Retention behavior of pyridinium oximes on PFP stationary phase in high-performance liquid chromatography

The chromatographic behavior was studied of a series of potential acetylcholinesterase reactivators, pyridinium oximes, bearing linear aliphatic chains of the length of the aliphatic bridge from 1 to 12 carbon atoms, on a pentafluorophenyl-modified stationary phase. The retention mechanisms and the dependence of the capacity factor on mobile phase composition, aliphatic chain bridge length and calculated log P were evaluated and discussed in detail. The separation of the studied oximes was found to be driven by hydrophobic interactions when a lower content of organic modifier was used in mobile phase; however, the ion-exchange mechanism was the leading one when a large portion of organic modifier was used. In addition, the lipophilicity was found to be a driving mechanism of the separation of oximes bearing a connecting chain of the length of 6-12 carbon atoms, whereas the retention of oximes with shorter connecting chains was significantly influenced by other separation mechanisms such as aromatic or π-π interaction. These results can be useful for the development of new, efficient acetylcholinesterase reactivators.