Ultrafast Spectroscopic Dynamics of Quinacrine-Riboflavin Binding Protein Interactions
2017
Yamazaki, Shiori | Diaz, Matthew A. | Carlino, Thomas M. | Gotluru, Chitra | Mazza, Mercedes M. A. | Scott, Amy M.
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 Kd = 264 nM. Steady-state absorption reveals a ∼ 10 nm red-shift in the ground state when QcH₃²⁺ is titrated with RfBP, and a Stern–Volmer analysis shows ∼84% quenching and a blue-shift of the QcH₃²⁺ photoluminescence to form a 1:1 binding ratio of the QcH₃²⁺–RfBP complex. Upon selective photoexcitation of QcH₃²⁺ in the QcH₃²⁺–RfBP complex, we observe charge separation in 7 ps to form ¹[QcH₃_ᵣₑd•⁺–RfBP•⁺], which persists for 138 ps. The FsTA spectra show the spectroscopic identification of QcH₃_ᵣₑd•⁺, determined from spectroelectrochemical measurements in DMSO. We correlate our results to 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.
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