Specific Intermolecular Interactions and Energies of Amino Acids and Esters

Abstract

The thermodynamic properties of important proteins amino acids have been studied a little in recent decades and the enthalpy and entropy values of the vaporization processes are still known for only a few (Table 9.1). These properties, along with the melting temperature carry important information because the process of phase transformation, with its chemical nature [1–3], is interconnected with the number of severe bonds and their energies. Therefore, even the melting temperature reflects the depth of the transformation of the crystal structure and its difference from the structure of the liquid condition [1–3]. On the basis of these ideas we have reasons to discuss the increase of the melting temperature of alanine at 2° in comparison with the same property of glycine amino acetic acid connected with stabilization of the same type of specific interaction. This stabilization is caused by the contribution to the structure of alanine of the methyl group, implementing the functional properties of the isostructural group, participating in the redistribution of the electron density in the molecule. Further replacement of the methyl group by the fragment СН₃(СН)СН– in the molecule of l-valine and СН₃(СН)СН–СН₂– in the molecule of d-(l)-leucine is accompanied by stabilization of the specific interaction, formed by these fragments with the oxygen atom of the hydroxyl group of the glycine fragment D–O → CH₃(CH₃)CH– < D–O → CH₃(CH₃)CH–CH₂–. Such a sharp reduction of the melting temperature of isoleucine points to the implementation of theethyl ligand with regard the functional properties of the isostructural group. Reduced melting temperatures of diacids l-ashartic and l-glutamic acids reflect the reduction of the energies of the hydrogen bonds with an increasing number of carboxyl groups and the significant change in the difference in the charges at the oxygen and hydrogen atomsof the hydroxyl groups. The hydrogen bonds introduced in a new type at asparagine and glutamine with reduced energy values, formed by the amide group D–O•••H–N > D–N•••H–N, and methylene groups, fringed by the amide groups, are responsible for the reduction of the melting temperature. In turn, the increasing number of methylene groups, fringed by the same amide groups, causes the reduction of the melting temperatures of the mentioned diacids in the sequence asparagine – glutamine. Similar changes in the energies of the specific interactions formed of crystalline and liquid dl-threonine, l-cysteine, l-arginine, and other compounds reflect the melting temperature of important protein amino acids. The insolubility of α-amino acids in hydrocarbons and ethers points to the high stability of the hydrogen bonds formed and the water solubility of most acids in this range indicates the comparability of the energies of the hydrogen bonds with a similar bond with water molecules (10.99 kJ mol⁻¹) [6].