Enthalpies of sublimation and fusion for N-acetyl substituted glycine, L-alanine, and D-leucine amides

Abstract

Enthalpies and entropies of sublimation for N-acetylglycine amide (NAGA), N-aeetyl-Lalanine 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.Enthalpies and temperatures of fusion were measured by differential scanning calorimetry (DSC) and entropies of fusion were derived. No solid-to-solid transitions were detected from r.t. to fusion. Enthalpies of sublimation and fusion were combined to evaluate enthalpies of vaporization of the melts.The experimental results decreased inversely to the molecular mass. An interpretation of this trend in terms of the crystalline structure of these compounds is proposed.Reliable thermodynamic data on evaporation processes of low-molar-mass organic compounds are of great importance in several fields, ranging from organic chemistry to biochemistry and biophysics. In particular, enthalpies of sublimation and vaporization can be combined with enthalpies of solution at infinite dilution of solid and liquid compounds, respectively, to obtain enthalpies of solvation [1,2]

Heat capacities, and enthalpies and entropies of fusion of some uncharged small peptides (N-acetylamino acid amides and 2,5-diketopiperazines)

“…This study is a part of a series of investigations on the thermodynamic properties of model compounds as pure substances. (9)(10)(11)(12)(13) In this work, we have determined the molar heat capacities as a function of temperature and molar heat capacities at T = 298.15 K for the solid N-acetyl substituted amides of glycine (NAGA), L-alanine (L-NAAA), L-proline (L-NAPA), L-valine (L-NAVA), L-leucine (L-NALA), and L-isoleucine (L-NAIA), and four cyclic anhydrides (diketo-piperazines) of glycine (c-GG), glycine and L-alanine (c-GA), L-alanine (c-AA), and sarcosine (c-SARSAR). Temperatures and enthalpies of fusion were also measured.…”

Structures of some N-acetylamides of amino acids

“…Moreover, the geometrical parameters obtained by X-ray analysis can be used, together with spectroscopic data, dipolar moments and sublimation energies, for deriving the parameters for semiempirical potential relations and for conformational analysis. The knowledge of intermolecular interactions in the solid state can give a simple explanation for the trends of enthalpies of melting and sublimation recently determined (Ferro, Della Gatta & Barone, 1988) and will be used as support for theoretical calculations concerning the weak nonbonding interactions in solution which have recently been the subject of extensive experimental studies (Barone, Castronuovo, Elia & Giancola, 1985;Blackburn, Lilley & Millburn, 1986;Barone, Castronuovo, Del Vecchio, Elia & Giancola, 1987).…”

Thermodynamics of sublimation ofN-acetyl amides of some amino acids

“…In the last columns, the standard molar sublimation enthalpies at T = 298.15 K, calculated according to equation (2), are reported. The experimental enthalpy values shown here differ appreciably from those obtained from a preceding series of measurements, in which an indirect tensimetric method was used to detect the average sublimation enthalpies of some of these substances at temperatures between T = 366 K and T = 445 K. (8,10) The standard molar enthalpies of sublimation at T = 298.15 K reported here for NAGA, L-NAAA, L-NAVA, and L-NALA represent an improvement over the results previously reported (24) because accurate temperature dependence of the corresponding C o p,m (cr) are now also available. (14) As outlined before, the (12,14) enthalpies of heating of solid L-NALA and L-NAIA, measured by calorimetry, are in good agreement with the values calculated from the integration of the C o p,m (cr) values.…”

“…Except for the N -acetyl-D-leucine amide (D-NALA), synthesized by us, (8,11) all the other substances studied, N -acetyl-glycine amide (NAGA)…”

“…(5,6) For these reasons, we are carrying on a program on the solid-state properties of several classes of organic model substances, with different approaches. (7)(8)(9)(10)(11)(12)(13)(14)(15) In this contribution, we present and discuss the enthalpies and entropies of sublimation, obtained by calorimetric methods, and Gibbs energies of an interesting set of amides of N -acetyl amino acids. These compounds are described by the general formula: CH 3 -CO-[NH-CHR-CO]-NH 2 , where -R represents the side chain of an amino acid residue: -H for glycine, -CH 3 for L-alanine, -CH(CH 3 ) 2 for L-valine, -CH 2 CH(CH 3 ) 2 for D-and L-leucine, and -CH(CH 3 )CH 2 -CH 3 for L-isoleucine.…”