A reaction temperature of 80 °C was initially chosen for the synthesis of Se-modified nucleosides as higher reaction rates are observed with the enzyme and it can be assumed that a high reaction temperature would further improve the solubility of 1 and 2. Initial studies, however, revealed product losses during the synthesis of 2'-deoxyribonucleosides (data not shown) which can be correlated to the hydrolysis of the transglycosylation intermediate b' at higher temperatures. Hence, reaction temperatures of 80 °C were applied for the synthesis of ribonucleosides, while 60 °C was chosen for reactions involving 2'deoxynucleosides.