Neither adriamycin nor actinomycin D displaces Tb3+ from DNA

The decay of fluorescence of Tb3+ bound to DNA was measured in the absence and presence of adriamycin and actinomycin D. The decay for Tb3+ bound to DNA was mainly exponential (lifetime: tau = 0.96 ms). In the presence of adriamycin or actinomycin D, the Tb3+ fluorescence decayed much faster, indicating that excitation energy was transferred from Tb3+ to the drugs. Extrapolation of the decay curves to zero time showed that the number of strongly emitting, DNA-bound terbium ions was not reduced by the presence of adriamycin or actinomycin D. Hence, these drugs do not seem to displace Tb3+ bound to DNA.