On the structure of prilocaine in aqueous and amphiphilic solutions.

The solvation of prilocaine has been investigated in pure water and in an amphiphilic methanol/water solution using a combination of neutron diffraction with isotopic substitution augmented by Empirical Potential Structure Refinement (EPSR) simulations. This combination of techniques allows for details of the solvation structure on the atomic scale to be unravelled. The hydration of prilocaine is significantly altered relative to when this molecule is in pure water (as a hydrochloride salt) or in an amphiphilic environment (as a freebase compound). Interestingly, there is not a significant change in hydration around the amine group on prilocaine hydrochloride compared with prilocaine as a freebase. Despite this group being an ammonium group in water and an amine group in methanol/water solutions, the hydration of this motif remains largely intact. The changes in hydration between prilocaine as a free base and prilocaine·HCl instead appears to arise from a change in hydration around the aromatic ring and the amide group in the prilocaine molecule.