Experimental and computational thermochemical study of N-benzylalanines

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.Thanks are due to the Conselho de Reitores das Universidades Portuguesas (CRUP), Portugal, and to Consejo Superior de Investigaciones Cientificas (CSIC), Madrid, Spain, for the joint research project CRUP/CSIC E39/08. The support of the Spanish Ministerio de Ciencia e Innovacion under Projects CTQ2007-60895/BQU and CTQ2010-16402 is gratefully acknowledged. Thanks are also due to Fundac)~ao para a Ci^encia e Tecnologia (FCT), Lisbon, Portugal, and to FEDER for financial support given to Centro de Investigac)~ao em Química da Universidade do Porto. A.F.L.O.M.S. thanks FCT and The European Social Fund (ESF) under the Community Support Framework (CSF) for the award of the postdoctoral fellowship (SFRH/BPD/41601/2007)