Effects of Intra-Base Pair Proton Transfer on Dissociation and Singlet Oxygenation of 9-Methyl-8-Oxoguanine-1-Methyl-Cytosine Base-Pair Radical Cations.

8-Oxoguanosine is the most common oxidatively generated base damage and pairs with cytidine within duplex DNA.  The 8-oxoguanosine-cytidine lesion, if not recognized and removed, not only leads to G-to-T transversion mutations but renders the base pair being more vulnerable to the ionizing radiation and singlet oxygen (1O2) damage.  Herein, reaction dynamics of a prototype Watson-Crick base pair [9MOG·1MC]·+, consisting of 9-methyl-8-oxoguanine radical cation (9MOG·+) and 1-methylcystosine (1MC), was examined using mass spectrometry coupled with electrospray ionization.  We first detected base-pair dissociation in collisions with the Xe gas, which provided insight into intra-base pair proton transfer of 9MOG·+·1MC[[EQUATION]][9MOG - HN1]··[1MC + HN3']+ and subsequent non-statistical base-pair separation.  We then measured the reaction of [9MOG·1MC]·+ with 1O2, revealing the two most probable pathways, C5-O2 addition and HN7-abstraction at 9MOG.  Reactions were entangled with the two forms of 9MOG radicals and base-pair structures as well as multi-configurations between open-shell radicals and 1O2 (that has a mixed singlet/triplet character).  These were disentangled by utilizing approximately spin-projected density functional theory, coupled-cluster theory and multi-referential electronic structure modeling.  The work delineated base-pair structural context effects and determined relative reactivity toward 1O2 as [9MOG - H]· > 9MOG·+ > [9MOG - HN1]··[1MC + HN3']+ ≥ 9MOG·+·1MC.