Abstract
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
= 264 nM. Steady-state absorption reveals a ∼ 10 nm red-shift in the ground state when QcH
is titrated with RfBP, and a Stern-Volmer analysis shows ∼84% quenching and a blue-shift of the QcH
photoluminescence to form a 1:1 binding ratio of the QcH
-RfBP complex. Upon selective photoexcitation of QcH
in the QcH
-RfBP complex, we observe charge separation in 7 ps to form
[QcH
-RfBP
], which persists for 138 ps. The FsTA spectra show the spectroscopic identification of QcH
, 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 QcH
and nearby redox active tyrosine (Tyr) or tryptophan (Trp) residues.
