title: Deprotonated guanine·cytosine and 9-methylguanine·cytosine base pairs and their "non-statistical" kinetics: a combined guided-ion beam and computational study.
abstract: We report a guided-ion beam mass spectrometric study on collision-induced dissociation (CID) of deprotonated guanine(G)·cytosine(C) base pairs and their 9-methylguanine (9MG) analogue with Xe, including measurements of product cross sections as a function of collision energy and determination of dissociation thresholds. DFT, RI-MP2 and DLPNO-CCSD(T) calculations and Rice-Ramsperger-Kassel-Marcus (RRKM) modeling were performed to elucidate structures and kinetics. The experiment and theoretical study have provided considerable insight into tautomerization, intra-base-pair proton transfer and dissociation of deprotonated G·C and 9MG·C. In contrast to the previously reported lowest-energy deprotonated base pair structure G·[C-H1]-that consists of H-bonded neutral guanine and N1-deprotonated cytosine, we found that proton transfer from guanine N1 to cytosine N3 within G·[C-H1]-(or 9MG·[C-H1]-) leads to another slightly more stable conformer denoted as G·[C-H1]-_PT1 (or 9MG·[C-H1]-_PT1). The conventional (non-proton-transferred) and the proton-transferred conformers are close in energy and interconvert quickly, but they can be distinguished by dissociation products. The conventional structure dissociates into deprotonated cytosine and neutral guanine, while the other dissociates into deprotonated guanine and neutral cytosine. The two dissociation asymptotes have similar threshold energies, but surprisingly the CID product mass spectra of deprotonated G·C and 9MG·C are both overwhelmingly dominated by deprotonated G or 9MG, with their branching ratios greater than RRKM predictions by one to two orders of magnitude. The proton-transferred structures of deprotonated base pairs and the "unexpected" non-statistical kinetics provide new leads for understanding purine-pyrimidine interactions, forming rare nucleobase tautomers, and base pair opening.
authors.affiliation: Department of Chemistry and Biochemistry, Queens College of the City University of New York, 65-30 Kissena Blvd., Queens, NY 11367, USA. jianbo.liu@qc.cuny.edu and Ph.D. Program in Chemistry, The Graduate Center of the City University of New York, 365 5th Ave., New York, NY 10016, USA.
authors.forename: Wenchao
authors.initials: W
authors.lastname: Lu
authors.affiliation: Department of Chemistry and Biochemistry, Queens College of the City University of New York, 65-30 Kissena Blvd., Queens, NY 11367, USA. jianbo.liu@qc.cuny.edu and Ph.D. Program in Chemistry, The Graduate Center of the City University of New York, 365 5th Ave., New York, NY 10016, USA.
authors.forename: Jianbo
authors.identifier: http://orcid.org/0000-0001-9577-3740
authors.initials: J
authors.lastname: Liu
mesh_terms: D020029:Base Pairing; D003596:Cytosine / Q000737:chemistry*; D006147:Guanine / Q000031:analogs & derivatives* / Q000737:chemistry*; D006860:Hydrogen Bonding; D007700:Kinetics; D011522:Protons
chemical_list: D011522:Protons; C116701:9-methylguanine; D006147:Guanine; D003596:Cytosine