Novel Au-III(dien)(N-heterocycle) compounds, including the first (AuN3)-N-III(N-purine) examples, are reported. The acidity of the dien ligand is affected by the nature of the fourth ligand as a leaving group. The metal center of [Au(dien)(N-heterocycle)](3+) compounds was shown to be more stable to reduction than when Cl- is present, with consequences for reactivity with biomolecules: specifically, significant enhancement of pi-pi-stacking interactions with tryptophan relative to isostructural and isoelectronic platinum(H) and palladium(II) compounds.