1. Disulfide bond linked peptide ions from an ESI source show oxidative cleavage of the disulfide bond after exposure to a low pressure mercury lamp. 2. Mechanism experiments indicate that the 185 nm emission from the lamp photodissociates molecular oxygen inducing radical reactions. 3. It is hypothesized that ·OH or O· cleaves the disulfide bond creating a thiol and sulfinyl radical. 4. Use of the lamp with a nanoESI source can show solution reactions resulting in disulfide bond cleavage forming a thiol group in solution. 5. An ESI source does not show any solution reaction after exposure to the UV lamp. 6. Rich sequence information is achieved after CID on the peptide ion with the thiol group. 7. Gas phase production of sulfinyl radical is also achieved which can be used for radical/ion studies.
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