Oleg V. Kulikov

Excess enthalpies and apparent molar volumes of aqueous solutions of crown ethers and cryptand(222) at 25°C

The enthalpies of dilution and densities of aqueous solutions of 12-crown-4, 15-crown-5, 18-crown-6, 1,10-diaza-18-crown-6 and cryptand (222) were measured at 25°C. The excess enthalpies and enthalpic coefficients of solute-solute interactions were calculated by the McMillan-Mayer theory formalism. Values for the apparent molar volumes at infinite dilution were determined by extrapolation. The contributions of the-CH2CH2O-group to values of h2 and to the limiting partial molar volume were calculated for the series of crown ethers studied. It is concluded that the hydrophobic hydration and the hydrophobic solute-solute interaction are predominant in the solutions investigated.

Physico-chemical properties of peptides and their solutions

Abstract Complex investigations of a series of dipeptides and their solutions in H 2 O were conducted by the methods of differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), densimetry, and IR and Raman spectroscopy. The results of investigations by DSC and TGA show that the differences in the spatial structure of solid dipeptides of a homologous series sharply influences the character of phase transformations. Linear correlations between the values of the heat capacity of crystalline amino acids and peptides and the number of atoms, number and length of interatomic bonds. The dipeptides studied are classified into three groups: those with predominantly hydrophilic, those with predominantly hydrophobic and those with hydrophobic-hydrophilic (at different concentrations) characters of hydration in aqueous solutions. The ratio between hydrophilic and hydrophobic contributions to the hydration of dipeptides, and role of separate fragments of the molecules is considered.

Complexation of peptides with crown ethers. Structure and thermal behaviour of ternary compounds: 18-crown-6/glycylglycine/water and 18-crown-6/L-α-alanyl-L-α-alanine/water

Abstract X-ray structures of ternary complexes composed of small dipeptides, i.e. glycylglycine or L-α-alanyl-L-α-alanine, 18-crown-6 and water molecules are determined based on single-crystal studies. It has been concluded that both dipeptides exist in the complex as zwitterions and are bonded to the 18-crown-6 macrocycle by the -NH+ 3 end group through N-H…O hydrogen bonds. The carboxylic groups at the opposite side of the peptide molecules take part in hydrogen bonding with water molecules. This scheme of water-peptide hydrogen bonding is particularly interesting in the glycylglycine compound where the water content is rather high (approximately 16 moles per mole of crown/dipeptide, 6 of which are found in the X-ray study) and may have interesting relevance with respect to the structure of gel solutions which easily form in these systems. The DSC and TGA thermograms of different hydrated forms of the complexes were obtained. Two types of water, i.e. weakly and strongly bonded, were observed upon heatin...

Thermodynamic investigations of interactions in aqueous solutions of glycine and some small peptides with caffeine and 1,3-dimethyluracil at 298.15 K

Abstract Enthalpies of dilution for ternary aqueous solutions of glycine, glycyl-glycine, triglycine, and l -α-alanyl- l -α-alanine with caffeine, as well as of glycyl-glycine, and l -α-alanyl- l -α-alanine with 1,3-dimethyluracil have been determined. The results have been analysed in terms of the McMillan-Mayer formalism in order to obtain the enthalpic virial coefficients. Weak interactions were observed. The densities and molar volumes at infinite dilution were determined for the systems mentioned, as well as for aqueous solutions of glycine, glycyl-glycine and triglycine.

Thermodynamics and mechanism of complexation of peptides with 18-crown-6 in water

Taking into account the unusual crystalline structures of 18-crown-6/peptides/water complexes, the links between their thermokinetic behaviour and the structural peculiarities are discussed. It has been found that the peptide chain lengthening leads to an increase in the temperature and activation energy of decomposition. The thermodynamics of complexation between small peptides and 18-crown-6 in water has been studied as well. It is shown that the influence of the peptide lengthening and the presence of a methyl group in the peptide side chain on the binding constant can be used as a the basis for the separation of peptide mixtures in solution.

