Computational challenges toward an accurate determination of the enthalpies of formation of amino acids are partly due to the nonavailability of systematic error-canceling thermochemical procedures for such biomonomers. Recently, we developed the connectivity-based hierarchy (CBH) to accurately compute the enthalpies of formations of organic molecules composed of main group elements. Advancing the applicability of CBH to biologically relevant molecules, we have computed the enthalpies of formation of the naturally occurring sulfur-containing amino acids cysteine and methionine which act as fertile testing grounds for the error-canceling ability of thermochemical schemes for biomolecules. We establish herein using the sophisticated error-canceling isoatomic scheme (CBH-2) that relatively inexpensive computational methods with modest basis sets can be used to accurately obtain the enthalpies of formations of the amino acids. Overall, we recommend the use of the isoatomic scheme over the currently popular isodesmic bond separation scheme in future applications in theoretical thermochemistry.
Cysteine Methionine