Corona discharge electrospray ionization of formate-containing solutions enables in-source reduction of disulfide bonds.

Disulfide bonds are critical linkages for maintaining protein structure and enzyme activity. These linkages, however, can limit peptide sequencing efforts by mass spectrometry (MS) and often require chemical reduction and alkylation. Under such conditions, information regarding cysteine connectivity is lost. Online partial disulfide reduction within the electrospray (ESI) source has recently been established as a means to identify complex cysteine linkage patterns in a liquid chromatography-MS experiment without the need for sample pre-treatment. Corona discharge (CD) is invoked as the causative factor of this in-source reduction (ISR); however, evidence remains largely circumstantial. In this study, we demonstrate that instrumental factors-nebulizing gas, ESI capillary material, organic solvent content, ESI spray needle-to-MS distance-all modulate the degree of reduction observed for the single disulfide in oxytocin, further implicating CD in ISR. Rigorous analysis of solution conditions, however, reveals that corona discharge alone can induce only minor disulfide reduction. We establish that CD-ESI of peptide solutions containing formic acid or its conjugate base results in a dramatic increase in disulfide reduction. It is also determined that ISR is exacerbated at low pH for complex peptides containing multiple disulfide bonds and possessing higher-order structure, as well as for a small protein. Overall, our results demonstrate that ESI of formate/formic acid-containing solutions under corona discharge conditions facilitates disulfide ISR, likely by a similar reduction pathway measured in γ-radiolysis studies nearly three decades ago.