Quantum yield for preferential photodimerization in long pyrimidine tracts

The quantum yield for cyclobutyl-pyrimidine dimerization in DNA has been observed to increase approximately linearly with increasing pyrimidine tract length. A model without adjustable parameters for this yield is proposed based on energy delocalization, vibronic symmetry switching, and saturation statistics that describe the average number of (base pairwise) breathing modes in a given tract of pyrimidines. This average number of modes is an approximately linear function of the tract length. Monte Carlo techniques are used to simulate base sequences and photochemical events, and indicate that this model is consistent with experiment for Tetrahymena pyriformis DNA.