Synthesis, Redox Chemistry and Antioxidant Activity of Sulfenic Acids
MCGRATH, ALAINA J
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Sulfenic acids figure prominently in biological and natural products chemistry as important intermediates. For example, cysteine derived sulfenic acids are key intermediates in cell signaling and play both catalytic and structural roles in enzymes. Due to the ubiquitous nature of protein sulfenic acids in cells, methods have been developed to detect and quantitate them. Although they can be detected, the mechanisms by which they form and react remain unclear. In addition, sulfenic acids are important enzymatic intermediates in Allium chemistry. Garlic, a member of the Allium genus, is known to have powerful antioxidant activity and this has recently been attributed to allyl sulfenic acid. Allicin, the thisolufinate that gives garlic its characteristic odor and flavor, decomposes to yield allyl sulfenic acid, which is believed to trap chain-carrying peroxyl radicals by readily donating a hydrogen atom, thus inhibiting autoxidations of hydrocarbons. Despite their important biological roles, little is known of the physicochemical properties of sulfenic acids. This is primarily due to their instability in air and high reactivity as both electrophiles and nucleophiles, giving them the tendency to self-condense and form thiosulfinates. Few persistent sulfenic acids, stabilized by alkyl steric protecting groups surrounding the sulfenic acid functional group, have been reported in the literature. Herein we report our synthetic efforts toward two such sulfenic acids, 9-triptycene sulfenic acid, and trans-9-decalinsulfenic acid, which were expected to be appropriate models for cysteine-derived and allyl sulfenic acids. Using 9-triptycene sulfenic acid, we were able to provide insight into the thermodynamics (O-H BDE) and kinetics (kinh) of the reactions of sulfenic acids with peroxyl radicals, which provide a clear connection between the antioxidant activity in garlic and sulfenic acids. We also preliminarily characterized the electrochemical behaviour of this compound, as well as determined its pKa.