Effect of crosslinking on the secondary structure of DNA. I. Crosslinking by photodimerization

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Abstract
An investigation was made into the effect produced by photo-induced pyrimidine cross-links upon the secondary structure of [calf thymus] DNA. The effect of UV irradiation upon the B .dblarw. A transition in DNA brought about by a change of solvent from 70-80% ethanol was studied. Circular dichroism (CD) was used to monitor the conformational changes. Laser Raman spectroscopy showed that CD is a reliable monitor of the conformational change, even though the DNA is aggregated in 80% alcohol solutions. This aggregation may stabilize the A-form through lateral interaction between the helices. The UV irradiation experiments show that pyrimidine-dimer cross-links induced into the B-form DNA will lock it irreversibly into that conformation and prevent it from going to the A-form in 80% EtOH solution. The A-form DNA can tolerate a few cross-links but converts cooperatively to the B-form if a larger number of cross-links is introduced. Irradiation-induced pyrimidine cross-links create locally denatured regions in B-form DNA. Upon continued irradiation, the entire DNA molecule becomes denatured.

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