Abstract

The conformation of acetylcholine (Ach) and its muscarinic analogue beta-methyl acetylcholine (beta-MeAch) on an alumina surface was analyzed by inelastic electron tunneling spectroscopy (IETS). This method detects vibrational modes of organic molecules that are active in both Raman (R) and IR spectroscopies. By using previously recorded and interpreted R and IR spectra of Ach and beta-MeAch in solid-state and aqueous solutions we studied the perturbations due to adsorption. The results were used to interpret the interaction of both molecules with the alumina surface, and a comparison to that with receptors or with acetylcholinesterase was attempted. In the case of nonhydrolytic interaction, the positive trimethylammonium groups of both molecules seemed to be attracted by the negative oxygen ions of the surface. There was evidence that the O--C--C--N skeleton of Ach changed its conformation in aqueous solution and adopted the solid-state conformation, which is very similar to that of beta-MeAch. This conformation once established, Ach appeared to interact with the alumina surface in the same way as did beta-MeAch: both tunneling spectra were very similar. There was also evidence that in the acetyl part of both molecules the C=O double bond was broken and that the oxygen atom coordinated with an Al+ cation. The acetyl skeleton did not show important conformational changes for either molecule. In the case of hydrolytic interaction of Ach or beta-MeAch, the products of the hydrolysis, acetate ion and choline--the latter also adsorbed in ionic form--were found on the alumina surface. In both cases the conformation of the lateral groups bonded to the choline and acetyl skeletons was also analyzed.