Ultrafast Spectroscopic Dynamics of Quinacrine-Riboflavin Binding Protein Interactions

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

University of Miami

Quinacrine (Qc) is a potential riboflavin (Rf) antagonist upon binding to riboflavin binding protein (RfBP). Here, we report the interaction between Qc and RfBP using a combination of photoluminescence spectroscopy, spectroelectrochemistry, and transient absorption (TA) spectroscopy. Stern-Volmer analysis reveals a 1:1 binding ratio between Qc and RfBP, as well as strong electronic coupling of the complex, confirmed by the observed red-shift of the ground state absorption. Upon selective photoexcitation of QcH₃²⁺ in the QcH₃²⁺-RfBP complex with femtosecond resolution, we observe a formation of a charge transfer state within 7 ps to form, [QcH_3-red2-RfBP^•+], that persists for 138 ps based on the combined spectroelectrochemical and TA detection of QcH_{3 -red2}. Driving force (ΔG⁰) calculations using multiple reduction waves of Qc suggest the favorable formation of a charge transfer state from both tyrosine (Tyr) and tryptophan (Trp) residues in RfBP.