On the Formation of Thymine Photodimers in Thymine Single Strands and Calf Thymus DNA

Solar light leads to thymine dimers that are mutagenic and primary cause of skin cancer. Here, we report absorption and synchrotron radiation circular dichroism (CD) spectra of T-n single strands with different number n of bases (n=2-7, 10, 11) recorded after various 254nm irradiation times. From a principal component analysis of the CD spectra, we extract fingerprint spectra of both the cyclobutane pyrimidine dimer (CPD) and the pyrimidine (6-4) pyrimidone photoadduct (64PP). Extending the CD measurements to the vacuum ultraviolet region in combination with systematic examinations of size effects is a new approach to gain insight on the dimeric photoproducts. We find a simple linear correlation between n and average number of dimers formed after 1h of irradiation. The probability for a thymine to engage in a dimer increases from 32% for n=2 to 41% for n=11, which implies limited effects of terminal thymines, i.e., the reaction does not occur preferentially at the extremities of the single strands as previously stated. It is even possible to form two dimers with only two bridging thymines. Finally, experiments conducted on calf thymus DNA provided a similar signature of the photodimer, but differences are also evident.