Ultrafast Spectroscopic Dynamics of Quinacrine-Riboflavin Binding Protein Interactions

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.

Excited-State Dynamics of Photoluminescent Gold Nanoclusters and Their Assemblies with Quantum Dot Donors

“…Femtosecond transient absorption (fsTA) measurements were performed on a commercial Ti:sapphire oscillator (MaiTai, Spectra Physics) and a mode-locked Ti:sapphire regenerative amplifier (Spitfire Ace, Spectra Physics) pumped by a 527 nm Nd:YLF laser (Empower, Spectra Physics). , The laser system provides ∼5 W of 35 fs (FWHM) light centered at 800 nm at a repetition rate of 1 kHz. The output of the ultrafast laser system was divided into pump and probe beams.…”