INTERACTION OF ANTICANCEROUS DRUG DAUNOMYCIN WITH NUCLEIC - ACIDS

Abstract

Nature has evolved a diverse set of antibiotics that bind to DNA in a variety of ways, but with the common ability to act as potent inhibitors of DNA transcription and replication. As a consequence, these natural products have been of considerable interest as potential anti cancer agents. Many synthetic compounds have been added to this list in the search for more potent drugs for use in chemotherapy. While it is appreciated that DNA is a primary target for many potent antitumor agents, data that pin-point the exact mechanism of action are generally unavailable. A substantial body of research has been directed towards understanding the molecular basis for DNA sequence specificity for binding, by identifying the preferred binding sequences of many key drugs with DNA. Structural tools such as x-ray crystallography and NMR spectroscopy, coupled with molecular modelling techniques have had considerable impact in advancing our understanding of the microscopic structural heterogeneity of DNA and the molecular basis for drug-DNA interactions. Daunomycin is an anticancer antibiotic isolated from Streptomyces peucetius; active mainly against acute lymphocytic leukemia. It inhibits in vitro growth of both normal and cancer cell lines and as a consequence nuclear damage occurs. Chromosomal and genetic aberrations of several types are produced by daunomycin. KV The present study has been undertaken to get a new insight of the conformational features of daunomycin in solution using one-dimensional and two-dimensional NMR techniques at 500 MHz The three-dimensional structure obtained by spin-spin coupling constraints, interproton distances has been compared with that obtained by x-ray crystallography. The conformation of deoxyhexanucleotide d-(TGATCA) is investigated in D_0. Geometry * 2 of the deoxyribose sugar of each residue is deduced using double quantum filter COSY (DQF) cross peak patterns, spin-spin coupling constants and sums of couplings obtained from one-dimensional NMR. The relative intensities of cross peaks obtained in NOESY spectra recorded at various mixing times have been used independently to deduce the geometry of deoxyribose sugar. The base to sugar proton NOE connectivities have been used to get information on glycosidic bond rotation. The complex of daunomycin and d-(TGATCA)2 has been obtained at various drug to DNA concentration ratio by adding daunomycin to d-(TGATCA)2 in steps to achieve 2:1 complex. Titration studies have been carried out using ID proton NMR. While two-dimensional NMR technique serves as a tool to assign all proton NMR signals unambiguously and determine the conformational features such as sugar puckering, helix sense, glycosidic bond rotation in complexed form. The changes in chemical shifts are attributed to stacking, alterations in base-base overlap etc. The NOE connectivities within the drug and DNA molecules are used to obtain the change in conformation of DNA and daunomycin due to interaction. Several intermolecular NOEs between protons of ring D and base protons of 5'-3' d-TpG step are direct proof of intercalation of aglycon chromophore of the drug between base pairs. Several intermolecular NOEs between daunosamine sugar protons and A3 and T4 residues of d-(TGATCA) indicate the proximity of sugar protons to the third base pair in the sequence. The results are discussed with a view to understand the implications of binding.