Investigation of interaction between human plasmatic albumin and potential fluorinated anti-trypanosomal drugs

Abstract In the present work, the interaction between HSA with three potential anti-trypanosomal drugs – ( E )-2-benzylidene-hydrazine-carbothioamide (UTS); ( E )-2-(3,4-difluoro-benzylidene)-hydrazine-carbothioamide (DFTS) and ( E )-2-(2,3,4-trifluoro-benzylidene)-hydrazine-carbothioamide (TFTS) – was studied under physiological conditions by spectroscopic techniques (UV–vis, circular dichroism, steady state, time resolved and synchronous fluorescence) at 296 K, 303 K and 310 K, and theoretical calculations (molecular docking). The bimolecular quenching constant values ( k q ) were higher than 5.00 × 10 9  M −1  s −1 , suggesting a ground state association between albumin and the ligands (static quenching mechanism). Time-resolved fluorescence measurements confirmed the static quenching mechanism, despite the high probability of Forster resonance energy transfer (FRET) ( r  ≈ 4.00 nm for TFTS and r  ≈ 3.00 nm for UTS and DFTS). Modified Stern–Volmer binding constant values ( K a  = 10 4  M −1 ), CD results and synchronous fluorescence (Δ λ  = 15 and 60 nm) indicated moderate association between HSA:UTS, HSA:DFTS and HSA:TFTS, with weak perturbation on the secondary structure of albumin, without perturbation on the microenvironment of the Trp and Tyr residues. The calculated thermodynamic parameters (Δ H °, Δ S ° and Δ G °) are in agreement with a spontaneous binding process, both entropically and enthalpically driven with hydrogen bonding and hydrophobic interactions as the main binding forces. Competitive binding experiments indicated site I as the main binding site of the protein for all studied compounds and molecular docking results suggested the same intermolecular interactions indicated by the thermodynamic parameters.