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Aiming to improve the spectral sensitivity (\u0394a<jats:sub>X<\/jats:sub>) of these probes (i.e., the difference between the EPR hyperfine splitting (hfs) in their protonated and unprotonated forms), we characterized a series of novel linear \u03b1\u2010carboxy, \u03b1\u2032\u2010diethoxyphosphoryl nitroxides constructed on an amino acid core and featuring an (\u03b1 or \u03b1\u2032)\u2010C\u2212H bond. In buffer, the three main hfs (a<jats:sub>N<\/jats:sub>, a<jats:sub>H<\/jats:sub>, and a<jats:sub>P<\/jats:sub>) of their EPR spectra vary reversibly with pH and, from a<jats:sub>P<\/jats:sub> or a<jats:sub>H<\/jats:sub> titration curves, a two\u2010 to fourfold increase in sensitivity was achieved compared to reference imidazoline or imidazolidine nitroxides. The crystallized carboxylate <jats:bold>10\u2009b<\/jats:bold> (p<jats:italic>K<\/jats:italic><jats:sub>a<\/jats:sub> \u22483.6), which demonstrated low cytotoxicity and good resistance to bioreduction, was applied to probe stomach acidity in rats. The results pave the way to a novel generation of highly sensitive EPR pH markers.<\/jats:p>","DOI":"10.1002\/cbic.201600550","type":"journal-article","created":{"date-parts":[[2016,11,25]],"date-time":"2016-11-25T02:42:48Z","timestamp":1480041768000},"page":"300-315","update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":6,"title":["New Amino\u2010Acid\u2010Based \u03b2\u2010Phosphorylated Nitroxides for Probing Acidic pH in Biological Systems by EPR Spectroscopy"],"prefix":"10.1002","volume":"18","author":[{"given":"Sophie","family":"Th\u00e9tiot\u2010Laurent","sequence":"first","affiliation":[{"name":"Aix Marseille Univ CNRS ICR UMR 7273  Avenue Escadrile Normandie Niemen 13397 Marseille France"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Ga\u00eblle","family":"Gosset","sequence":"additional","affiliation":[{"name":"Aix Marseille Univ CNRS ICR UMR 7273  Avenue Escadrile Normandie Niemen 13397 Marseille 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