Synthesis and spectroscopic characterization of site-specific 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine oligodeoxyribonucleotide adducts

The aim of the present study is to determine the chemical structure and con formation of DNA adducts formed by incubation of the bioactive form of 2-am ino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhlP), N-acetoxy-PhlP, with a single-stranded 11mer oligodeoxyribonucleotide. Using conditions optimized to give the C8-dG-PhlP adduct as the major product, sufficient material was synthesized for NMR solution structure determination. The NMR data indicat e that in duplex DNA this adduct exists in equilibrium between two differen t conformational states. In the main conformer, the covalently bound PhlP m olecule intercalates in the helix, whilst in the minor conformation the Phl P ligand is probably solvent exposed. In addition to the C8-dG-PhlP adduct, at least eight polar adducts are found after reaction of N-acetoxy-PhlP wi th the oligonucleotide. Three of these were purified for further characteri zation and shown to exhibit lowest energy UV absorption bands in the range 342-347 nm, confirming the presence of PhlP or PhlP derivative. Accurate ma ss determination of two of the polar adducts by negative ion MALDI-TOF MS r evealed ions consistent with a spirobisguanidino-PhlP derivative and a ring -opened adduct. The third adduct, which has the same mass as the C8-dG-PhlP oligonucleotide adduct, may contain PhlP bound to the N-2 position of guan ine.