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

A series of new mono- and di-cationic 4,9-diazapyrenium derivatives were sy nthesized, for which in vitro anticancer activity has been demonstrated alr eady. Their spectroscopic (NMR, electronic absorption and fluorescence) pro perties and the influence of positive charges, aromatic surface and substit uents on the formation of stacked complexes with major nucleotides in water (this article) and on their interaction with nucleic acids (following arti cle 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 equi librium constants, expressed as pK(DMOH), were determined. Binding of nucle otides (AMP, ADP, ATP, GMP and CMP) was studied in buffered (pH 5) aqueous solution by fluorescence. The formation of stacked nucleic base-diazapyreni um complexes with 1 : 1 stoichiometry (log K-s 1.6-2.8) was observed. The s tability constants were found to be independent of nucleotide charge, showi ng the dominance of aromatic stacking over coulombic interactions in such c omplexes. The presence of 6-phenyl and 5,10-diphenyl substituents on the ph enanthridinium and 4,9-diazapyrenium systems, respectively, diminish nucleo tide binding in both cases. The larger aromatic surface of the 4,9-diazapyr enium system, relative to the phenanthridinium (log K-s < 1 to 2.3), does n ot considerably enhance nucleotide binding by the former. The nucleotide bi nding is somewhat stronger compared to neutral pyrene (log K-s 1.1-1.7) pos sessing a comparable aromatic surface. Singly charged 4,9-diazapyrenium der ivatives 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.