Photochemical alterations in DNA revealed by DNA-based liquid crystals.

The preparations of chicken erythrocyte linear double-stranded DNA and superhelical plasmid pBR322 DNA were irradiated by continuous low-intensity UV radiation (I = 25-50 W/m2, lambda = 254 nm) as well as by high-intensity picosecond laser UV radiation (I = 10(11)-10(13) W/m2, lambda = 266 nm). The effect of DNA secondary structure alterations on the formation of liquid-crystalline dispersions from UV-irradiated DNA preparations was studied. It was shown that in the case of linear DNA, watching the disappearance of abnormal optical activity characteristic for cholesteric liquid crystal we managed to detect the presence of photochemical alterations in DNA irradiated by low-intensity UV radiation at an absorbed energy of more than 20 quanta per nucleotide. In the case of superhelical DNA using enzyme treatment of liquid-crystalline dispersions and monitoring the appearance of abnormal optical activity, we detected the presence of photochemical alterations in DNA molecules after low-intensity UV irradiation at an absorbed energy of less than 4 quanta per nucleotide. Under the latter approach using picosecond UV laser irradiation at three different light intensities we were able to distinguish the different mechanisms of fine alterations in DNA secondary structure at an absorbed energy value of about 3 quanta per nucleotide.