LAUSR

Lanthanide complexes have found extensive use as luminescent probes for biological and medical investigations. Recently, a di-europium complex that exhibits pH-dependent luminescence-decay was reported, and the ligand in that complex includes a large number of ionizable sites. To better understand the pH-dependence of luminescence-decay of this complex, the pKa's of all tautomers of the di-Lu3+ version of this complex were calculated computationally. The calculated Boltzmann-averaged pKa's of the complex are 5.85, −0.21, and −1.47 for the di-Lu3+ complex in its first, second, and third protonation states, respectively. These pKa values across protonation states indicate that changes in luminescence-decay rate at physiologically relevant pH may be related to first protonation event of the complex exclusively.