A Comprehensive Study of the Ca2+ Ion Binding of Fluorescently Labelled BAPTA Analogues

Abstract Since its development, the ionophore BAPTA (1,2‐bis(2‐aminophenoxy)‐ethane‐N,N,N’,N’‐tetraacetic acid) has been used unchanged in calcium sensing applications. In this work we present a comprehensive experimental and theoretical study of novel alterations in the structure of BAPTA, with a focus on the systematic modification of the chain connecting the two aromatic rings of the molecule (denoted as “linker”). A bis‐(diethylamino)xantene fluorophore was also attached to the structures in a fixed position and the structure‐fluorescence response relationship of these molecules was investigated in addition. The effect of the linker's length, the number of oxygen atoms in this chain and even the removal of one of the rings was tested; these all proved to significantly alter the characteristics of the compounds. For example, it was found that the second aromatic ring of BAPTA is not essential for the turn‐on of the fluorescence. We also demonstrated that successful sensing can be realized even by replacing the chain with a single oxygen atom, which suggests the availability of a new calcium binding mode of the chelator. The reliable turn‐on characteristic, the steep Ca 2+ fluorescence titration curve and the intense fluorescence emission combine to make this compound a prospective candidate as a calcium sensing molecular probe in diagnostic neurobiological applications.