Table 3.
Summary of mono- (Mo) and bifunctional (Bi) adducts obtained by treating bovine carbonic anhydrase II (CA) with either PDG or BDG, followed by digestion with trypsin (T) or chymotrypsin (C). Monoisotopic masses were calculated from the incremental masses described in Scheme 2 and from theoretical peptide masses provided by the PeptideMass calculator [67]. Note that the species with mass 2392.0974 Da corresponds to products in which either R56 or R57 were mono-functionally modified (ΔM = 284.07 Da). The formation of a bifunctional conjugate bridging these residues was ruled out by the absence of diagnostic products with ΔM = 230.04 or 274.05 Da.
| Reagent | Obs. mass (Da) | Calc. mass (Da) | Digestion product | Modified site(s) |
|---|---|---|---|---|
| PDG | 5995.9628 | 5995.9518 | 36–88 + Bi, T | R56 ↔ R57 |
| 2391.0906 | 2391.1138 | 51–58/88–96 + Bi, C | R56 (R57) ↔ R88 | |
| 3815.7660 | 3815.7484 | 57–88 + Mo, T | R57 | |
| 4592.1720 | 4592.1684 | 89–125 + Mo, T | R110 | |
| 2985.4593 | 2985.4615 | 18–35/253–259 + Bi, T | R26 ↔ R255 | |
| BDG | 2392.0974 | 2392.0911 | 47–65 + Mo, C | R56 or R57 |
| 4271.0332 | 4271.0166 | 49–69/79–92 + Bi, C | R56 (R57) ↔ R88 | |
| 4612.1528 | 4612.1509 | 49–78/88–94 + Bi, C | R56 (R57) ↔ R88 | |
| 3105.4148 | 3105.4001 | 89–112 + Mo, T | R110 |