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Abstract  

In this paper a review of recent results concerning thermodynamic properties of solid uncharged derivatives of some amino acids and small peptides is reported. The experimental data obtained by different calorimetric methods are: sublimation enthalpies, heat capacities, enthalpies and temperatures of fusion and, in few cases, enthalpies and temperatures of solid-to-solid transitions. The standard molar and specific enthalpies and entropies of sublimation at 298.15 K have been calculated integrating the heat capacities of solids and vapours as function of temperature or directly measuring by calorimetry the heating enthalpies. The first ones have been obtained by interpolation of the values calculated according to the group additivity method of Benson. The sublimation thermodynamic properties have regarded N-acetylamides of glycine (NAGA), L-alanine (L-NAAA), L-valine (L-NAVA), D- and L-leucine (D-NALA and L-NALA, respectively) and L-isoleucine (L-NAIA) as well as the cyclic dipeptides glycyl-glycine (c-Gly-Gly), glycyl-L-alanine (c-Gly-L-Ala), L-alanyl-L-alanine (c-L-Ala-L-Ala) and sarcosyl-sarcosine (c-Sar-Sar). Heat capacities of the solid phases have been included also for N-acetylamide of L-proline (L-NAPA), N-methyl derivatives of the N-acetylamides previously cited and other amino acids, such as phenylalanine (F), isobutyric acid (isoBu), norvaline (norV) and norleucine (norL). In the text these substances are indicated as NAFAMe, etc. The heat capacities of their racemes are also reported. The fusion properties have concerned only two raceme mixtures (D,L-NAAA and D,L-NALA) and N-acetylamides of the cited amino acids, sarcosine (NASarA) and the following di-or tripeptides: glycyl-L-alanine (NAGAA), L-alanyl-L-alanine (NAA2A), glycyl-L-proline (NAGPA), L-prolyl-glycine (NAPGA), L-leucyl-L-proline (NALPA) and L-prolyl-L-leucyl-glycine (NAPLGA). Finally, solid-to-solid transitions have been found and characterized for L-NALA and NAGPA. All thermodynamic properties are discussed in the light of the crystal packing parameters determined during parallel crystallographic studies. It allows a comprehensive rationale of the behaviour of the solid state and its transitions for this interesting family of substances.

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A miniaturized effusion cell adapted to a Sorption LKB microcalorimeter has been designed, built and tested. Vaporization is performed isothermally into a vacuum through a small orifice permitting a vapour pressure very close to the equilibrium values. The cell has been tested by measuring the enthalpies of vaporization at 298.15 K of reference liquid compounds (water, benzene, propanol-1, propanol-2) with a reproducibility better than 1%. Enthalpies of vaporization of butanol-1 and deuterated water have also been determined.

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Abstract  

In this paper we try to perform a thermodynamic analysis of the temperature-induced transition from the molten globule to the unfolded state of globular proteins. A series of calorimetric investigations showed that this process is not associated with an excess heat capacity absorption peak, and cannot be regarded as a first-order phase transition. This result contrasts with the well-established conclusion that the thermal unfolding of the native tertiary structure of globular proteins is a first-order phase transition. First, the theoretical approach developed by Ikegami is outlined to emphasize that a second-order or gradual transition induced by temperature is expected for globular proteins when the various secondary structure elements do not interact cooperatively. Secondly, a simple thermodynamic model is presented which, taking into account the independence of the secondary structure elements among each other, is able to rationalize the shape of the experimental DSC profiles.

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Enthalpies and entropies of sublimation for N-acetylglycine amide (NAGA), N-acetyl-L-alanine amide (L-NAAA), and N-acetyl-D-leucine amide (D-NALA) were determined from the dependence of their vapour pressures on temperature, as measured by the torsion-effusion method.

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The heats of dilution in water of binary and ternary solutions of the two enantiomeric forms ofN-acetylalanineamide have been measured at 25°. The excess enthalpies, expressed as virial expansion series, permit evaluation of the pairwise self and cross enthalpic coefficients. As for the chiral forms of some monosaccharides, the cross coefficient for the interaction between theD andL forms ofN-acetylalanineamide is slightly but significantly different from the corresponding self coefficient. A weak, water-mediated chiral recognition can be assumed to exist between pairs of amide molecules.

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Abstract  

A general feature of temperature-induced reversible denaturation of small globular proteins is its all-or-none character. This strong cooperativity leads to think that protein molecules, possessing only two accessible thermodynamic states, the native and the denatured one, resemble ‘crystal molecules’ that melt at raising temperature. An analysis, grounded on mean field theory, allows to conclude that the two-state transition is a first-order phase transition. The implication of this conclusion are briefly discussed.

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Abstract  

The thermodynamic stability of pancreatic ribonuclease B (RNase B), which possesses identical protein structure of pancreatic ribonuclease A (RNase A), but differs by the presence of a carbohydrate chain attached to Asn 34, was studied by means of differential scanning calorimetry (DSC) at different pH conditions. The comparison between the two proteins has shown a little but significant stabilization of RNase B with respect to the unglycosylated one at pH values higher than 7.0. The thermodynamic analysis reveals the carbohydrate moiety to have a small stabilization effect of 3 kJ mol–1 at pH 8.0 and 63°C on the protein.

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Journal of Thermal Analysis and Calorimetry
Authors: C. Giancola, A. Buono, G. Barone, L. De Napoli, D. Montesarchio, D. Palomba, and G. Piccialli

Abstract  

In this work we report a thermodynamic characterization of stability and melting behaviour of two 24-mer DNA triplexes. The third strand, that binds the Watson-Crick double helix with Hoogsteen hydrogen bonds, contains 3′-3′ phosphodiester junction that determines the polarity inversion. The target double helix is composed of adjacent and alternate fragments of oligopurine-oligopyrimidine tracts. The two helices differ from the substitution of the cytosine, involved in the junction, with the thymine. Calorimetric data reported here provide a quantitative measure of the influence of pH and base modification on the stability of a DNA triplex.

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