A rapid and sensitive detection of D-Aspartic acid in Crystallin by chiral derivatized liquid chromatography mass spectrometry

- A derivatization and LC-MS method for D-Asp in α-Crystallin has been developed.
- The DBD-Py-NCS derivatized peptide diastereomers were separated by an ADME colum.
- This method was linear in the D/L peptide concentration ratio from 1/100 to 50/50.
- We applied this method to identify D-Asp residues in the UV irradiated α-Crystallin.

A method for the determination of D-Aspartic acid (D-Asp) and its D/L ratio in peptides and proteins has been developed. This method was carried out with good separation of the D/L chiral peptide pairs by combination of a chiral derivatization and an ADME column separation. Furthermore, a cationic derivatization reagent, DBD-Py-NCS, increased the sensitivity of the ESI-MS/MS detection. To confirm the comprehensive peptide analysis, synthesized α-Crystallin tryptic peptides, which included D-Asp residues, were analyzed. The 5 pairs of D/L-Asp that included peptide diastereomers were well separated. Their peak resolutions were more than 1.5 and the results were reproducible (RSD < 0.05, n = 5). As an application of this method, we analyzed the α-Crystallin standard and UV irradiated α-Crystallin. After trypsin digestion and DBD-Py-NCS derivatization, the tryptic peptide derivatives were applied to LC-MS/MS. Based on the results of peptide sequence identification, almost all the tryptic peptides of the αA- and αB-Crystallin homologous subunits of α-Crystallin were detected as DBD-Py NCS derivatives. However, there was no D-Asp residue in the standard proteins. In the case of the UV irradiated α-Crystallin, Asp76 and Asp84 in the αA-Crystallin and Asp96 in αB-Crystallin were racemized to D-Asp. These results show that this proposed chiral peptide LC-MS/MS method using chiral derivatization provides a rapid and sensitive analysis for post translational Asp racemization sites in aging proteins.