In chemistry, sulfoxidation refers to two distinct reactions.In one meaning, sulfoxidation refers to the reaction of alkanes with a mixture of sulfur dioxide and oxygen. This reaction is employed industrially to produce alkyl sulfonic acids, which are used as surfactants. The reaction requires UV-radiation. The reaction favors secondary positions in accord with its free-radical mechanism. Mixtures are produced. Semiconductor-sensitized variants have been reported. Sulfoxidation can also refer to the oxidation of a thioether to a sulfoxide. A typical source of "O" is hydrogen peroxide.

Reference
Wiki: Methionine sulfoxidation