Molar absorption coefficients and stability constants of Zincon metal complexes for determination of metal ions and bioinorganic applications

Abstract Zincon (ZI) is one of the most common chromophoric chelating probes for the determination of Zn 2 + and Cu 2 + ions. It is also known to bind other metal ions. However, literature data on its binding properties and molar absorption coefficients are rather poor, varying among publications or determined only in certain conditions. There are no systematic studies on Zn 2 + and Cu 2 + affinities towards ZI performed under various conditions. However, this widely commercially available and inexpensive agent is frequently the first choice probe for the measurement of metal binding and release as well as determination of affinity constants of other ligands/macromolecules of interest. Here, we establish the spectral properties and the stability of ZI and its complexes with Zn 2 + , Cu 2 + , Cd 2 + , Hg 2 + , Co 2 + , Ni 2 + and Pb 2 + at multiple pH values from 6 to 9.9. The obtained results show that in water solution the MZI complex is predominant, but in the case of Co 2 + and Ni 2 + , M(ZI) 2 complexes are also formed. The molar absorption coefficient at 618 nm for ZnZI and 599 nm for CuZI complexes at pH 7.4 in buffered ( I  = 0.1 M) water solutions are 24,200 and 26,100 M − 1  cm − 1 , respectively. Dissociation constants of those complexes are 2.09 × 10 − 6 and 4.68 × 10 − 17  M. We also characterized the metal-assisted Zincon decomposition. Our results provide new and reassessed optical and stability data that are applicable to a wide range of chemical and bioinorganic applications including metal ion detection, and quantification and affinity studies of ligands of interest. Synopsis Accurate values of molar absorption coefficients of Zincon complex with Zn 2 + , Cd 2 + , Hg 2 + , Co 2 + , Ni 2 + , Cu 2 + , and Pb 2 + for rapid metal ion quantification are provided. Zincon stability constants with Zn 2 + and Cu 2 + in a wide pH range were determined.