All chemicals and solvents were of analytical grade or higher and purchased, if not stated otherwise, from Sigma-Aldrich (Steinheim, Germany), Carl Roth (Karlsruhe, Germany), TCI Deutschland (Eschborn, Germany), Carbosynth (Berkshire, UK) or VWR (Darmstadt, Germany). 2-Se-uracil (1) and 2-Se-thymine (2) were prepared according to literature procedures. The thermostable nucleoside phosphorylase PyNP-04 (E-PyNP-0004) was obtained from BioNukleo GmbH (Berlin, Germany) and used as recommended by the manufacturer. PyNP-04 was heterologously expressed in Escherichia coli and purified by affinity chromatography. The provided stock solution (1.12 mg mL -1 ) was stored at 4 °C. The enzyme originates from a thermophilic bacterium with an optimum growth temperature of 80 °C.