What are the Differently Paired Forms of Adenine (A)-Thymine (T)?

Some tautomers of adenine (A) and thymine (T) are selected to study the existence of differently paired TA pairs unlike Watson Crick (WC) AT. In this investigation DFT (B3LYP/6-31+G (d,p)) calculations are carried out to obtain the optimum structures of all tautomers and tautomer TA pairs. Schematic studies have been performed to analyse how the effect of proton on basic sites of A and T can destabilize the Watson Crick (WC) AT. Certain basic sites of A and T acquire strong affinity for proton that may lead to destabilization of hydrogen bonds in WC AT. Several tautomer TA pairs formed due to paring of A and T tautomers through H-bonding in a different manner are found quite stable. These tautomer TA pairs are formed through hydrogen bonding between compatible counter tautomers. So, destabilization of WC AT usually generates less stable tautomers of A and T, which in turn may combine to form quite stable tautomer TA pairs. Moreover, direct reaction pathways from WC AT to tautomer TA pairs are not feasible, since the reaction pathways pass through high energy barriers.