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A PARACEST agent responsive to inner- and outer-sphere phosphate ester interactions for MRI applications.

ABSTRACT: Eu(S-THP)(3+) is the first PARACEST agent that functions through exchange of hydroxyl protons with water protons in aqueous solution. The CEST spectrum of this complex is strongly pH dependent and modulated by the presence of phosphate esters, as shown for diethyl phosphate and methyl phosphate, which form outer- and inner-sphere complexes, respectively, with Eu(S-THP)(3+). The sensitivity of the alcohol proton environment to interactions with these anions shows that this complex has potential as a responsive PARACEST MRI contrast agent.

SUBMITTER: Huang CH

PROVIDER: S-EPMC2744982 | biostudies-literature | 2009 Apr

REPOSITORIES: biostudies-literature

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A PARACEST agent responsive to inner- and outer-sphere phosphate ester interactions for MRI applications.

Huang Ching-Hui CH Morrow Janet R JR

Journal of the American Chemical Society 20090401 12

Eu(S-THP)(3+) is the first PARACEST agent that functions through exchange of hydroxyl protons with water protons in aqueous solution. The CEST spectrum of this complex is strongly pH dependent and modulated by the presence of phosphate esters, as shown for diethyl phosphate and methyl phosphate, which form outer- and inner-sphere complexes, respectively, with Eu(S-THP)(3+). The sensitivity of the alcohol proton environment to interactions with these anions shows that this complex has potential a ...[more]

PMID: 19317496

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Project description:Ln(S-THP)(3+) complexes are paramagnetic chemical exchange saturation transfer (PARACEST) agents for magnetic resonance imaging (MRI; S-THP = (1S,4S,7S,10S)-1,4,7,10-tetrakis(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane, Ln(III) = Ce(III), Eu(III), Yb(III)). CEST spectra at 11.7 T show that the PARACEST effect of these complexes is enhanced at neutral pH in buffered solutions containing 100 mM NaCl upon the addition of 1-2 equiv of diethylphosphate (DEP). CEST images of phantoms at 4.7 T confirm that DEP enhances the properties of Yb(S-THP)(3+) as a PARACEST MRI agent in buffered solutions at neutral pH and 100 mM NaCl. Studies using (1)H NMR, direct excitation Eu(III) luminescence spectroscopy, and UV-visible spectroscopy show that DEP is an outersphere ligand. Dissociation constants for [Ln(S-THP)(OH(2))](DEP) are 1.9 mM and 2.8 mM for Ln(III) = Yb(III) at pH 7.0 and Eu(III) at pH 7.4. Related ligands including phosphorothioic acid, O,O-diethylester, ethyl methylphosphonate, O-(4-nitrophenylphosphoryl)choline, and cyclic 3,5-adenosine monophosphate do not activate PARACEST. BNPP (bis(4-nitrophenyl phosphate) activates PARACEST of Ln(S-THP)(3+) (Ln(III) = Eu(III), Yb(III)), albeit less effectively than does DEP. These data show that binding through second coordination sphere interactions is selective for phosphate diesters with two terminal oxygens and two identical ester groups. A crystal structure of [Eu(S-THP)(OH(2))]((O(2)NPhO)(2)PO(2))(2)(CF(3)SO(3)) x 2 H(2)O x iPrOH has two outersphere BNPP anions that form hydrogen bonds to the alcohol groups of the macrocycle and the bound water ligand. This structure supports (1)H NMR spectroscopy studies showing that outersphere interactions of the phosphate diester with the alcohol protons modulate the rate of alcohol proton exchange to influence the PARACEST properties of the complex. Further, DEP interacts only with the nonionized form of the complex, Ln(S-THP)(OH(2))(3+) contributing to the pH dependence of the PARACEST effect.

Dataset Information

A PARACEST agent responsive to inner- and outer-sphere phosphate ester interactions for MRI applications.

Publications

A PARACEST agent responsive to inner- and outer-sphere phosphate ester interactions for MRI applications.

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