Giuseppina Castronuovo

Thermodynamics of the interaction of cyclodextrins with aromatic and α, ω-amino acids in aqueous solutions: a calorimetric study at 25°C

Abstract The interaction in water of α- and β-cyclodextrins with l -phenylalanine, l -tyrosine, l -tryptophan, and l -histidine has been studied calorimetrically at 25°C in pure water and in a phosphate buffer (pH 11.3). The interaction in water of α-cyclodextrin with some α, ω-amino acids was also studied. When a complex forms, calorimetry allows the calculation of both the enthalpy and the association constant, from which the free energy and the entropy of the process can be obtained. Aromatic amino acids form 1:1 inclusion complexes, characterized by low values of the association constants. The association occurs through the insertion of the guests aromatic ring into the hosts cavity, and is stronger at pH 11.3 than in pure water. For α, ω-amino acids the association constants increase with increasing lengths of the alkyl chains between the functional groups. For this class of substances the association is supposed to occur through an interaction mechanism different from inclusion, which involves mainly the exterior of cyclodextrin. The analysis of the signs and values of the thermodynamic parameters obtained permits hypotheses to be made about the forces involved in the association process.

Excess enthalpies of aqueous solutions of mono- and oligo-saccharide at 25°

Abstract The heats of dilution in water of d -xylose, d -fructose, d -galactose, d -mannose, lactose, and raffinose have been determined calorimetrically at 25°. The calorimetric data, expressed in terms of excess enthalpy, lead to an evaluation of pair- and triplet-interaction coefficients. Osmotic data, where known, permit the analogous coefficients of the excess free energy to be obtained and thence those of the excess entropy. Analysis of the excess functions and comparison with spectroscopic properties permits some qualitative hypotheses to be formulated on the molecular interactions occurring in these solutions.

Chiral recognition between enantiomeric α-aminoacids. A calorimetric study at 25°C

Homotactic and heterotactic enthalpic pair interaction coefficients were determined at 25°C from the enthalpies of dilution of binary and ternary aqueous solutions containing the L and D forms of α-aminoacids bearing alkyl chains of increasing length (alanine, α-aminobutyric acid, valine, norvaline, leucine, isoleucine and norleucine). Significant differences were found between the values of the homotactic (hDD or hLL) and heterotactic (hDL) pairwise coefficients, for the enantiomers of leucine and its isomers. The role of the zwitterionic interactions is discussed to explain the nature of the chiral recognition.

The hydrophobic effect in aqueous solutions of nonelectrolytes. I. Self-interactions of alkylureas

In order to clarify some aspects of the hydrophobic interactions, the enthalpies of dilution of monoethylurea, 1,3-dimethylurea, and 1,3-diethylurea have been determined calorimetrically at 25°C. The calorimetric data, expressed in terms of excess enthalpy, permit the evaluation of the pair and triplet interaction coefficients. The analyses of these and of the analogous coefficientsgxx andgxxx, derived from osmotic data, indicate a driving force favorable to the interactions among the hydrated solute molecules. Nevertheless, the positive values of thehxx andhxxx coefficients seem to suggest that the source of the effect is a rearrangement of the water molecules rather than a direct association of the solute molecules. There are evidences of a strict correlation between the enthalpic and the entropic effects.

Interactions in aqueous solutions of urea-like compounds. Heats of mixing of urea, monomethylurea and thiourea at 298.15 K

The heats of mixing of aqueous solutions of monomethylurea, urea and thiourea were determined at 298.15 K and the excess enthalpies obtained were expressed in terms of pair- and triplet-interaction coefficients.The validity of the empirical rule: hij2≈hij hjj, relating the self- and cross-interaction coefficients, was tested for the solutes under examination, for which both hii, hjj < 0.

Excess enthalpies of aqueous solutions of polyols at 25

The excess enthalpies of aqueous solutions of nine polyols were determined at 25°C and reported in the virial form. The most interesting and new feature of this family of solutes is that the sign of the enthalpic pair interaction coefficients hxx is positive for the first members of the series and negative for the higher homologues. Other points are the large differences found among the values of hxx for stereoisomers, whereas pairs of enantiomers show the same values within experimental errors. An application of the group additivity method is also discussed.

Inclusion compounds in water: Calorimetric and spectroscopic studies of the interaction of cyclomaltohexaose (α-cyclodextrin) with alkanols at 25°

Abstract A study of the interaction in water of some alkanols and alkanediols with cyclomaltohexaose (α-cyclodextrin, αCD), by calorimetry and 1 H-n.m.r. spectroscopy at 25°, indicates that the alkyl chains are included in the cavity of αCD, that 1:1 complexes are formed, and that the formation constants and enthalpies are markedly dependent on the number and position of the hydroxyl groups in the chain. For α,ω-alkanediols, the interaction is more complex than a simple inclusion.

Hydrophobic interactions of alkanols. A calorimetric study in water at 298.15 K

Enthalpies of dilution in water of binary aqueous solutions of n-diols of increasing alkyl chain length are performed using a flow microcalorimeter. The pairwise enthalpic coefficients of these hydrophobic solutes depend mainly on the mutual positions of the —OH groups in the molecule. A rationalization of the excess enthalpies of positional isomers is attempted.

Chiral interaction in aqueous solutions of aminoacids. A calorimetric study of the protonated forms in H2O−HCl mixed solvents at 25°C

The enthalpies of dilution in H2O−HCl mixed solvents containing the protonated forms of α-aminoacids bearing alkyl chains of increasing length (glycine, alanine, aminobutyric acid, valine, norvaline, leucine, isoleucine, norleucine) were measured at 25°C. The dilution enthalpies were the same for solutions containing only one or both the enantiomeric forms of the same aminoacid. The hypothesis of the main role played in pure water by the zwitterions on the possibility of chiral recognition was confirmed.

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

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.