Unexpected Changes in the Population of Coordination Isomers for the Lanthanide Ion Complexes of DOTMA-Tetraglycinate.
Lanthanide complexes with DOTA-tetraglycinate (DOTA-(gly)4) heavily favor the square antiprismatic (SAP) coordination isomer in aqueous solution, a structural feature that has made them useful as water-based paraCEST agents. In an effort to create amide-based paraCEST agents with rapid water exchange rates, we prepared the analogous tetraglycinate complexes with DOTMA, a ligand known to favor the twisted square antiprismatic (TSAP) coordination structures. Unexpectedly, NMR investigations show that the LnDOTMA-(gly)4 complexes, like the LnDOTA-(gly)4 complexes, also favor the SAP isomers in solution. This observation led to density functional theory (DFT) calculations in order to identify the energy terms that favor the SAP structures in lanthanide complexes formed with macrocyclic DOTA- and DOTMA-tetraamide ligands. The DFT calculations revealed that, regardless the nature of the ligand, the TSAP isomers present more negative hydration energies than the SAP counterparts. The extent to which the TSAP isomer is stabilized varies, however, depending on the ligand structure, resulting in different isomeric populations in solution.
Department of Chemistry, University of Texas at Dallas , Dallas, Texas 75080, United States.
Cemile
C
Kumas
Department of Chemistry, University of Texas at Dallas , Dallas, Texas 75080, United States.
W Shirangi
WS
Fernando
Department of Chemistry, University of Texas at Dallas , Dallas, Texas 75080, United States.
Piyu
P
Zhao
Centro de Investigaciones Científicas Avanzadas (CICA) and Departamento de Química Fundamental, Universidade da Coruña , Campus da Zapateira, Rúa da Fraga 10, 15008 A Coruña, Spain.
Martín
M
Regueiro-Figueroa
Department of Chemistry, University of Texas at Dallas , Dallas, Texas 75080, United States.|Macrocyclics, Inc. , Dallas, Texas 75235, United States.
Garry E
GE
Kiefer
Department of Chemistry, University of Texas at Dallas , Dallas, Texas 75080, United States.
André F
AF
Martins
Centro de Investigaciones Científicas Avanzadas (CICA) and Departamento de Química Fundamental, Universidade da Coruña , Campus da Zapateira, Rúa da Fraga 10, 15008 A Coruña, Spain.
Carlos
C
Platas-Iglesias
Department of Chemistry, University of Texas at Dallas , Dallas, Texas 75080, United States.|Advanced Imaging Research Center, University of Texas Southwestern Medical Center , Dallas, Texas 75390, United States.
A Dean
AD
Sherry