Gas-PhaseEnthalpies of Formation and Enthalpies ofSublimation of Amino Acids Based on Isodesmic Reaction Calculations.

Accurate gas-phase enthalpies offormation (ΔfH298°) of 20 common α-aminoacids, seven uncommon aminoacids, and three small peptides were calculated by combining G4 theorycalculations with an isodesmic reaction approach. The internal consistencyover a set of ΔfH298°(g) values was achievedby sequential adjustment of their values through the isodesmic reactions.Four amino acids, alanine, β-alanine, sarcosine, and glycine,with reliable internally self-consistent experimental data, were chosenas the key reference compounds. These amino acids together with about100 compounds with reliable experimental data (their accuracy wassupported by G4 calculations) were used to estimate the enthalpiesof formation of remaining amino acids. All of the amino acids withthe previously established enthalpies of formation were later usedas the reference species in the isodesmic reactions for the otheramino acids. A systematic comparison was made of 14 experimentallydetermined enthalpies of formation with the results of calculations.The experimental enthalpies of formation for 10 amino acids were reproducedwith good accuracy, but the experimental and calculated values for4 compounds differed by 11–21 kJ/mol. For these species, thetheoretical ΔfH298°(g) values were suggestedas more reliable than the experimental values. On the basis of theoreticalresults, the recommended values for the gas-phase enthalpies of formationwere also provided for amino acids for which the experimental ΔfH298°(g) were not available. The enthalpiesof sublimation were evaluated for all compounds by taking into accountthe literature data on the solid-phase enthalpies of formation andthe ΔfH298°(g) values recommended in our work.A special attention was paid to the accurate prediction of enthalpiesof formation of amino acids from the atomization reactions. The problemsassociated with conformational flexibility of these compounds andharmonic treatment of low frequency torsional modes were discussed.The surprisingly good agreement between the ΔfH298°(g) values calculated from the atomization and isodesmic reactionsis largely the result of a fortuitous mutual compensation of variouscorrections used in the atomization reaction procedure. [ABSTRACT FROM AUTHOR]