Acta Phys. -Chim. Sin. ›› 2012, Vol. 28 ›› Issue (12): 2803-2808.doi: 10.3866/PKU.WHXB201209284


Semiclassical Dynamics Simulation and CASSCF Calculation for 5-Methyl Cytosine and Cytosine

YUAN Shuai, MA Jing, ZHANG Wen-Ying, SHU Kun-Xian, DOU Yu-Sheng   

  1. Institute of Bioinformatics, Chongqing University of Posts and Telecommunications, Chongqing 400065, P. R. China
  • Received:2012-08-24 Revised:2012-09-28 Published:2012-11-14
  • Supported by:

    The project was supported by the National Natural Science Foundation of China (21073242), Natural Science Foundation of Chongqing, China (CSTC2011jjA00009, CSTC2009BB5419), and Project of the Science Technology Foundation of Chongqing Education Committee, China (KJ120516).


We performed a semiclassical dynamics simulation study of the photophysical deactivation of 5m-cytosine (5m-Cyt) and cytosine (Cyt) induced by ultraviolet radiation of 267 nm. The results show that deactivation of the excited state of 5m-Cyt and Cyt results from the distortion of the C5-C6 bond and the out-of-plane vibration of methyl (or H5) and the H6 atom. A so-called“biradical state”, in which the methyl (or H5) and H6 atoms are nearly perpendicular to the average ring plane and displaced in opposite directions, is formed at the decay point. The vibration frequency of the methyl derivative is less than that of the H atom derivative because of its increased volume relative to the H atom. The results indicated that molecular deformation at the C5 atom of 5m-Cyt will be weakened and will result in a longer excited lifetime of 5m-Cyt. Complete active space self-consistent field (CASSCF) calculations show that the energy of the conical intersection (CI) of 5m-Cyt is 0.3 eV higher than that of Cyt. This suggests that promotion to the CI point for 5m-Cyt requires the molecule to overcome a larger energetic barrier, which results in a longer excited state lifetime than Cyt.

Key words: 5-Methylcytosine, Cytosine, Semiclassical dynamic simulation, CASSCF, Lifetime of excited state


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