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Their Gd<jats:sup>III<\/jats:sup> chelates were studied in aqueous solution through variable\u2010temperature <jats:sup>1<\/jats:sup>H nuclear magnetic relaxation dispersion (NMRD) and <jats:sup>17<\/jats:sup>O NMR spectroscopy to determine their relaxivities and the parameters that govern them. The relaxivity varied from 5.1 to 6.5 m<jats:sc>M<\/jats:sc><jats:sup>\u20131<\/jats:sup>\u2009s<jats:sup>\u20131<\/jats:sup> (37 \u00b0C and 60 MHz) as the molecular weight of the polyethylene glycol (PEG) chain increased and was slightly higher than that of the parent chelate Gd(DOTA\u2010AHA), owing to a small contribution of a slow global rotation of the complexes. A variable\u2010temperature <jats:sup>1<\/jats:sup>H NMR spectroscopy study of several Ln<jats:sup>III<\/jats:sup> chelates of DOTA\u2010A(PEG<jats:sub>750<\/jats:sub>)HA allowed the determination of the isomeric <jats:italic>M<\/jats:italic>\/<jats:italic>m<\/jats:italic> ratio (<jats:italic>M<\/jats:italic> = square\u2010antiprismatic isomer and <jats:italic>m<\/jats:italic> = twisted square\u2010antiprismatic isomer; the latter presents a much faster water exchange), which was estimated to be ca. 1:0.2 for the Gd<jats:sup>III<\/jats:sup> chelate, very close to that of [Gd(DOTA)]<jats:sup>\u2013<\/jats:sup>. This explains why the PEGylated Gd<jats:sup>III<\/jats:sup> chelate has a water rate exchange similar to that of [Gd(DOTA)]<jats:sup>\u2013<\/jats:sup>. The predominance of the <jats:italic>M<\/jats:italic> isomer is a consequence of the bulky PEG moiety, which does not favor the stabilization of the <jats:italic>m<\/jats:italic> isomer in sterically crowded systems at the substituent site, in contrast to what happens with less packed asymmetrical DOTA\u2010type chelates with substitution at one of the four acetate C(<jats:italic>\u03b1<\/jats:italic>) atoms.<\/jats:p>","DOI":"10.1002\/ejic.201500688","type":"journal-article","created":{"date-parts":[[2015,9,4]],"date-time":"2015-09-04T06:47:25Z","timestamp":1441349245000},"page":"4798-4809","update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":7,"title":["PEGylated DOTA\u2010AHA\u2010Based Gd<sup>III<\/sup> Chelates: A Relaxometric Study"],"prefix":"10.1002","volume":"2015","author":[{"given":"Andr\u00e9","family":"Fontes","sequence":"first","affiliation":[]},{"given":"Shima","family":"Karimi","sequence":"additional","affiliation":[]},{"given":"Lothar","family":"Helm","sequence":"additional","affiliation":[]},{"given":"Paula M.","family":"Ferreira","sequence":"additional","affiliation":[]},{"given":"Jo\u00e3o P.","family":"Andr\u00e9","sequence":"additional","affiliation":[]}],"member":"311","published-online":{"date-parts":[[2015,9,4]]},"reference":[{"volume-title":"The Chemistry of Contrast Agents in Medical Magnetic Resonance Imaging","year":"2001","author":"Mansson S.","key":"e_1_2_6_1_2"},{"volume-title":"The Chemistry of Contrast Agents in Medical Magnetic Resonance Imaging","year":"2013","author":"Helm L.","key":"e_1_2_6_2_2"},{"key":"e_1_2_6_3_2","doi-asserted-by":"publisher","DOI":"10.1021\/cr00081a003"},{"key":"e_1_2_6_4_2","doi-asserted-by":"publisher","DOI":"10.1002\/cmmi.327"},{"key":"e_1_2_6_5_2","doi-asserted-by":"publisher","DOI":"10.1002\/jmri.1880050102"},{"key":"e_1_2_6_6_2","doi-asserted-by":"publisher","DOI":"10.1021\/cr980440x"},{"key":"e_1_2_6_7_2","unstructured":"\u00a0"},{"key":"e_1_2_6_8_2","doi-asserted-by":"publisher","DOI":"10.1021\/ic0200390"},{"key":"e_1_2_6_9_2","doi-asserted-by":"publisher","DOI":"10.1021\/ic050743r"},{"key":"e_1_2_6_10_2","doi-asserted-by":"publisher","DOI":"10.1039\/b518147j"},{"key":"e_1_2_6_11_2","doi-asserted-by":"publisher","DOI":"10.1021\/ja0777990"},{"key":"e_1_2_6_12_2","doi-asserted-by":"publisher","DOI":"10.1021\/ja1099616"},{"key":"e_1_2_6_13_2","doi-asserted-by":"publisher","DOI":"10.1039\/c0dt01370f"},{"key":"e_1_2_6_14_2","unstructured":"\u00a0"},{"key":"e_1_2_6_15_2","volume":"36","author":"van Uffelen I.","year":"1998","journal-title":"Magn. 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