The conformation of the CYP2D6 model substrate sparteine (1) is investigated by NMR techniques over a wide pH range. Regioselectively deuterated analogues are used to aid assignments. At pH > 6, i.e. under physiological conditions, 1 adopts an all-chair conformation (mono-protonated cage structure). Below pH 4, ring C is inverted to a boat conformation, allowing for double protonation at both nitrogens of 1.