4-Dialkylamino-2,5-dihydroimidazol-1-oxyls with Functional Groups at the Position 2 and at the Exocyclic Nitrogen: The pH-Sensitive Spin Labels

Abstract

Local acidity and electrostatic interactions are associated both with catalytic properties and the adsorption activity of various materials, and with the vital functions of biomolecules. The observation of acid–base equilibria in stable free radicals using EPR spectroscopy represents a convenient method for monitoring pH changes and the investigation of surface electrostatics, the advantages of which are especially evident in opaque and turbid samples and in porous materials such as xerogels. Imidazoline nitroxides are the most commonly used pH-sensitive spin probes and labels due to the high sensitivity of the parameters of the EPR spectra to pH changes, their small size, and their well-developed chemistry. In this work, several new derivatives of 4-(N,N-dialkylamino)-2,5-dihydrioimidazol-1-oxyl, with functional groups suitable for specific binding, were synthesized. The dependence of the parameters of their EPR spectra on pH was studied. Several showed a pKa close to 7.4, following the pH changes in a normal physiological range, and some demonstrated a monotonous change of the hyperfine coupling constant by 0.14 mT upon pH variation by four units.

Synthesis and Properties of (1R(S),5R(S),7R(S),8R(S))-1,8-Bis(hydroxymethyl)-6-azadispiro[4.1.4.2]tridecane-6-oxyl: Reduction-Resistant Spin Labels with High Spin Relaxation Times

“…Exploring the possible ways to functional derivatives of 5, we tried several alkylation and acylation reactions. A reaction of nitroxide alcohols with carbonyldiimidazole (CDI) was used for binding to primary amino groups [37,38]. A reaction of 5 with CDI afforded the nitroxide 26 with an excellent yield (Scheme 7).…”

“…Scheme 9. Synthesis of 35.2,2,5,5-Tetramethyl-3-(((1-(2,3,4,6-tetra-O-acetyl-β-D-galactopyranosyl)-1H-1,2,3-triazol-4-yl)methoxy)methyl)pyrrolidine-1-oxyl(38). 2,3,4,6-Tetra-O-acetyl-β-D-galactopyranosyl azide (0.570 g, 1.53 mmol) and 2,2,5,5-tetramethyl-3-((prop-2-ynyloxy)methyl)pyrrolidine 1oxyl (330 mg, 1.57 mmol) were dissolved in a mixture of EtOH (5 mL) and H2O (1 mL) at 40 °C.…”

Editorial on the Special Issue: “Advances in Xerogels: From Design to Applications”

“…The responsibility of porous materials toward external stimuli is crucial for the obtainment of smart innovative materials, and the chemistry of N-oxide moieties offers a wide range of possibilities. In this context, Trofimov and co-workers report the synthesis of 4-dialkylamino-2,5-dihydroimidazol-1-oxyls with moieties at position 2 and at the exocyclic nitrogen able to act as pH-Sensitive spin labels [ 8 ], while Jayabhavan and colleagues study the stimuli-responsivity supramolecular gels based on pyridyl-N-oxide amides [ 9 ].…”