Correcting the Experimental Enthalpies of Formation of Some Members of the Biologically Significant Sulfenic Acids Family

Sulfenic acids are important intermediates in the oxidation of cysteine thiol groups in proteins by reactive oxygen species. The mechanism is influenced heavily by the presence of polar groups, other thiol groups, and solvent, all of which determines the need to compute precisely the energies involved in the process. Surprisingly, very scarce experimental information exists about a very basic property of sulfenic acids, the enthalpies of formation. In this Article, we use high level quantum chemical methods to derive the enthalpy of formation at 298.15 K of methane-, ethene-, ethyne-, and benzenesulfenic acids, the only ones for which some experimental information exists. The methods employed were tested against well-known experimental data of related species and extensive CCSD(T) calculations. Our best results consistently point out to a much lower enthalpy of formation of methanesulfenic acid, CH₃SOH (Δ_fH⁰(298.15K) = −35.1 ± 0.4 kcal mol^–1), than the one reported in the NIST thermochemical data tables. The enthalpies of formation derived for ethynesulfenic acid, HCCSOH, +32.9 ± 1.0 kcal/mol, and benzenesulfenic acid, C₆H₅SOH, −2.6 ± 0.6 kcal mol^–1, also differ markedly from the experimental values, while the enthalpy of formation of ethenesulfenic acid CH₂CHSOH, not available experimentally, was calculated as −11.2 ± 0.7 kcal mol^–1.