. "Interacting selectively and non-covalently with a purine nucleobase, an organic nitrogenous base with a purine skeleton." . . "purine nucleobase binding" . "purine base binding" . "purine binding" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .