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Isoguanine Formation from Adenine
Author(s) -
Cheng Qianyi,
Gu Jiande,
Compaan Katherine R.,
Schaefer Henry F.
Publication year - 2012
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201102415
Subject(s) - tautomer , chemistry , isomerization , dehydrogenation , enol , molecule , exothermic reaction , hydrogen bond , hydrogen , photochemistry , hydrogen atom , bond cleavage , computational chemistry , stereochemistry , catalysis , organic chemistry , alkyl
Several possible mechanisms underlying isoguanine formation when OH radical attacks the C 2 position of adenine (A C 2) are investigated theoretically for the first time. Two steps are involved in this process. In the first step, one of two low‐lying A C 2⋅⋅⋅OH reactant complexes is formed, leading to C 2 H 2 bond cleavage. Between the two reactant complexes there is a small isomerization barrier, which lies well below separated adenine plus OH radical. The complex dissociates to free molecular hydrogen and an isoguanine tautomer ( isoG 1 or isoG 2 ). The local and activation barriers for the two pathways are very similar. This evidence suggests that the two pathways are competitive. After dehydrogenation, there are two possible routes for the second step of the reaction. One is direct hydrogen transfer, via enol–keto tautomerization, which has high local barriers for both tautomers and is not favored. The other option is indirect hydrogen transfer involving microsolvation by one water molecule. The water lowers the reaction barrier by over 20 kcal mol −1 , indicating that water‐mediated hydrogen transfer is much more favorable. Both A+OH . →isoG+H . reactions are exothermic and spontaneous. Among four isoguanine tautomers, isoG 1 has the lowest energy. Our findings explain why only the N 1 H and O 2 H tautomers of isolated isoguanine and isoguanosine have been observed experimentally.