Amidinoquinoxaline N-oxides: spin tapping of O- and C-centered radicals

Amidinoquinoxaline N-oxides represent a novel family of heterocyclic spin traps. In this work, their ability to trap O- and C-centered radicals was tested using selected derivatives with different structural modifications. All the studied nitrones were able to trap radicals forming persistent spin adducts, also in the case of OH and OOH radicals which are of wide biological interest as examples of ROS. Adducts stability was mainly attributed to the wide delocalization of the unpaired electron over the whole quinoxaline moiety. The nitroxides spectral parameters (hfccs and g-factors) were analyzed and the results were supported by DFT calculations. N-19 hfccs and g-factors were characteristic of each aminoxyl and could aid in the identification of the trapped radical. The enhanced stability of OH adducts in the employed reaction conditions could be ascribed to their possible stabilization by IHB with two different acceptors: the N-O moiety or the amidine functionality. DFT calculations indicate that the preferred IHB is strongly conditioned by the amidine ring size. While five membered homologues show a clear preference for the IHB with the N-O group, in six membered derivatives this stabilizing interaction is preferentially established with the amidine nitrogen as IHB acceptor.