Lajos Barcza

Complexes of short-chain alcohols withβ-cyclodextrin

The complex formation of β-cyclodextrin with short chain aliphatic alcohols has been studied. The stability constants of 1∶1 complexes, determined spectrophotometrically, increase monotonically in the series of normal alcohols, while branching in the chain results in a further increase in stability. With smaller monoalcohols, complexes containing more than one guest molecule are formed as well, but the relative stability of these complexes decreases with increasing chain length and branching. The stabilities of 1∶1 complexes decrease with an increasing number of OH-groups, probably due to a stronger hydration. This reflects both the roles of space filling and hydrogen bridging

Mechanochemistry: a novel approach to the synthesis of fullerene compounds. Water soluble buckminsterfullerene - γ-cyclodextrin inclusion complexes via a solid-solid reaction

Abstract It has been shown that the solid-solid mechanochemical reaction between crystalline buckminsterfullerene and gamma cyclodextrin at room temperature yields an inclusion complex of more intact nature than the procedure previously suggested by Swedish researchers. Experimental evidence suggests that the reaction proceeds via the mechanochemical amorphization of the reactants and dissolution of C 60 in the amorphous γ-CD phase. Preliminary investigations show that mechanochemistry offers a novel approach to the synthesis of many fullerene compounds and derivatives.

Complex formation of phenolphthalein and some related compounds with β-cyclodextrin

The complex formation of phenolphthalein and β-cyclodextrin (cyclohepta-amylose) has been investigated by spectrophotometric, c.d., and potentiometric methods. It has been shown that complexes are formed with the protonated and dissociated forms of phenolphthalein as well; the dianion is complexed in alkaline solution in a colourless form. The complexes formed have the largest stability constants among cyclodextrin complexes known up to now. The unusual stabilities and colour change can be explained by the combined effect of the accessibility of the phenolic-phenolate moiety and presence of the carboxy–carboxylate group simultaneously, providing optimal space filling and the possibility for the cyclodextrin molecule to interact with three functional groups of phenolphthalein at the same time.

Determination of binding constants and the influence of methanol on the separation of drug enantiomers in cyclodextrin modified capillary electrophoresis

Abstract Binding constants of the optical isomers of phenylalkylamine derivatives and (2,6-di-O-methyl)-β-cyclodextrin were determined using electrophoretic mobility data gained from separations performed by capillary electrophoresis, and assuming a simple equilibrium model. Dependence of the optimum separation conditions and parameters on the binding constants were investigated and the influence of methanol was estimated.

Complex formation of inorganic salts withβ-cyclodextrin

The interactions between some alkali halides, perchlorates and sulfates andβ-cyclodextrin (cycloheptaamylose, CD) have been investigated by solubility, spectrophotometric, thermoanalytical and preparative methods. The main conclusions are as follows. (i) The most pronounced interaction occurs with the anions; in dilute solutions this can be characterized as the formation of 1 : 1 complexes. In more concentrated solutions ternary associates and more complicated ones (involving the cations as well) are also formed. (ii) Changes in the activity coefficients or of the activity of water may also have some role in the phenomena, but it cannot be dominant: differences according to the individual properties of the anions are much more pronounced than to those of the cations or to changes of the ionic strength. (iii) Regarding the type of complex formation, real inclusion and ‘outer sphere’ interactions (via hydrogen bonds) are also probable.

Complex formation of phenol, aniline, and their nitro derivatives with β-cyclodextrin

The interactions of phenol, aniline, and their nitro derivatives with β-cyclodextrin (cyclohepta-amylose) have been studied by a spectrophotometric method. The complex formation constants have been determined for both the conjugate acid and base forms. The para isomer of nitrophenol is the only case when a more stable complex is formed with the ionic species than with the undissociated one, supporting the concept that resonance charge delocalization and London dispersion interactions are important factors influencing the stability of the complexes.

Hydrogen bonded associates in the Bayer process (in concentrated aluminate lyes): the mechanism of gibbsite nucleation

The highly alkaline aluminate lye, which is produced in the first step of Bayer process, is characterized by low water activity and strong competition for water molecules. The consequence of these effects is that two (or more) negatively charged species can be brought together and stabilized in the form of anionic hydrogen bonded complexes. Depending on temperature, total concentration and aluminate to hydroxide ratio, aluminate–hydroxide as well as aluminate–aluminate associates can be formed. Among the oligomerized aluminates, the cyclic hexamer seems to play a key role, as it contains octahedrally coordinated aluminate ions and is able to form nuclei in further polymerization for the partial precipitation of Al(OH)3 . The interactions have been experimentally investigated using specially developed methods and a detailed computational analysis.

Colour change and tautomerism of some azo-indicators on complex formation with cyclodextrins

Complex formation of methyl orange and some other azobenzene derivatives with cyclodextrins has been studied in acidic and alkaline solutions by a spectrophotometric method. A correlation has been established between the stability constants of the complexes, the spectral changes accompanying complex formation and a tautomeric equilibrium between protonation on the azo and dimethylamino group. Our own results have been compared with some literature data, and equilibrium constants for the tautomeric rearrangement have been calculated. It has been demonstrated that the significant colour changes observed with the protonated forms only (and not with the corresponding bases) is really caused by a shift of the tautomeric equilibrium.

Cyclodextrin assisted nanophase determination of alkaloid salts

The poor water solubility of the free base and the high dissociation constant (K(a)) hinder mainly the assay of alkaloid salts. We have elaborated an environment friendly method that can be carried out in aqueous media. The stability difference of the cyclodextrin (CD) complexes of free and protonated bases were used for this purpose. The base is included into the hydrophobic cavity of the CD (which serves as an apolar solvent phase on molecular level) and its solubility in water is increased. Since the base forms more stable inclusion complex than its protonated species, the pK(a) is decreased and the potentiometric titration is promoted by this way, too. Six different hydrohalide alkaloid salts have been investigated and the most appropriate CDs were chosen (depending on the size of the molecules and/or substituents). The results of the assays agree well with those obtained by the direct nonaqueous titrations. The stability constants of the inclusion complexes have been also computed.

Hydroxypropyl-β-cyclodextrins: Correlation between the stability of their inclusion complexes with phenolphthalein and the degree of substitution

The stability constants of the inclusion complexes between 2-hydroxypropyl-β-cyclodextrin and phenolphthalein have been found to decrease with increasing degree of substitution; the hydroxypropylation of the primary rim has a relatively higher effect. The change of specific rotations seems to support this finding. The decrease can be explained by the disturbance of the three-site interaction characteristic for the molecular recognition of phenolphthalein by cyclodextrins.