The exceptional hydrogen-bond properties of neutral and protonated lobeline

The X-ray diffraction structure of (-)-lobeline, a high affinity nicotinic ligand, has been determined. A comparison with its hydrobromide and hydrochloride salts shows the great flexibility of the two lateral chains of the N-methylpiperidine ring. Infrared studies carried out on the same species, in the solid state and in solution, reveal the propensity of this molecular framework to accommodate very specific hydrogen bonds (HBs) depending on the state-neutral or protonated-of the molecule. In solution, a strong internal HB between the hydroxyl group and the piperidine nitrogen gives an exceptionally high HB affinity to the hydroxyl oxygen of the lobeline base. In the ionic form, both NH+ and OH groups of the molecule cooperate as HB donors to chelate the counterion. These interactions provide very stable structures and indicate that protonated lobeline can also act as a strong HB donor.