The other enzyme monomer was observed in an 'open' conformation with the side chain of Q46 displaced because of a movement of the protein backbone between residues L43 and Y58. This active site featured electron density consistent with the covalent 2ʹ-deoxyribosyl intermediate attached via an ester linkage from E98, and also two molecules of the nucleobase 17 in overlapping locations in a purine binding site (Figure ). The purine interactions within the active site side chains are similar to those previously reported in the paper by Ealick et al., however no coordinates are available in the PDB for a thorough comparison. For the molecule of 17 situated closest to the 2'-deoxyribosyl adduct, N9 is 3.9 Å from the C1ʹ of the sugar, to which it would be bonded in an intact nucleoside; the N7 atom is 3.0 Å from the Cterminus of Y157, and N3 is 3.0 Å from the side-chain of D72. Another molecule of 17 is also present in the active site and is accommodated by movement of the Q46 side chain, forming a non-covalent interaction with the bromine atom at C6.