Characterization of IgG2 Disulfide Bonds with LC/MS/MS and Postcolumn Online Reduction.

The complication of IgG2 disulfide connections demands advances in techniques for disulfide bond determination. We have developed a new LC/MS/MS method for improved disulfide analysis. With postcolumn introduction of dithiothreitol (DTT) and ammonium hydroxide, each disulfide-containing peptide eluted out of LC in an acidic mobile phase can be rapidly reduced prior to MS analysis. The reduction can be driven to near completion. The reagents are MS-friendly, and the reaction occurs at no cost of separation (little is added to the postcolumn dead volume of the LC system). Comparing LC/MS data with and without online reduction, a direct correlation can be established between a disulfide peptide and its composing peptides using retention time. With disulfide online removal, high-quality MS/MS fragmentation data can be acquired and allows for definitive determination of the disulfide peptide. This technique is especially valuable in determining the disulfide bond linkage of complicated molecules such as the hinge-containing disulfide peptides produced from IgG2 disulfide isoforms. Due to over/under enzymatic cleavages, multiple hinge-containing disulfide peptides are produced from each isoform. Twenty-two hinge-containing disulfide peptides in total have been confidently identified with this technique. Without the method, successful identification to many of these peptides would have become extremely difficult.