complex (35,36). A similar result was also obtained for the sulfate complex.…”

Section: Resultssupporting

confidence: 74%

Conformation and anion binding properties of cyclic hexapeptides containing l -4-hydroxyproline and 6-aminopicolinic acid subunits

Kubik

Goddard

2002

Proc. Natl. Acad. Sci. U.S.A.

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Two cyclic hexapeptides containing alternating all R and all S configured L-(4R͞S)-hydroxyproline and 6-aminopicolinic acid subunits are presented, and the influence of the hydroxyl groups on the solubility, conformation, and receptor properties is investigated. Cyclopeptide 2, containing the natural 4R configured hydroxyproline, adopts a conformation similar to that of the unsubstituted peptide 1, which is able to bind anions such as halides and sulfate in aqueous solution. 2 also interacts with these anions, but whereas 1 forms sandwich type 2:1 complexes, in which the anion is bound by two cyclopeptide moieties, 2 forms 1:1 complexes. The stabilities of the halide and sulfate complexes of 2 range between 10 0 and 10 2 M ؊1 in 80% D2O͞CD3OD. Complex formation is detectable even in water, but with slightly smaller stability constants. Using this information a quantitative evaluation of the stability of the 2:1 complexes of 1, for which overall stability constants in the order 10 4 to 10 5 M ؊2 in 80% D2O͞CD3OD were observed, was made. In contrast to 2, the conformation of 3, containing the non-natural 4S configured hydroxyproline, is strongly affected by the presence of the hydroxyl groups. In d 6-DMSO and methanol͞water mixtures a slow conformational equilibrium between two C3-symmetrical conformers is observed, and 3 is thus much less preorganized for anion binding than either 1 or 2.T he crystal structures of anion binding proteins with the substrate resting in the active site have provided valuable information on the principles nature uses for the recognition of negatively charged compounds (1). Important binding interactions are, for example, multiple hydrogen bonds formed between the substrate and NH groups of the protein backbone or hydrogen bond donors in amino acid side chains (2, 3). A way of mimicking these interactions in an artificial system is to arrange amide or urea NH groups around the cavity of a suitable host molecule. This strategy has led to the development of many artificial hosts for anions, some of which possess remarkable substrate affinity, even in highly competitive solvents such as DMSO or acetonitrile (4-12). The ultimate goal has, of course, been the design of receptors that bind anions by hydrogen bonds in water. However, despite the recent advances in the development of anion receptors (4-7), most systems described so far require strong electrostatic or coordinative interactions to be effective in this solvent. One of the few exceptions is the cyclic hexapeptide 1 that has recently been described by us (13). This neutral macrocyclic receptor interacts with anions such as halides or sulfate even in water͞methanol mixtures, a behavior that can largely be attributed to the special geometry of the complexes formed. We have found that the presence of a suitable anion in solution induces an aggregation of two cyclopeptide molecules, whereby a cavity is formed into which the anion is included (Fig. 1). In this complex, the anion is shielded from the surrounding solvent and can thus inte...

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“…3. The maximum of the curve at x ϭ 0.5 for 2 clearly illustrates the formation of a 1:1 complex (35,36). A similar result was also obtained for the sulfate complex.…”

Section: Resultssupporting

confidence: 74%

Conformation and anion binding properties of cyclic hexapeptides containing l -4-hydroxyproline and 6-aminopicolinic acid subunits

Kubik

Goddard

2002

Proc. Natl. Acad. Sci. U.S.A.

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“…The stoichiometry of the inclusion complex was provided by NMR titrations using the continuous variation technique (Jobs method -see Supporting Information). [22] The Jobs plot shows a maximum at r = 0.5 and a highly symmetrical shape, indicating that the stoichiometry of inclusion complex is 1:1. The association constant was evaluated at 298 K from UV-Vis spectroscopic data (spectral displacement method -see Supporting Information) and was found to be equal to 2750 M…”

Section: Synthesis Of the Water-soluble Phosphanesmentioning

confidence: 94%

Biphasic Aqueous Organometallic Catalysis Promoted by Cyclodextrins: How to Design the Water‐Soluble Phenylphosphane to Avoid Interaction with Cyclodextrin

et al. 2008

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The ability of cyclodextrin to interact with meta-trisulfonated triphenylphosphane derivatives bearing one or two methyl (or methoxy) groups on the aromatic ring has been investigated by NMR and UV-vis spectroscopy. In the case of native b-cyclodextrin (b-CD), the presence of one methyl or methoxy group in the ortho-position on each aromatic ring is necessary to hamper the formation of an inclusion complex between the b-CD and meta-trisulfonated triphenylphosphane derivatives. In the case of methylated b-CD, the formation of an inclusion complex is only observed when the meta-trisulfonated triphenylphosphane contains a methyl group in the para-position. The poor affinity of methylated b-CD towards modified trisulfonated triphenylphosphanes was attributed to the steric hindrance generated by the methyl groups on the CD secondary face. The absence or presence of an interaction between phosphanes and methylated b-CD was also confirmed by catalytic experiments. Thus, the phosphanes that do not interact with the methylated CD were the most efficient mass-transfer promoters in an aqueous biphasic palladium-catalyzed Tsuji-Trost reaction.

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“…It is more reasonable to expect 34 that other complex species are also present in the solution, along with the principle 2:2:2 complex, when the complexation interaction is weak. In this case, the continuous variation curve loses its symmetric character (see curve 3a, Figure 4); and this inevitably will reflect on the reliability of the results 14,52,53 obtained by the Likussar-Boltz method. Table 4 shows that the influence of side reactions on the complex formation with INT can be neglected when the extraction is performed at the optimum conditions.…”

Section: Constants Of Extraction and Other Characteristics Concerningmentioning

confidence: 99%

Liquid-Liquid Extraction-Spectrophotometric Investigations of Three Ternary Complexes of Vanadium(V)

Gavazov¹,

Stefanova²

2014

Croat. Chem. Acta

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Abstract. Complex formation and liquid-liquid extraction (LLE) were studied in systems containing vanadium(V), 5-methyl-4-(2-thiazolylazo)resorcinol (TAO), tetrazolium salt (TZS), water and chloroform. The following three TZSs were used: 3-(4,5-dimethyl-2-thiazol)-2,5-diphenyl-2H-tetrazolium bromide (MTT), 3-(2-naphtyl)-2,5-diphenyl-2H-tetrazolium chloride (Tetrazolium violet, TV), and 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyl-2H-tetrazolium chloride (INT). Concentration of the reagents (TAO and TZS), pH of the aqueous medium, and shaking time were subjects of optimization experiments. Formation of ternary complexes with a composition of 2:2:2 was demonstrated by a set of different methods. Some key characteristics concerning the analytical application of the studied LLE-chromogenic systems were established as well.

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On the use of the method of continuous variations

Abstract: In this paper, the authors recall and systemize the restrictions to the application of Job's method, illustrate its use associated to proton magnetic resonance spectral intensities, and present some results of a computer program they developed for the case of multiple equilibria.

Cited by 241 publications

References 11 publications

Conformation and anion binding properties of cyclic hexapeptides containing l -4-hydroxyproline and 6-aminopicolinic acid subunits

Conformation and anion binding properties of cyclic hexapeptides containing l -4-hydroxyproline and 6-aminopicolinic acid subunits

Biphasic Aqueous Organometallic Catalysis Promoted by Cyclodextrins: How to Design the Water‐Soluble Phenylphosphane to Avoid Interaction with Cyclodextrin

Liquid-Liquid Extraction-Spectrophotometric Investigations of Three Ternary Complexes of Vanadium(V)

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