Modeling predicts facile release of nitrite but not nitric oxide from the thionitrate CH₃SNO₂ with relevance to nitroglycerin bioactivation

Nitroglycerin is a potent vasodilator in clinical use since the late 1800s. It functions as a prodrug that is bioactivated by formation of an enzyme-based thionitrate, E–Cys–NO₂. This intermediate reportedly decomposes to release NO and NO₂⁻ but their relative yields remain controversial. Hence, we determined barriers for NO and NO₂⁻ production from the model thionitrate, CH₃SNO₂, using comprehensive high-level quantum chemistry calculations [CCSD(T)//MP2/aug-cc-pVTZ]. We find that the sulfenyl nitrite, CH₃SONO, readily releases NO on (S)O–N bond homolysis but CH₃SONO formation from CH₃SNO₂ either by S–NO₂ bond homolysis or concerted rearrangement faces prohibitively high barriers (ΔH_calc/ΔH^‡_calc > 42 kcal/mol). Dramatically lower barriers (ΔH^‡_calc ~ 17–21 kcal/mol) control NO₂⁻ release from CH₃SNO₂ by gas-phase hydrolysis or nucleophilic attack by OH⁻ or CH₃S⁻ on the sulfur atom within the C–S–NO₂ molecular plane. Moreover, attack by either anion along the S–NO₂ bond results in barrierless NO₂⁻ release (ΔH^‡_calc ~ 0 kcal/mol) since a σ-hole (i.e., area of positive electrostatic potential) extends from this bond. Consistent with our high-level calculations, ALDH2 and GAPDH, enzymes implicated in nitroglycerin bioactivation via an E–Cys–NO₂ intermediate, catalyze mainly or exclusively NO₂⁻ release from the prodrug.