1 Center for Research in Molecular Modeling (CERMM), Quebec Network for Research on Protein Function, Engineering, and Applications (PROTEO), and Department of Chemistry and Biochemistry, Concordia University, 7141 Sherbrooke Street West, Montreal, Quebec H4B 1R6, Canada. ann.english@concordia.ca.
2 Current address: Laboratory of Membrane Proteins and Structural Biology, Biochemistry and Biophysics Center, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD, USA.

Cysteine sulfenic acid (CysOH), formed on oxidation of Cys residues, is an intermediate in the catalytic cycles of numerous antioxidant enzymes and participates in oxidative-stress sensing and redox signaling. Proteins control CysOH reactivity in part by its interactions with aromatic residues. To characterize such interactions, we performed extensive ab initio quantum mechanical calculations with MP2(full)/6-311++G(d,p) on complexes of CH₃SOH as a CysOH model with side-chain models for Phe (toluene), Trp (3-methylindole), Tyr/Tyr^- (4-methylphenol/4-methylphenolate) and His/HisH⁺ (4-methylimidazole/4-methylimidazolium) residues. The gas-phase global minima conformers extracted from the 67 aromatic complexes found exhibit binding energies of ~-5 to -25 kcal mol^-1. In the neutral CH₃SOH-aromatics, the center oxidized is the stronger H-bond donor, which varies with the geometry of the complex as does the ionization potential (IPV). While CH₃SOH (IPV = 9.20 eV) is exclusively oxidized when complexed to 4-methylimidazolium (IPV = 14.64 eV), the phenol ring is oxidized in all CH₃SOH complexes with 4-methylphenolate (IPV = 3.31 eV). To perform molecular dynamics (MD) simulations of the aqueous complexes, a potential model was optimized for CH₃SOH and calibrated for its interactions with the aromatic ions. The MD simulations reveal that in bulk water the S atom preferentially adopts en-face or intermediate binding geometry with binding free energies of ~-0.6, -2.5 and -5 kcal mol^-1 for the neutral, imidazolium and phenolate complexes, respectively. Overall, the gas-phase and aqueous CH₃SOH complexes are 40-170% more stable and 0-40% less stable, respectively, than their CH₃SH counterparts. Exceptionally, aqueous 4-methylphenolate binds CH₃SOH ~50% more tightly than CH₃SH due to strong s-type O-H?O_ar H-bond bonding. Examination of a subset of CysOH-aromatics from the Protein Data Bank highlight their role in CysOH formation and stabilization in proteins.