Effect of intercalator substituent and nucleotide sequence on the stability of DNA- and RNA-naphthalimide complexes.

DNA intercalators are commonly used as anti-cancer and anti-tumor agents. As a result, it is imperative to understand how changes in intercalator structure affect binding affinity to DNA. Amonafide and mitonafide, two naphthalimide derivatives that are active against HeLa and KB cells in vitro, were previously shown to intercalate into DNA. Here, a systematic study was undertaken to change the 3-substituent on the aromatic intercalator 1,8-naphthalimide to determine how 11 different functional groups with a variety of physical and electronic properties affect binding of the naphthalimide to DNA and RNA duplexes of different sequence compositions and lengths. Wavelength scans, NMR titrations, and circular dichroism were used to investigate the binding mode of 1,8-naphthalimide derivatives to short synthetic DNA. Optical melting experiments were used to measure the change in melting temperature of the DNA and RNA duplexes due to intercalation, which ranged from 0 to 19.4°C. Thermal stabilities were affected by changing the substituent, and several patterns and idiosyncrasies were identified. By systematically varying the 3-substituent, the binding strength of the same derivative to various DNA and RNA duplexes was compared. The binding strength of different derivatives to the same DNA and RNA sequences was also compared. The results of these comparisons shed light on the complexities of site specificity and binding strength in DNA-intercalator complexes. For example, the consequences of adding a 5'-TpG-3' or 5'-GpT-3' step to a duplex is dependent on the sequence composition of the duplex. When added to a poly-AT duplex, naphthalimide binding was enhanced by 5.6-11.5°C, but when added to a poly-GC duplex, naphthalimide binding was diminished by 3.2-6.9°C.