Fig. 17.12 The distribution of H-bond-donor groups (carbon is white, oxygen is red, and nitrogen is blue) around a carboxylate group or a carbonyl group are superimposed with the 3D structure of the complex of methotrexate with dihydrofolate reductase (Fig. 17.7). The distributions are imposed onto the acid group of Asp26 and the carbonyl groups of Leu4 and Ala97. The hydrogen bonds formed between protein and ligand coincide geometrically with ranges often found in small organic molecules in the crystal structures.