Accelerated Oxidation of Organic Sulfides by Microdroplet Chemistry

We report the rapid oxidation of organic sulfides to sulfoxides by means of microdroplet chemistry at room temperature using a spray solution containing an organic sulfide dissolved in water/methanol, dilute (11%-14%) sodium hypochlorite (NaClO), and 5% chloroauric acid (HAuCl₄). Ultrasonic nebulization, easy ambient sonic-spray ionization, or electrosonic spray ionization serves as the microdroplet source. High-resolution mass spectrometry was used as an online detector, and nuclear magnetic resonance was used as an offline detector. We found that the sulfoxide yields vary between 66 and 95%, the highest rate of product formation is 195 mg/min for benzyl phenyl sulfoxide, and the time required is a few minutes, which is much less than that required for the conventional means of achieving this chemical transformation. We also applied this microdroplet method to protein fingerprinting. We found that protein sequences containing methionine can be quickly oxidized, providing useful information for protein structure determinations.