Inclusion Complexes of Nitroxides of Pyrrolidine and Imidazoline Series with Cucurbit[7]uril

Abstract

Inclusion of nitroxides into the cavity of molecular nanocontainers may prevent or significantly retard nitroxide reactions with cellular reductans and increase nitroxide lifetime. This paper is aimed at studying possible applications of cucurbit[7]uril (CB7) as a molecular nanocontainer for pH-sensitive nitroxides probes. Inclusion complexes of CB7 with different nitroxides--3-aminomethyl-proxyl (AMP), 3-hydroxymethyl-proxyl (HMP), 4-amino-2,2,5,5-tetramethyl-2,5-dihydroimidazole-1-oxyl (ATI), and 2,2,4,5,5-pentamethyl-2,5-dihydroimidazole-1-oxyl (MTI)--were studied. It was found that reversible formation of inclusion complexes AMP@CB7, AMPH(+)@CB7, HMP@CB7, ATIH(+)@CB7, and MTIH(+)@CB7 is accompanied by a decrease of hyperfine interaction (HFI) constant on nitrogen of the nitroxide group and by a 5-7-fold increase of rotational correlation time monitored by electron paramagnetic resonance (EPR). The binding constants of nitroxide@CB7 complexes were determined, and the influence of alkali metal ions and pH on the equilibrium between free and encapsulated nitroxides was studied. The EPR spectra of CB7 mixtures with protonable nitroxides were found to be more sensitive to pH changes than the spectra of pure nitroxides, and the apparent pK of these mixtures was found to increase with CB7 concentration. The nitroxide@CB7 complexes showed higher resistance to chemical reduction with ascorbic acid compared to free nitroxides. Thus, CB7 can be used for the improvement of functional properties of nitroxides.