Comparison of Lanthanide Macrocyclic Complexes as 23 Na NMR Sensors.

Nuclear magnetic resonance (NMR) agents, composed of paramagnetic lanthanide ions (Ln 3+ ) complexed with negatively charged cyclic chelating agents (Che ( n +3)- ) forming polyanionic lanthanide complexes (LnChe n - ), perturb sodium-23 ( 23 Na) signals, a phenomenon which depends sodium ions (Na + ) exchanging with LnChe n - . We analyzed 23 Na shiftability and broadening due to hyperfine and bulk magnetic susceptibility (BMS) effects that arise from LnChe n - designs using selective Ln 3+ ions ( i.e. , thulium, Tm 3+ ; gadolinium, Gd 3+ ; and europium, Eu 3+ ) and macrocyclics derived from 1,4,7,10-tetraazacyclododecane (cyclen) [ i.e. , with phosphonate (DOTP 8- ) and carboxylate (DOTMA 4- ) arms] and 1,4,7-triazacyclononane (TACN) [ i.e. , with phosphonate (NOTP 6- ) arms]. All LnChe n - complexes showed downfield shifts, but Gd 3+ and Tm 3+ agents, respectively, were dominated by BMS and hyperfine effects, in good agreement with theory. While 23 Na shiftability and broadening were minimally affected by pH and competing cations (K + , Ca 2+ , and Mg 2+ ) within physiological ranges, the 23 Na shiftability and broadening were most sensitive to LnChe n - concentration in relation to the interstitial Na + level in vivo . Greatest 23 Na shiftability and broadening were obtained with Tm 3+ and Gd 3+ agents, respectively. While BMS contribution to shiftability was most impacted by the number of unpaired electrons on Ln 3+ , negative charge on LnChe n - regulated Na + exchange for line broadening. In brain tumor models, TmDOTP 5- with 23 Na-NMR has been used previously to separate Na + in intracellular, blood, and interstitial pools, while evidence here shows that GdDOTP 5- can distinguish Na + within intracellular and extracellular ( i.e. , blood and interstitial) pools. Given the biological importance of Na + in vivo , future macrocyclic designs of LnChe n - should be sought for 23 Na-NMR biomedical applications.