Structural characteristics of TOT and To

The inversion of the cyclobutane ring leads to another isomer, which is a mirror image of the one described here. The barrier height of the inversion of the cyclobutane ring of T T is calculated to be 5.2 kcal/mol. Each mirror image isomer would induce a distinct tertiary structure around the lesion site of DNA. 

The HOMO (highest occupied molecular orbital) of T T has bonding character on the C6A-C6B bond ionization of T T is excepted to involve electron removal from the HOMO orbital. Removal of an electron from this orbital is expected to result in a weakening of the C6A-C6B bond, leading to an increase in the C6A-C6B bond length. Accordingly, one of the striking features of the optimized structure of ToT( +) is that bond. The HOMO of 

ToT has no population on the C5A-C5B bond the bond length of the C5A-C5B in T T( +) is close to that in T T (see Table 1). These characteristics show that the fragmentation of the T T( +) most likely proceeds in a stepwise fashion, where an initial bond cleavage begins at the C6A-C6B bond. 

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