biostudies-literatureGalazzo LDeutsche Forschungsgemeinschaft8141-8149https://www.ebi.ac.uk/biostudies/studies/S-EPMC12359196biostudies-literatureUnknown16(32)Protonatable nitroxides are electron paramagnetic resonance (EPR) molecular probes employed for pH measurements in bulk aqueous media. The change in the protonation state of the molecule induces a measurable change in the <i>g</i>- and hyperfine (<i>A</i>) parameters used as pH indicators. The quantitative understanding of the origin of the change of the EPR parameters in terms of electronic structure and different solvation patterns is still lacking. Here, we delve into the origins of the changes in the <i>g</i>- and hyperfine (<i>A</i>) parameters of ¹⁴N upon protonation of the heterocyclic nitrogen of the pH-sensitive nitroxide probe HMI (2,2,3,4,5,5-hexamethylimidazolidin-1-oxyl, C₉H₁₉N₂O) by means of combined experimental and theoretical tecThe journal of physical chemistry lettersOrigin of the pH Dependency of EPR Parameters: The Case of a Protonatable Nitroxide in Aqueous Solution.PMC12359196271512359EXC-2033 390677874Kast SMNeese FMaste SSharma BBordignon EGalazzo LPongratz TTran VATeucher MMarx DfalseOrigin of the pH Dependency of EPR Parameters: The Case of a Protonatable Nitroxide in Aqueous Solution.Protonatable nitroxides are electron paramagnetic resonance (EPR) molecular probes employed for pH measurements in bulk aqueous media. The change in the protonation state of the molecule induces a measurable change in the <i>g</i>- and hyperfine (<i>A</i>) parameters used as pH indicators. The quantitative understanding of the origin of the change of the EPR parameters in terms of electronic structure and different solvation patterns is still lacking. Here, we delve into the origins of the changes in the <i>g</i>- and hyperfine (<i>A</i>) parameters of ¹⁴N upon protonation of the heterocyclic nitrogen of the pH-sensitive nitroxide probe HMI (2,2,3,4,5,5-hexamethylimidazolidin-1-oxyl, C₉H₁₉N₂O) by means of combined experimental and theoretical tec2025-01-01T00:00:00Z2025 Aug2026-05-01T22:56:28.132Z2026-04-07T17:21:25.385ZS-EPMC123591964074888510.1021/acs.jpclett.5c01053