Cr3+ Binding to DNA Backbone Phosphate and Bases: Slow Ligand Exchange Rates and Metal Hydrolysis

Abstract

The interaction between chromium ions and DNA is of great interest in inorganic chemistry, toxicology, and analytical chemistry. Most previous studies focused on in situ reduction of Cr(VI), producing Cr3+ for DNA binding. Recently, Cr3+ was reported to activate the Cel3d DNAzyme for RNA cleavage. Herein, the Ce13d is used to study two types of Cr3+ and DNA interactions. First, Cr3+ binds to the DNA phosphate backbone weakly through reversible electrostatic interactions, which is weakened by adding competing inorganic phosphate. However, Cr3+ coordinates with DNA nucleobases forming stable cross-links that can survive denaturing gel electrophoresis condition. The binding of Cr3+ to different nucleobases was further studied in terms of binding kinetics and affinity by exploiting carboxyfluorescein-labeled DNA homopolymers. Once binding takes place, the stable Cr3+/DNA complex cannot be dissociated by EDTA, attributable to the ultraslow ligand exchange rate of Cr3+. The binding rate follows the order of G > C > T approximate to A. Finally, Cr3+ gradually loses its DNA binding ability after being stored at neutral or high pH, attributable to hydrolysis. This hydrolysis can be reversed by lowering the pH. This work provides a deeper insight into the bioinorganic chemistry Of Cr3+ coordination with DNA, clarifies some inconsistency in the previous literature, and offers practically useful information for generating reproducible results.