Infrared Spectrum of the Ag+–(Pyridine)2 Ionic Complex: Probing Interactions in Artificial Metal-Mediated Base Pairing

Abstract

The isolated pyridine–Ag+–pyridine unit (PyAg+Py) is employed as a model system to characterize the recently observed Ag+-mediated base pairing in DNA oligonucleotides at the molecular level. The structure and infrared (IR) spectrum of the Ag+Py2 cationic complex are investigated in the gas phase by IR multiple-photon dissociation (IRMPD) spectroscopy and quantum chemical calculations to determine the preferred metal-ion binding site and other salient properties of the potential-energy surface. The IRMPD spectrum has been obtained in the 840–1720 cm−1 fingerprint region by coupling the IR free electron laser at the Centre Laser Infrarouge d’Orsay (CLIO) with a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer equipped with an electrospray ionization source. The spectroscopic results are interpreted with quantum chemical calculations conducted at the B3LYP/aug-cc-pVDZ level. The analysis of the IRMPD spectrum is consistent with a σ complex, in which the Ag+ ion binds to the nitrogen lone pairs of the two Py ligands in a linear configuration. The binding motif of PyAg+Py in the gas phase is the same as that observed in Ag+-mediated base pairing in solution. Ag+ bonding to the π-electron system of the aromatic ring is predicted to be a substantially less-favorable binding motif.