The structures and isomerization reactions of 5-methylcytosine-BH₃ complexes have been investigated by the B3LYP and MP2 methods with 6-311＋G(d) basis set. 6 complexes and 4 transition states were located. Natural bond orbital analysis was performed to reveal the origin of intermolecular interaction. To obtain the accurate binding energies and potential energy surface, single point energy calculations and BSSE correction were performed. The results indicate that a, b₁, b₂ and c with BH₃ directly combining N(3), O or N(4) are stable complexes with the binding energies of 104.58, 92.25, 63.49 and 43.41 kJ•mol^－1, respectively. The complexes may be formed with BH₃ directly combining to 5-methylcytosine as well as with BH₃ entirely transferring on 5-methylcytosine-BH₃. The influence of cytosine methylation was not obvious on stabilization orders and the energy barriers of isomerization.

Key words: B3LYP, MP2, 5-methylcytosine, BH₃, complex, isomerization