Excess enthalpies of aqueous solutions of small peptides at 25°C

Heats of dilution of aqueous solutions of the following di-and tripeptides were determined at 25°C over a wide concentration range: β-alanyl-glycine, β-alanyl-β-alanine, DL-α-alanyl-glycine, glycyl-DL-α-alanine, L-α-alanyl-Lα-alanine, DL-α-alanyl-DL-α-alanine, DL-α-alanyl-DL-valine, DL-α-alanyl-β-alanine, glycyl-γ-aminobutyric acid, glycyl-L-leucine and DL-α-alanyl-glycylglycine. The excess enthalpies Hex and partial molar relative enthalpies L2 were calculated and enthalpy coefficients of intermolecular interaction were analysed using the additivity principle of Savage and Wood. The concentration dependence of the enthalpic characteristics of peptide-peptide interactions is discussed based on of their hydrophobicity and hydrophilicity. The three-stage model of peptide association is described using enthalpic coefficients of intermolecular interactions.

Thermodynamics of the interactions of peptides with α- and β-cyclodextrins

The enthalpies of solution of α- and β-cyclodextrins is aqueous peptide solutions were determined experimentally at 298.15 K. The obtained results were used to calculate pair cross interaction parameters between solutes. The results are discussed in terms of the likelysolute–solute interactions. For systems α-cyclodextrin+peptide and β-cyclodextrin+peptide the diametrically opposite character of interaction defined by structure and solvation of the molecules is observed.

Complexation of peptides with crown ethers: Part 1. Composition and thermal behaviour of compounds consisting of oligopeptides and some crown ethers

Abstract Crystals of the complexes 18-crown-6/diglycine, 18-crown-6/diglycine/water, 18-crown-6/triglycine, 18-crown-6/ l -α-alanyl- l -α-alanine and 1,10-diaza-18-crown-6/diglycine/water were obtained. The thermal behaviour of these compounds was investigated by DSC and TGA methods. It was found that the anhydrous complexes have at least two physicochemical transitions in the range 320–470 K: melting of the complexes at 360–380 K and decomposition above 410 K. Two types of water, i.e. weakly and strongly bound with the complex, were observed on heating the hydrated complexes. The enthalpic and entropic changes of the melting process are discussed in terms of the cationic and anionic binding of the peptide molecule to crown ether.

Thermodynamics of α-Cyclodextrin Complexation with Bases of Nucleic Acids and Their Derivatives

The interactions of α-cyclodextrin with adenine, thymine, uracil, cytosine, and caffeine in water at 298.15 K have been studied by the calorimetric method. α-Cyclodextrin is found to interact selectively with nucleic bases and their derivatives to form complexes with uracil, cytosine, and caffeine only. The influence of the structure and solvation of the reactants on the thermodynamic characteristics of their complexation in a solution has been examined.

Enthalpy parameters for interaction of small peptides with 18-crown-6 and aza-18-crown-6 in water at 25°C

Enthalpies of dilution have been determined for binary aqueous solutions of 1-aza-18-crown-6 as well as for ternary aqueous solutions containing glycine, glycylglycine, glycyl-L-α-alanine, L-α-alanyl-glycine, L-α-alanyl-L-α-alanine, DL-α-alanyl-DL-α-alanine, trialanine and 18-crown-6 and/or 1-aza-18-crown-6 and or 1,10-diaza-18-crown-6 at 25°C. The results have been treated by the McMillan-Mayer approach in order to obtain enthalpic virial coefficients for homotactic and heterotactic interactions. A significant exo-effect is demonstrated by the enthalpically favorable interaction between peptides and the 18-crown-6. The additivity of the positive alanyl group contribution tohxy has been confirmed on the basis of oligomeric data. The influence on the enthalpy of the 18-crown-6-peptide interaction of the methyl group position, in relation to the ammonium group in peptides, has been found to result in the exo-effect decreasing with a decrease of this distance. Some decrease in enthalpy of L-α-alanyl-L-α-alanine and DL-α-alanyl-DL-α-alanine by 18-crown-6 has been observed as well.