Reversed-phase high-performance liquid chromatography (RP-HPLC), which is routinely used to detect and quantitate levels of protein oxidation, was used to analyze a free cysteine-containing protein. However, the RP-HPLC method appeared to induce dimerization of the oxidized protein. The purpose of this study was to understand the role of RP-HPLC conditions in inducing protein dimerization. Samples were also analyzed by orthogonal size-based analytical methods such as size-exclusion high-performance liquid chromatography and sodium dodecyl sulfate polyacrylamide gel electrophoresis. These methods indicated the presence of dimer and confirmed that the acidic solvent conditions induced the dimer formation of the oxidized protein. Furthermore, the dimerization was observed only when the protein was mildly oxidized and not when the protein was severely oxidized or in its native form. The sulfenic acid form of cysteine is a likely precursor to the disulfide formation. The amount of dimers increased with increasing concentration of trifluoroacetic acid (TFA) or formic acid is in the range of 0%-0.3%. The effect of the organic solvent was less than the effect of TFA/formic acid on dimer formation. Given that RP-HPLC is typically run with low-pH mobile phase containing an ion-pairing acid for improved resolution, its potential for inducing artifacts needs to be taken into consideration during method development.
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