Separation of Contact and Pseudocontact Contributions to ¹³C Lanthanide Induced Shifts in Non-Axially-Symmetric Lanthanide Ethylenediaminetetraacetate Chelates

Lanthanide-induced shifts in the carboxyl carbon resonance of 12 paramagnetic lanthanide [1-¹³C]ethylenediaminetetraacetate chelates have been measured from 298 to 358 K. Separate carbon resonances are observed for bound and excess free chelate carbons over this range and the bound peak sharpens considerably at higher temperatures. Attempts were made to separate the contact and pseudocontact contributions to the paramagnetic shifts by fitting the data to several temperature-dependent approximations but none yielded consistent results. The shifts at 318 K were successfully separated into their contact and pseudocontact components by using the method of Reilley. The analysis indicates that a structural change occurs between the terbium and dysprosium chelates which results in a larger pseudocontact component for the heavier lanthanide chelates but with no corresponding change in the scalar hyperfine coupling constant. These results show that the Reilley method of separation is preferred even in slowly exchanging, nonaxial lanthanide chelate systems and the temperature- dependent separation is unacceptable.