To α-oxo aldehyde LYRAG 2 in pH 7.5 sodium phosphate buffer was added a premixed stock of SCALP donor and 4 in DMSO such that the final concentrations/conditions were 0.1 mM 2, 2 mM of 4, 2 mM SCALP donor, 20 mM sodium phosphate, pH 7.5. The resultant solutions were incubated at 37 °C for 1 h prior to analysis by mass spectrometry. Relative rates of reaction were calculated by comparing the total ion counts of the SCALPproducts for both the SCALP-donor under assay and that of phenylacetaldehyde, giving an approximate relative rate of reaction compared to 4 (Figure ). α-oxo aldehyde JVZ007 was prepared and characterised exactly as previously reported. Azide-JVZ007. To 100 µL of a 50 µM solution of α-oxo aldehyde JVZ007 in pH 7.5 buffer (25 mM sodium phosphate) was added 25 equivalents of 15 (via the delivery of 6.5 µL of a 20 mM solution of 15 in DMSO). The resultant solution was incubated at 37 °C in darkness for 2 hours, after which time the azide-JVZ007 sample was desalted into HPLC grade water using a PD MiniTrap™ G-25 desalting column (Cytiva), flash-frozen using liquid nitrogen, and lyophilised.