Heats of formation of the amino acids re-examined by means of W1-F12 and W2-F12 theories

We have obtained accurate heats of formation for the twenty natural amino acids by means of explicitly correlated high-level thermochemical procedures. Our best theoretical heats of formation, obtained by means of the ab initio W1-F12 and W2-F12 thermochemical protocols, differ significantly (RMSD = 2.3 kcal/mol, maximum deviation 4.6 kcal/mol) from recently reported values using the lower-cost G3(MP2) method. With the more recent G4(MP2) procedure, RMSD drops slightly to 1.8 kcal/mol, while full G4 theory offers a more significant improvement to 0.72 kcal/mol (max. dev. 1.4 kcal/mol for glutamine). The economical G4(MP2)-6X protocol performs equivalently at RMSD = 0.71 kcal/mol (max. dev. 1.6 kcal/mol for arginine and glutamine). Our calculations are in excellent agreement with experiment for glycine, alanine and are in excellent agreement with the recent revised value for methionine, but suggest revisions by several kcal/mol for valine, proline, phenylalanine, and cysteine, in the latter case confirming a recent proposed revision. Our best heats of formation at 298 K ( $\Delta H_{f,298}^{\circ }$ ) are as follows: at the W2-F12 level: glycine ?94.1, alanine $-$ 101.5, serine $-$ 139.2, cysteine $-$ 94.5, and methionine $-$ 102.4  kcal/mol, and at the W1-F12 level: arginine $-$ 98.8, asparagine $-$ 146.5, aspartic acid $-$ 189.6, glutamine $-$ 151.0, glutamic acid $-$ 195.5, histidine $-$ 69.8, isoleucine $-$ 118.3, leucine $-$ 118.8, lysine $-$ 110.0, phenylalanine $-$ 76.9, proline $-$ 92.8, threonine $-$ 149.0, and valine $-$ 113.6 kcal/mol. For the two largest amino acids, an average over G4, G4(MP2)-6X, and CBS-QB3 yields best estimates of $-$ 58.4 kcal/mol for tryptophan, and of $-$ 117.5 kcal/mol for tyrosine. For glycine, we were able to obtain a “quasi-W4” result corresponding to $\hbox {TAE}_e$  = 968.1, $\hbox {TAE}_0$  = 918.6, $\Delta H_{f,298}^{\circ }=-90.0$ , and $\Delta H_{f,298}^{\circ }=-94.0$  kcal/mol.