Effect of osmolytes-TMAO and serine on the binding and stability of 5-fluorouracil with calf thymus DNA (CT-DNA): biophysical insight.

Osmolytes play major role in stabilization of macromolecules like proteins and nucleic acids. The major force responsible for stabilization of these macromolecules are preferential binding and preferential exclusion or hydration. Here, we have determined intermolecular interactions of 5-fluorouracil (5FU) with calf thymus DNA (CT-DNA) with addition of osmolytes trimethylamine oxide (TMAO) and serine (Ser). The thermal stability and conformational changes of CT-DNA with 0.08 mM TMAO and 0.08 mM Ser showed that they stabilize DNA. It has been observed that 0.5 M TMAO and 0.5 M Ser showed more stability as compared to high molality (2 M). TMAO and Ser interact with CT-DNA via electrostatic and hydrogen bond interactions. The binding constant values of 5FU increased with TMAO due to hydration effects, whereas in the presence of serine, it decreased due to presence of common residue at binding site of CT-DNA. ITC data showed that binding mode of 5FU with CT-DNA with addition of TMAO and Ser changes from two and three sequential mode to independent one. The presence of ethidium bromide at intercalation and osmolytes at minor groove site, reduces the binding affinity of 5FU from 103 to 101 M-1. From docking it has been observed that 5FU intercalates in between bases of CT-DNA by forming hydrogen bonds and electrostatic interactions with phosphate groups of DNA when TMAO or Ser added to it. The study of 5FU-osmolyte-DNA interaction using spectroscopic, calorimetric and docking methods is crucial for elucidating the underlying mechanisms of ligand-macromolecule binding and optimizing chemotherapy treatments at primary level.