After purification of our peptibody with MabSelect Protein A chromatography resin (molecular weight ≈60 kDa, pI=8.3), we have two RP-HPLC chromatograms from a biosimilar sample and a standard. It was shown a shoulder in the right hand of the main peak of the biosimilar sample. But it was not detected in the standard main peak. One of the possibilities can be aspartic acid (Asp) isomerization in our recombinant peptibody.
Actually, the conditions required for Asp isomerization seem to be pretty harsh. Do you have hard evidence (e.g. MS data) for it?
This paper might be helpful: Kinetics of Aspartic Acid Isomerization and Enantiomerization in Model Aspartyl Tripeptides under Forced Conditions, https://www.sciencedirect.com/science/article/abs/pii/S0022354915324448
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