A new fluorescent chelator Oxa, having two metal-binding sites, was designed and synthesized in six steps. Oxa exhibited two distinctive dissociation constants for Zn2+ (K-d(1) = 1.50 muM and K-d(2) = 140 muM), with considerable fluorescence increase in aqueous buffer at pH 7.2. Affinities of Oxa for the other biologically important ions such as Ca (Kd' = 250 VM and Kd 275 mM) and (K-d(1) - 975 muM and K-d(2) could not be determined) in the same conditions were also obtained. (C) 2004 Elsevier Ltd. All rights reserved.