Nonenzymatic Hydrolysis of Acetylthiocholine by Silver Nanoparticles

Abstract

Since five decades, acetylthiocholine, an O → S substituted synthetic analogue of the natural neurotransmitter acetylcholine, has become a key element in various assays used for probing the presence and activity of a highly important enzyme, that is, acetylcholinesterase, in different biological media. A large number of these assays are now using plasmonic nanostructures because thiocholine, issued from the enzymatic hydrolysis of acetylthiocholine, is able to bind to the surface of both silver and gold nanoparticles by its end groups (trimethylammonium and thiol groups). Herein, by following the characteristic thiocholine surface-enhanced Raman scattering markers, it is shown that a nonenzymatic hydrolysis of acetylthiocholine is also possible at the surface of silver nanoparticles, presumably because of (i) the silver reactivity toward the sulfur atom, and especially to the chemical bonds in which it is involved, and (ii) the conformational flexibility of acetylthiocholine for giving the adequate orientation to its scissile S-C bond with respect to the silver surface in order to facilitate its cleavage. Nevertheless, being less efficient than the enzymatic degradation, the nonenzymatic one appears to be incomplete and concentration-dependent and occurs within the time interval generally required for sample preparation and data accumulation in acetylcholinesterase assays. Therefore, precaution should be used to avoid any distortion of the acquired data by selecting adequate protocols and substrate concentrations.