4,9-Diazapyrenium cations. Synthesis, physico-chemical properties and binding of nucleotides in water

Abstract

A series of new mono- and di-cationic 4,9-diazapyrenium derivatives were synthesized, for which in vitro anticancer activity has been demonstrated already. Their spectroscopic (NMR, electronic absorption and fluorescence) properties and the influence of positive charges, aromatic surface and substituents on the formation of stacked complexes with major nucleotides in water (this article) and on their interaction with nucleic acids (following article in this issue) was investigated. A reversible pH dependent formation of 5-hydroxy-4,9-diazapyrenium mono-pseudobase (DMOH) was observed for mono- and di-cationic 4,9-diazapyrenium derivatives in aqueous solution. The equilibrium constants, expressed as pK_DMOH, were determined. Binding of nucleotides (AMP, ADP, ATP, GMP and CMP) was studied in buffered (pH 5) aqueous solution by fluorescence. The formation of stacked nucleic base–diazapyrenium complexes with 1∶1 stoichiometry (log K_s 1.6–2.8) was observed. The stability constants were found to be independent of nucleotide charge, showing the dominance of aromatic stacking over coulombic interactions in such complexes. The presence of 6-phenyl and 5,10-diphenyl substituents on the phenanthridinium and 4,9-diazapyrenium systems, respectively, diminish nucleotide binding in both cases. The larger aromatic surface of the 4,9-diazapyrenium system, relative to the phenanthridinium (log K_s <1 to 2.3), does not considerably enhance nucleotide binding by the former. The nucleotide binding is somewhat stronger compared to neutral pyrene (log K_s 1.1–1.7) possessing a comparable aromatic surface. Singly charged 4,9-diazapyrenium derivatives bind nucleotides with a strength comparable to doubly charged 2,7-diazapyrenium derivatives. Some diazapyrenium derivatives exhibit high AMP/CMP selectivity (13, 11.7 and 15, 9.3) and moderate GMP/CMP selectivity (13, 6.0 and 12, 5.5). The observed selectivities are considerably higher than those exhibited by pyrene, ethidium or 2,7-diazapyrenium.