Fingerprints of both Watson-Crick and Hoogsteen isomers of isolated (Cytosine-Guanine)H+ pair

F. BERTHIAS; G. A. PINO; M. ROSSA; P. MAITRE; A. CRUZ ORTIZ; M. BERDAKIN

Carlos Paz

Encuentro; XVII ELAFOT; 2017

One decade after Watson and Crick (WC) discovered the double-helixstructure of DNA,[i] Hoogsteen (Hoo)[ii]reported a crystal structure inwhich the base pair had a different geometry to that reported by WC. WhileWC isomers are considered as the canonical paring forms in DNA transporting thegenetic information, Hoo isomers are associated to the formation of triplexrelated to several human diseases.[iii] Therefore, the development of simple techniques that allowidentifying the existence of WC or Hoo isomers is of a broad interest. In this we report a method to prepare the protonated Cytosine-Guanine(CGH+) pair in solution, either in the WC or Hoo isomeric structuresthat retain their structure after been transferred to the gas phase by ElectroSpray Ionization (ESI) where their finger prints are characterized by massspectrometry (MS) in an 7T FT-ICR Bruker Apex Qe mass spectrometer and by infraredmultiphoton dissociation (IRMPD) spectroscopy with the Free-Electron-Laser(FEL) at CLIO. Calculations at the DFT level are performed to help theinterpretation of the experimental results.Briefly, the mass fragmentation pattern as well asthe IRMPD spectra of the parent ion m/z = 263 (CGH+), stronglydepends on the pH of the solution (Figure 1), indicating that different isomersare produced. Therefore, the protonated CGH+ WC or Hoo isomers canbe selectively prepared in the solution and this structure is preserved uponvaporization in an ESI source. The MS and IRMPD fingerprints of each isomerallow to unequivocally assigning them. This is expected to be applicable as aneasy methodology based on MS and/or IRMPD to determine the existence of Hoopairs associated to diseases and mutations, in real samples.