Redox active cofactors play a dynamic role inside protein binding active sites because the amino acids responsible for binding participate in electron transfer (ET) reactions. Here, we use femtosecond transient absorption (FsTA) spectroscopy to examine the ultrafast ET between quinacrine (Qc), an antimalarial drug with potential anticancer activity, and riboflavin binding protein (RfBP) with a known K-d = 264 nM. Steady-state absorption reveals a similar to 10 nrri red-shift in the ground state when QcH(3)(2+) is titrated with RfBP, and a Stern-Volmer analysis shows similar to 84% quenching and a blue-shift of the QcH(3)(2+) photoluminescence to form a 1:1 binding ratio of the OsH32+-RfBP complex. Upon selective photoexcitation of QcH(3)(2+) in the QcH(3)(2+)-RfBP complex, we observe charge separation in 7 ps to form (1)[QcH(3)_(center dot+)(red)RfBP(center dot+)],; which persists for 138 ps. The FsTA spectra show the spectroscopic identification of QcH(3)_(center dot+)(red), determined from spectroelectrOchemical measurements in DMSO. We correlate our results to literature and report lifetimes that are 10-20x slower than the natural riboflavin, Rf-RfBP, complex and are oxygen independent. Driving force (AG) calculations, corrected for estimated dielectric constants for protein hydrophobic pockets, and Marcus theory depict a favorable one-electron ET process between QH(3)(2+) and nearby redox active tyrosine (Tyr) or tryptophan (Trp) residues.