To rationalize the variable temperature proton NMR spectra, the energy barriers for the inter-conversion in Lu- 5 and Lu- 6 via arm rotation were also calculated. These analyses indicate that the electrostatic repulsion between the arm methyl and the neighboring carboxylate significantly destabilizes the square antiprism (SAP) isomer of Lu- 5 and the twisted square antiprism (TSAP) isomer of Lu- 6, while the steric repulsion between the ring and arm methyl groups attenuates the stability of both TSAP of Lu- 5 and SAP of Lu- 6.
Herein, we have investigated the geometries, energetics, and electrostatic potentials of Lu complexed with DOTA ( 1), ring methylated M4DOTA ( 2), and arm methylated R-DOTMA ( 3) and S-DOTMA ( 4), as well as, both ring and arm methylated 4 S-4 S-M4DOTMA ( 5) and 4 S-4 R-M4DOTMA ( 6) at the level of M06-L/6-31+G(d)-SDD, to elucidate the origin of the isomer stability. DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid)-based chelates that give only a single isomer in solution when complexed with lanthanide (Ln 3+) ions is of value for studying protein dynamics and interactions via NMR.
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