We showed that biologically important planar Hyp*center dot Hyp base pair (C-s) formed by the enol and keto tautomers of the hypoxanthine tautomerises via the synchronous concerted mechanism through the TS (C-2v). The five key points were detected and completely investigated along the IRC of the Hyp*center dot Hyp <-> Hyp center dot Hyp* tautomerisation via the DPT. It was found that intermolecular antiparallel O6H center dot center dot center dot O6 and N1H center dot center dot center dot N1 H-bonds are cooperative and mutually reinforce each other. It was proved that the Hyp*center dot Hyp/Hyp center dot Hyp* base pair is dynamically stable structure with a lifetime 8.2 x 10(-12) s and all its six low-frequency intermolecular vibrations are able to develop during this period of time. (C) 2013 Elsevier B. V. All rights reserved.