Deamidation of -Asn-Gly- sequences during sample preparation for proteomics: Consequences for MALDI and HPLC-MALDI analysis.

We find that peptides containing -Asn-Gly- sequences typically show approximately 70-80% degree of deamidation after standard overnight (approximately 12 h) tryptic digestion at 37 degrees C. This emphasizes the need for more detailed information about the deamidation reaction in -Asn-Gly- sequences, in which two deamidated species are produced, one containing an aspartic acid (-Asp-Gly-) residue and the other containing an isoaspartic acid (-betaAsp-Gly-) residue. For the peptide SLNGEWR (54-60 beta-galactosidase, E. coli), all three components of the reaction mixture were separated by HPLC on C18 300-A sorbent, with trifluoroacetic acid as an ion-pairing modifier. Their intensity ratios suggested the elution order -betaAsp-/-Asn-/-Asp-, which was subsequently confirmed by MALDI MS and MS/MS analysis. The kinetics of the deamidation was studied in detail for the synthetic SLNGEWR parent using RP HPLC with UV detection. The half-life of this peptide was found to be approximately 8 h under digestion conditions. Analysis of a large pool of peptide retention data shows that the -betaAsp-/-Asn-/ -Asp- retention order is normally observed under the above conditions, especially if the original -NG- sequence is surrounded by hydrophobic amino acids. However, changing chromatographic conditions to 100-A pore size sorbents, or using formic acid as a modifier, increases the retention time of -betaAsp- relative to the -Asn-/-Asp- pair, so the order can sometimes be different.