In situ crystallization of N-hydroxycytidine 4 in reactions catalyzed by CAT3 leads to product enrichment. Reaction conditions: 750 mM 2, 7.5 µM CAT3, 10% NH2OH (pH 7), 4°C. b) HPLC trace of the product isolated from the biotransformation described in (a). N-hydroxy-cytidine 4 is isolated in 91% yield and >98% purity as confirmed by HPLC analysis and analysis of the 1 H NMR spectra. c) Stacked 1 H NMR traces of Nhydroxy-cytidine 4, commercial standard (top) and the product isolated from the biotransformation described in (a) (bottom).