TY - JOUR
T1 - Activation of a PARACEST agent for MRI through selective outersphere interactions with phosphate diesters
AU - Huang, Ching Hui
AU - Hammell, Jacob
AU - Ratnakar, S. James
AU - Sherry, A. Dean
AU - Morrow, Janet R.
PY - 2010/7/5
Y1 - 2010/7/5
N2 - 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 1H NMR, direct excitation Eu(III) luminescence spectroscopy, and UV?visible spectroscopy show that DEP is an outersphere ligand. Dissociation constants for [Ln(S-THP)(OH2)](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)(OH2)]((O2NPhO) 2PO2)2(CF3SO3) ·2H2O·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 1H 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)(OH2)3+ contributing to the pH dependence of the PARACEST effect.
AB - 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 1H NMR, direct excitation Eu(III) luminescence spectroscopy, and UV?visible spectroscopy show that DEP is an outersphere ligand. Dissociation constants for [Ln(S-THP)(OH2)](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)(OH2)]((O2NPhO) 2PO2)2(CF3SO3) ·2H2O·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 1H 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)(OH2)3+ contributing to the pH dependence of the PARACEST effect.
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U2 - 10.1021/ic1004616
DO - 10.1021/ic1004616
M3 - Article
C2 - 20509631
AN - SCOPUS:77954104613
SN - 0020-1669
VL - 49
SP - 5963
EP - 5970
JO - Inorganic Chemistry
JF - Inorganic Chemistry
IS - 13
ER -