Table 1.
Putative modifications identified by spectral networks on the Lens data set
| Location | Modification mass, Da | Type | Putative annotation | Comment | Reference |
|---|---|---|---|---|---|
| M | −48 | Neutral loss | Loss of methane sulfenic acid | Reported on the same site | 24 |
| W | 4 | PTM | Kynurenine | Reported in cataractous lenses | 22 |
| S | 30/73 | Unknown | Unknown | ||
| W | 32 | PTM | Formylkynurenine | Reported in cataractous lenses | 23 |
| N terminus | 57 | Unknown | Carboxyamidomethylation | Possible chemical artifact | 26 |
| N terminus | 229/271 | Unknown | Unknown |
All of these modifications occur on peptides that were previously identified in this sample. However, most of these modifications are rare in that they occur only on specific sites and thus tend to have low spectral counts. Two of these modifications can be explained as artifacts, two are known to occur in the context of lens, and two remain unexplained to date (see Discussion). Spectral networks and annotated MS/MS spectra figures supporting these modifications can be found in SI Appendix B. Our approach identified all previously found modifications except deamidation on N,Q (7).