Shiori Yamazaki, Matthew A. Diaz, Thomas M. Carlino, Chitra Gotluru, Mercedes M. A. Mazza, Amy M. Scott

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 transient absorption spectroscopy to examine the ultrafast ET between quinacrine (Qc), an antimalarial drug with potential anticancer activity, and riboflavin binding protein (RfBP) with a known Kd =264 nM. Steady-state absorption reveal a ~10 nm red-shift in the ground state when QcH32+ is titrated with RfBP, and a Stern-Volmer analysis shows ~84% quenching and blue-shift of the QcH32+ photoluminescence to form a 1:1 binding ratio of the QcH3(2+)-RfBP complex. Upon selective photoexcitation of QcH3(2+) in the QcH3(2+)-RfBP complex with ~70 fs pulses, we observe charge separation in 7 ps to form 1[QcH3_red●+-RfBP●+] that persist for 138 ps. The FsTA spectra show the spectroscopic identification of QcH3_red●+, determined from spectroelectrochemical measurements in DMSO. We correlate our results to recent literature and report lifetimes that are 10-20× slower than the natural riboflavin, Rf-RfBP, complex and are oxygen independent. Driving force (ΔG) calculations, corrected for estimated dielectric constants for protein hydrophobic pockets, and Marcus theory depict a favorable one electron ET process between QcH32+ and nearby redox active tyrosine (Tyr) or tryptophan (Trp) residues.