Application of electrospray ionization mass spectrometry for the evaluation of alkaloids binding to G-quadruplex of HIV-1 integrase inhibitors

The G-rich oligonucleotide T30695 and T30177 with the sequence of d(GGGTGGGTGGGTGGGT) and d(GTGGTGGGTGGGTGGGT) can inhibit HIV-1 IN activity at nanomolar concentration levels. The formation of G-quadruplex of T30695 and T30177 as well as the interaction of two natural alkaloids berberine and sanguinarine with G-quadruplex were investigated using circular dichroism spectrometry and electrospray ionization-mass spectrometry. The results indicated that T30695 and T30177 could fold to form intramolecular G-quadruplex at low concentrations of NH4OAc. As the concentration of NH4OAc increased to 40 mM, the intramolecular G-quadruplex could be completely transformed to dimeric G-quadruplex for T30695, however, this concentration needed to be increased to 60 mM for T30177. The inducing study illustrated that berberine and sanguinarine could induce T30695 and T30177 to form intramolecular G-quadruplex but not dimeric G-quadruplex. Berberine and sanguinarine could bind to intramolecular G-quadruplex, and the relative binding affinity of sanguinarine was greater than that of berberine for T30695, but the two alkaloids showed nearly identical relative binding affinities for T30177. Berberine and sanguinarine could also bind to dimeric G-quadruplex, and the relative binding affinity of berberine was greater than that of sanguinarine. Both berberine and sanguinarine exhibited good affinities toward intramolecular and dimeric G-quadruplex, which may be significant in the process of G-quadruplex formation against HIV.