The complexation properties towards Bi 3 + of benzoazacrown ligands with five (H 3 BA3A) and six (H 4 BATA) heteroatoms and corresponding number of acetic arms have been investigated. Complexation constants were determined via potentiometric titration for BA3ABi complex (logβ = 26.2) and refined for BATABi complex (logβ = 31.7). According to NMR, X-ray and EXAFS results, chelation of cation by H 3 BA3A appears to be outside the macrocyclic cavity both in the solid state and in the aqueous solution. This out-cage location despite the high stability constant causes serum and in vivo instability of complex of smaller azacrown H 3 BA3A in contrast to H 4 BATA. Whereas the larger macrocycle and four pendant arms of H 4 BATA provide long-range coordination effectively shielding the cation thereby ensuring high inertness in vitro, low accumulation of the radionuclide in the organs, and fast clearance.
Radiolabeled macrocyclic zinc complexes are more stable in serum than analogous copper compounds; an azacrown-derived cycle with five heteroatoms coordinates zinc, providing fast complexation and high in vivo stability.
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