Conversions were determined by quantifying the 2-Senucleosides 1a-2b and 2-Se-nucleobases 1 and 2 at 307 nm using equation (2), whereby is the peak area of compound X and is the sum of all peak areas at 307 nm. Substrates and products were identified based on their retention time and UV absorption spectra by comparison to authentic standards. Typical retention times under these conditions were as follows: 1: 3.4 min, 2: 6 min, a: 3.6 min, b: 8.6 min, a'': 3 min, b'': 4.5 min, 1a: 7 min, 1b: 12 min, 2a: 12 min and 2b: 13 min. a' and b' are not UV-active and cannot be detected with a DAD system.