Photoscission of DNA in the presence of ethidium bromide or riboflavin
(vitamin BZ) was studied. Laser irradiation of DNA in the presence of
these agents induced single- and double-stranded lesions in a superco
iled plasmid pBR322. Their formation was monitored by the content of r
elaxed and linear forms of the plasmid. The possibility of DNA scissio
n al the expense of the energy absorbed by a nonintercalating chromoph
ore (riboflavin) was demonstrated for the first time. The dependence o
f the number of radiation-induced lesions in the presence of riboflavi
n on the energy density in the impulse implies a nonlinear (two-quantu
m) mode of photoscission. It was shown that the quantum yield of DNA p
hotoscission in the presence of riboflavin is severalfold higher than
that for ethidium bromide.