Pasquale Cacace

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.

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 thermodynamic properties of aqueous solutions of l-fucose and l-rhamnose at 25°

Abstract The excess thermodynamic properties of aqueous solutions of l -fucose and l -rhamnose have been determined by means of microcalorimetry and isopiestic gravimetry. The data suggest that solute-solute interactions are unfavoured in binary solutions of these hydrophilic compounds, the solute-solvent interactions being predominant. These results lead to the conclusion that the specific function that the corresponding monomeric units seem to have, in some cases, in the reciprocal recognition of polysaccharide chains requires the presence of complexing ions ( e.g. , Ca 2+ ) and/or specific conformations of the segments of the chains involved in the process.

Excess thermodynamic properties of myo-inositol and polyhydric alcohols in water at 25°

Abstract The excess thermodynamic properties of aqueous solutions of myo -inositol have been determined at 25° and differ remarkably from those of pyranosides. New excess-enthalpy data for aqueous solutions of myo -inositol and acyclic polyhydric alcohols are treated by a group-contribution approach that reveals subtle, but probably significant, differences in the parameters characterising the same functional groups in both sugars and polyhydric alcohols.

Excess enthalpies of aqueous solutions of methyl α-d-aldopyranosides at 25°

Abstract The excess enthalpies of four methyl α- d -aldopyranosides were determined at 25° by calorimetry and expressed as virial expansion of the molality, according to the McMillan-Mayer approach. The signs and the values of the pair-interaction coefficients were attributed to the release of water from the hydration cospheres during the concentration process. The influence of the stereochemistry of the solutes on this release is discussed.

Weak stereospecific interactions in water. Excess enthalpies of aqueous solutions containing aldopyranosides at 25°C

The heats of dilution in water of methyl-β-D-galacto-, gluco-, and xylopyranosides and the heats of mixing with the respective α-isomers were determined calorimetrically at 25°C. The excess enthalpies, expressed as virial expansions as a function of the molalities, lead to the evaluation of the self- and cross-interaction coefficients. As for other saccharide derivatives, the pairwise interaction coefficients are positive and larger than those for the parent monosacharides. The cross coefficients, in turn, are smaller than the homogeneous coefficients resulting in a favorable contribution to the free energy of mixing. This indicates the existence of a weak, water-mediated, but still stereospecific recognition between pairs of solute molecules.

The peptide–urea interaction. Excess enthalpies of ternary aqueous solution of sarcosylsarcosine diketopiperazine and alkylureas at 298.15 K

The excess enthalpies of ternary aqueous solutions of sarcosylsarcosine diketopiperazine and alkylureas have been determined and compared with the properties of other diketopiperazine–urea systems at 298.15 K. A group-contribution analysis shows that cyclic dipeptides are characterized by more favourable peptide–peptide and peptide–urea interactions than the linear oligopeptides. The other contributions, and in particular the cross-interactions between peptides and alkyl chains, seem to assume values equal to those of non-cyclic solutes. The particular properties of sarcosylsarcosine diketopiperazine aqueous solutions are discussed and an unexpected analogy with monosaccharide aqueous solutions is found.

Role of hydrophilic interactions in sustaining preferential orientations: calorimetric and nuclear magnetic resonance studies of ternary aqueous systems containing a destructuring agent and hydroxylated compounds at 298.15 K

Enthalpies of dilution of ternary aqueous solutions containing biuret or urea and alkane-n, m-diols have been determined by flow microcalorimetry at 298.15 K. The pairwise cross-interaction coefficients of the virial expansion of the excess enthalpies were determined. The coefficients are positive and depend in a complex manner on the length of the alkyl chain of the hydroxylated substances. A balance between favourable hydrophilic–hydrophilic and repulsive hydrophilic–hydrophobic interactions is the factor that determines the behaviour of these systems. NMR relaxation time measurements have also been performed: however, this technique does not show the complexity unravelled by calorimetry. For binary solutions of α,ω-diols, a correlation is presented between the relaxation times and the enthalpic self-interaction coefficients.

Interactions in aqueous solutions of alkylureas. Excess enthalpies at 25• for butylurea and its isomers.

Abstract The heats of dilution in water of three isomeric alkylureas (normal-butylurea, tert-butylurea and tetramethylurea) were determined at 25°C. A comparison of the concentration coefficients of the excess enthalpies, with those of other alkylureas in water makes possible to identify, different behaviours, both determined primarily by hydrophobic interactions and urea-hydrocarbons, water-mediated, interactions. One, shown by the n-butylurea, is similar to that of diluted or moderately concentrated solutions of alkanols in water. Another is shown by the tert-butylurea and tetramethylurea. The kind of aliphatic substituents (linear or branched) and their position determine probably large differences in the cooperativity and effectiveness of the interactions.

Thermodynamics of aqueous solutions of isomeric sugars.

Abstract The excess enthalpies of ternary aqueous solutions, containing pairs of monosaccharides and their derivatives, were determined in order to ascertain whether it is possible (in spite of the prevailing solute-solvent interactions that mai n ly characterize the properties of these mixtures) to reveal the existence of weak stereospecific interactions. For the two pairs of xylose and arabinose enantiomers this investigation was successful.