Experimental and Computational Thermochemical Study of N-Benzylalanines

• Published in: The journal of physical chemistry. B 2011-08, Vol.115 (30), p.9401-9409
• Main Authors: Notario, Rafael, Roux, Maria Victoria, Foces-Foces, Concepción, Ribeiro da Silva, Manuel A. V, Ribeiro da Silva, Maria das Dores M. C, Santos, Ana Filipa L. O. M, Guzmán-Mejía, Ramón, Juaristi, Eusebio
• Format: Article
• Language: English
• Subjects: Amino acids; Aminobutyrates - chemistry; B: Statistical Mechanics, Thermodynamics, Medium Effects; Calorimetry, Differential Scanning; Combustion; Enthalpy; Formations; Isomerism; Isomers; Mathematical analysis; Models, Chemical; Molecular Conformation; Physical chemistry; Thermodynamics
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp200891v

Calorimetric measurements are expected to provide useful data regarding the relative stability of α- versus β-amino acid isomers, which, in turn, may help us to understand why nature chose α- instead of β-amino acids for the formation of the biomolecules that are essential constituents of life on earth. The present study is a combination of the experimental determination of the enthalpy of formation of N-benzyl-β-alanine, and high-level ab initio calculations of its molecular structure. The experimentally determined standard molar enthalpy of formation of N-benzyl-β-alanine in gaseous phase at T = 298.15 K is −(298.8 ± 4.8) kJ·mol–1, whereas its G3(MP2)//B3LYP-calculated enthalpy of formation is −303.7 kJ·mol–1. This value is in very good agreement with the experimental one. Although the combustion experiments of N-benzyl-α-alanine were unsuccessful, its calculated enthalpy of formation is −310.7 kJ·mol–1; thus, comparison with the corresponding experimental enthalpy of formation of N-benzyl-β-alanine, −(298.8 ± 4.8) kJ/mol, is in line with the concept that the more branched amino acid (α-alanine) is intrinsically more stable than the linear β-amino acid, β-alanine.