RESUMEN
Nitroxides are stable organic radicals with exceptionally long lifetimes, which render them uniquely suitable as observable probes or polarising agents for spectroscopic investigation of biomolecular structure and dynamics. Radical-based probes for biological applications are ideally characterized by both robustness towards reductive degradation and beneficial electron spin relaxation parameters. These properties are largely influenced by the molecular structure of the nitroxide scaffold, and also by the conformations it prefers to adopt. In this study we present the synthesis of the first nitroxides based on a spirocyclic pyrrolidine scaffold with an exocyclic methylene substituent. The conformations adopted by these nitroxides were evaluated by X-ray crystallography, both with single nitroxide crystals and by inclusion of nitroxides in a microporous crystalline sponge. The kinetic and thermodynamic stability of the new nitroxides towards reduction was investigated by electron paramagnetic resonance (EPR) spectroscopy and cyclic voltammetry (CV). In combination with EPR measurements of electron spin relaxation properties, these results suggest that this new family of nitroxides can provide access to multifunctionalized probes and polarising agents suitable for use in biological environments at elevated temperatures.
RESUMEN
Two new crystalline solids, namely, 5-aminotetrazole-3,5-dihydroxybenzoic acid-water (1/4/6), CH3N5·4C7H6O4·6H2O (I), and 5-aminotetrazolium 3,5-dinitrosalicylate, CH4N5+·C7H3N2O7- (II), have been synthesized and characterized by single-crystal X-ray diffraction and Hirshfeld surface analysis. The crystal packing arrangements of I and II are governed by N-H...O and O-H...O hydrogen-bonding interactions. In cocrystal I, adjacent acid molecules are linked through O-H...O hydrogen bonds, forming a dimer with an R22(8) motif. In salt II, the tetrazolium cation and acid anion are linked through N-H...O hydrogen bonds to also form a dimer with an R22(8) motif. Further N-H...O and O-H...O hydrogen bonds help to stabilize the crystal packing, along with aromatic π-π stacking interactions in I and carbonyl...π interactions in II. The Hirshfeld surface analysis and fingerprint plots reveal that O...H/H...O interactions contribute 34.4% of the total interactions in the crystal packing of cocrystal I and 36.7% in salt II.