We next investigated the reaction without co-substrate in more detail. As expected, no reaction was observed with only co-substrate 2a. Interestingly, the concentration of acetophenone 4a formed in the absence of co-substrate was only slightly increased compared to the reaction containing both substrates, and a significant loss of material was observed (Figure ). This implies that, in the absence of co-substrate, the fate of the α-acyl radical is not predominantly acetophenone 4a. Concurrently, we observed yellow colour, indicative of oxidized flavin, in the reaction containing only 1a (Figure ), implying enzyme inactivation. We carried out protein mass spectrometry on the band corresponding to GluER T36A (Figure ), and observed a decrease in the abundance of certain peptides containing active site amino acids (Figure ) that may become modified by highly reactive radicals in the active site (Figure ). Reactions containing either only 2a or both substrates showed no such decrease (Figures S5&6), demonstrating that the reaction with the co-substrate outcompetes this radical inactivation. While we observed a clear decrease in peptide abundance, we were unable to determine the nature of the modifications.