Kinetic studies on 5’-modified d(TGGGAG) endowed with prominent anti-HIV activity by ESI-MS

In ‘90s, Hotoda et al. identified a variety of tetramolecular G-quadruplexes based on the sequence d(TGGGAG) and modified at the 5’-end with aromatic groups, targeting the glycoprotein of HIV-1, gp120. In 2011 Di Fabio et al. reported the synthesis and characterization of a new mini-library of d(TGGGAG) ODNs carrying aryl groups at the 5’-end through a phosphodiester bond and endowed with prominent anti-HIV activity[2]. In order to have more information on the G-quadruplex folding depending on the modifications at 5’-end of d(TGGGAG), we have explored the kinetic aspects involved into G-quadruplex folding using electrospray mass spectrometry (ESI-MS). The studies are conducted on more active 5’-end modified ODNs in comparison with unmodified sequence d(TGGGAG). By ESI-MS experiments, we could obtain information about the number of strands, and the inner cations for each complex, moreover it is possible to detect all possible intermediate complexes and to determine their equilibrium binding constants. We followed the kinetics of G-quadruplex formation during 14 days in the presence of an internal reference (dT6), selecting some time points after addition of different buffers (150mM NH4OAc and 150mM TMAA/1mM KCl). Overall data, have reported a rapid conversion of the single strand into dimer and trimer, and slower conversion in a tetramer and into an unexpected G4-supramolecular structure: the octamer complex. The formation of octamer is confirmed also in solution by native PAGE.