Whereas in the WT-LlNDT-2-deoxycytidine complex (9F09) the 3'-OH of 2'-deoxycytidine is bound only by the OE2 atom of the D98 carboxylate, in the Y7F/D72N-WT-LlNDT-2 complex the 2'-OH and 3'-OH of the ribonucleoside interact with one carboxylate atom of E98 each. The 2'-OH also interacts with the side chain of N72 at a distance of 2.7 Å, but only in one rotameric orientation. The interaction between E98 and the 2'-OH alone would militate against the activity of E98 as a nucleophile for attack at C1' of the substrate; additionally, the even lower activity of the Y7F/D72N mutant versus the WT-LlNDT-2 for ribonucleosides would be compromised by the removal of the interaction between the phenol of Y7 and E98, which would suppress the ionisation of E98 for nucleophilic attack at C1. Broadening substrate scope: transglycosylation using 2′-modified nucleoside donors With the structural basis for the recognition of ribonucleosides established, we investigated the potential of WT-LlNDT-2 to accept a range of modified nucleoside donor substrates (Figure ). Both 2′-fluoro-2′-deoxyribonucleosides 25 and 26 were accepted as sugar donor substrates. Proof of concept was demonstrated by the formation of ribonucleosides 27-28 and arabinonucleosides 29-30 and also the formation of the antileukemia drug chlofarabine (30) in 25% conversion.