Changes in the circular dichroic spectrum of calf thymus solubilized chromatin caused by ultraviolet irradiation.

UV irradiation of the chromatin caused an increase of the positive circular dichroic band in the vicinity of 275 nm (corresponding to DNA) and a deepening of the negative band of proteins at about 225 nm. These changes in the circular dichroic spectrum are monotonous in the range of doses studied (less than 6 X 10(4) J.m-2). The increase of the positive circular dichroic band probably reflects the occurrence of local conformational changes in DNA, which include changes in base position (tilting, distance from helix axis) in the close neighborhood of photoproducts. The presence of photoproducts in chromatin reduces changes in its circular dichroic spectra with temperature.