Femtosecond dynamics of a drug-protein complex: Daunomycin with Apo riboflavin-binding protein

In this contribution, we report studies of the primary dynamics of the drug -protein complexes of daunomycin with apo riboflavin-binding protein. With femtosecond resolution, we observed the ultrafast charge separation between daunomycin and aromatic amino acid residues of the protein, tryptophan(s). Electron transfer occurs from tryptophan(s) to daunomycin with two reactio n times, 1 ps and 6 ps, depending on the local complex structure. The forma tion of anionic daunomycin radical is crucial for triggering a series of ch emical reactions in redox cycling. One of the subsequent reactions is the r eduction of dioxygen to form active superoxide by the reduced daunomycin. T his catalytic process was found to occur within 10 ps. In the absence of di oxygen, charge recombination takes a much longer time, more than 100 ps. Th ese results, along with similar findings in DNA and nucleotides, elucidate that the ultrafast generation of reduced daunomycin radicals by photoactiva tion is a primary step for the observed photoenhancement of drug cytotoxici ty by several orders of magnitude. We also studied the dependence of the dy namics on protein conformations at different ionic strengths and denaturant concentrations. We observe a sharp transition from the tertiary structure to the unfolding state at 2 M of denaturant concentration.