title: Protonated adenine: tautomers, solvated clusters, and dissociation mechanisms.
abstract: Ab initio and density functional theory calculations at the B3-MP2 and CCSD(T)/6-311 + G(3df,2p) levels of theory are reported that address the protonation of adenine in the gas phase, water clusters, and bulk aqueous solution. The calculations point to N-1-protonated adenine (1+) as the thermodynamically most stable cationic tautomer in the gas phase, water clusters, and bulk solution. This strongly indicates that electrospray ionization of adenine solutions produces tautomer 1+ with a specificity calculated as 97-90% in the 298-473 K temperature range. The mechanisms for elimination of hydrogen atoms and ammonia from 1+ have also been studied computationally. Ion 1+ is calculated to undergo fast migrations of protons among positions N-1, C-2, N-3, N-10, N-7, and C-8 that result in an exchange of five hydrogens before loss of a hydrogen atom forming adenine cation radical at 415 kJ mol(-1) dissociation threshold energy. The elimination of ammonia is found to be substantially endothermic requiring 376-380 kJ mol(-1) at the dissociation threshold and depending on the dissociation pathway. The overall dissociation is slowed by the involvement of ion-molecule complexes along the dissociation pathways. The competing isomerization of 1+ proceeds by a sequence of ring opening, internal rotations, imine flipping, ring closures, and proton migrations to effectively exchange the N-1 and N-10 atoms in 1+, so that either can be eliminated as ammonia. This mechanism explains the previous N-1/N-10 exchange upon collision-induced dissociation of protonated adenine.
authors.affiliation: Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, USA. turecek@chem.washington.edu
authors.forename: Frantisek
authors.initials: F
authors.lastname: Turecek
authors.forename: Xiaohong
authors.initials: X
authors.lastname: Chen
mesh_terms: D000225:Adenine / Q000032:analysis* / Q000737:chemistry*; D000465:Algorithms; D003198:Computer Simulation; D019281:Dimerization; D008956:Models, Chemical; D008958:Models, Molecular; D011522:Protons; D012997:Solvents / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*
chemical_list: D011522:Protons; D012997:Solvents; D000225:Adenine