Spectrophotometric observations on the changes in the Y-peak of actinomycin D complexed to DNA on gamma-irradiation.

An understanding of the pi-pi * transitions of the Y-peak of the DNA bound to actinomycin D, in the presence and absence of gamma-irradiation has been attempted. It was seen that in the bound DNA the presence of the Y-peak, without irradiation was mainly due to the pi-pi * transitions associated with the DNA bases. The Y-peak was found to be more sensitive to the binding of the drug than the X-peak. After gamma-irradiation the Y-peak characteristics of the complexed DNA were different from those of the X-peak. From these studies it has been concluded that the electronic transitions (pi-pi *) of the DNA, in the far U.V., can be divided into three distinct groups: (a) Electronic transitions which are only seen at the Y-peak, (b) Transitions common both to the X and Y-peaks, and (c) Transitions which are only seen at the X-peak. This explains the difference in the behaviour of the Y- and X-peaks of the DNA due to the binding of Act.D and gamma-irradiation.