| Abstract
| - Imidazoline-based nitroxide radicals are often used as spin probes for medium acidity and polarity in differentsystems. In this work, using the density functional theory (DFT) approach, we have studied howphysicochemical characteristics (geometry, atomic charges, and electron spin density distribution) of pH-sensitive spin-label 4-amino-2,2,5,5-tetramethyl-3-imidazoline-N-oxyl (ATI) depend on protonation and aqueoussurroundings. Our calculations demonstrate that ATI protonation should occur at the nitrogen atom of theimidazoline ring rather than at the amino group. Protonation of ATI leads to a decrease in a spin density onthe nitrogen atom of the nitroxide fragment >N−O•. For simulation of ATI hydration effects, we haveconstructed a water shell around a spin-label molecule by means of gradual (step-by-step) surrounding ofATI with water molecules (nH2O = 2−41). Calculated spin density on the nitrogen atom of the nitroxidefragment increased with an extension of a water shell around ATI. Both protonation and hydration of ATIcaused certain changes in calculated geometric parameters (bond lengths and valence angles). Investigatinghow structural and energy parameters of a system ATI−(H2O)n depend on a number of surrounding watermolecules, we came to the conclusion that a hydrogen-bonded cluster of nH2O ≥ 41 water molecules could beconsidered as an appropriate model for simulation of ATI hydration effects.
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