4. Conclusions

This paper presented for the first time accurate quantum mechanical calculations of the cyclobutane uracil dimer 1 as well as the corresponding radical cation 1^*+ and radical anion 1^*- . The puckering angle of the central cyclobutane ring is accurately described by these calculations. The position of the polar groups, which are likely to be important for binding of the substrate to the enzyme DNA photolyase, are largely determined by this angle. The calculations support the binding mode suggested by Park et.al.[4] The radical cation 1^*+ has a symmetric structure with a planarized cyclobutane ring and the charge and spin density delocalized over both ring systems. The radical anion is also almost symmetric, but the charge and spin density in 1^*- are localized. The differences in geometry and charge distribution between neutral 1 and the radical ions can be rationalized through the different topology of the frontier molecular orbitals.

5. Acknowledgments:

This work was supported by the Department of Chemistry and Biochemistry at the University of Notre Dame. We are grateful to the Office of Information Technologies at the University of Notre Dame and the National Center for Supercomputer Applications (NCSA) at the University of Illinois Urbana-Champaign (Grant CHE960004) for generous allocation of computing resources.

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