Protonation of carboxyl groups in EuDOTA-tetraamide complexes results in catalytic prototropic exchange and quenching of the CEST signal

Lei Zhang, Osasere M. Evbuomwan, Michael Tieu, Piyu Zhao, Andre F. Martins, A. Dean Sherry
2017 Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences  
One contribution of 10 to a discussion meeting issue 'Challenges for chemistry in molecular imaging'. The CEST properties of EuDOTA-tetraamide complexes bearing pendant carboxylate and carboxyl ethyl esters were measured as a function of pH. The CEST signal from the Eu 3+ -bound water molecule decreased in intensity between pH 8.5 and 4.5 while the proton exchange rates (k ex ) increased over this same pH range. In comparison, the CEST signal in the corresponding carboxyl ester derivatives was
more » ... er derivatives was nearly constant. Both observations are consistent with stepwise protonation of the four carboxylic acid groups over this same pH range. This indicates that negative charges on the carboxyl groups above pH 6 facilitate the formation of a strong hydrogen-bonding network in the coordination second sphere above the single Eu 3+ -bound water molecule, thereby decreasing prototropic exchange of protons on the bound water molecule with bulk water protons.
doi:10.1098/rsta.2017.0113 pmid:29038382 fatcat:stzume3vvjakpgcubeyzuaflz4