physical parameters and biological stability of yttriumiii diethylenetriaminepentaacetic acid derivative conjugates the solution equilibria acid dissociation and serum stability of a series of yiii complexes of dtpa ligands functionalized with pnitrobenzyl methyl and transcyclohexyl substituents were studied the thermodynamic stability of the complexes studied ranged from log k 2153 to 247 acid dissociation rates were found to decrease as the substitution on the carbon backbone increased and significant differences in dissociation rates were observed for the yiii complexes of a pair of diasteriomeric cyclohexyldtpa ligands while one diastereomer was found to have the slowest acid dissociation rate of the entire dtpa series it was remarkably labile in both serum stability and in vivo studies