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 Title: METAL ION INDUCED PAIRING OF CYTOSINE BASES: FORMATION OF I-MOTIF STRUCTURES IDENTIFIED BY IR ION SPECTROSCOPY Author(s): Oomens, J. Contributor(s): Berden, Giel; Gao, Juehan Subject(s): Mini-symposium: Accelerator-Based Spectroscopy Abstract: While the Watson-Crick structure of DNA is among the most well-known molecular structures of our time, alternative base-pairing motifs are also known to occur, often depending on base sequence, pH, or presence of cations. Pairing of two cytosine (C) bases induced by the sharing of a single proton (C-H$^+$-C) gives rise to the so-called $i$-motif, occurring particularly in the telomeric region of DNA, and particularly at low pH. At physiological pH, silver cations were recently suggested to form cytosine dimers in a C-Ag$^+$-C structure analogous to the hemiprotonated cytosine dimer, which was later confirmed by IR spectroscopy.$^1$ Here we investigate whether Ag$^+$ is unique in this behavior. Using infrared action spectroscopy employing the free-electron laser FELIX and a tandem mass spectrometer in combination with quantum-chemical computations, we investigate a series of C-M$^+$-C complexes, where M is Cu, Li and Na. The complexes are formed by electrospray ionization (ESI) from a solution of cytosine and the metal chloride salt in acetonitrile/water. The complexes of interest are mass-isolated in the cell of a FT ion cyclotron resonance mass spectrometer, where they are irradiated with the tunable IR radiation from FELIX in the 600 - 1800 wn range. Spectra in the H-stretching range are obtained with a LaserVision OPO. Both experimental spectra as well as theoretical calculations indicate that while Cu behaves as Ag, the alkali metal ions induce a clearly different dimer structure, in which the two cytosine units are parallelly displaced. In addition to coordination to the ring nitrogen atom, the alkali metal ions coordinate to the carbonyl oxygen atoms of both cytosine bases, indicating that the alkali metal ion coordination favorably competes with hydrogen bonding between the two cytosine sub-units of the $i$-motif like structure. {small 1. Berdakin, Steinmetz, Maitre, Pino, J. Phys. Chem. A 2014, 118, 3804} Issue Date: 25-Jun-15 Publisher: International Symposium on Molecular Spectroscopy Citation Info: ACS Genre: CONFERENCE PAPER/PRESENTATION Type: Text Language: English URI: http://hdl.handle.net/2142/79245 Date Available in IDEALS: 2016-01-05
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