In the North of Brazil (Pará and Amazonas states) the leaves of the plant Talinum triangulare (popular: cariru) replace spinach as food. From a phytochemical point of view, they are rich in compounds of the group of pheophytins. These substances, related to chlorophyll, have photophysical properties that give them potential application in photodynamic therapy. Human serum albumin (HSA) is one of the main endogenous vehicles for biodistribution of molecules by blood plasma. Association constants and thermodynamic parameters for the interaction of HSA with pheophytin from Talinum triangulare were studied by UV-Vis absorption, fluorescence techniques, and molecular modeling (docking). Fluorescence quenching of the HSA’s internal fluorophore (tryptophan) at temperatures 296 K, 303 K, and 310 K, resulted in values for the association constants of the order of 10⁴ L∙mol⁻¹, indicating a moderate interaction between the compound and the albumin. The negative values of ΔG° indicate a spontaneous process; ΔH° = 15.5 kJ∙mol⁻¹ indicates an endothermic process of association and ΔS° = 0.145 kJ∙mol⁻¹∙K⁻¹ shows that the interaction between HSA and pheophytin occurs mainly by hydrophobic factors. The observed Trp fluorescence quenching is static: there is initial non-fluorescent association, in the ground state, HSA:Pheophytin. Possible solution obtained by a molecular docking study suggests that pheophytin is able to interact with HSA by means of hydrogen bonds with three lysine and one arginine residues, whereas the phytyl group is inserted in a hydrophobic pocket, close to Trp-214.

The interaction between the main carrier of endogenous and exogenous compounds in the human bloodstream (human serum albumin, HSA) and a potential anticancer compound (the capsaicin analogue RPF101) was investigated by spectroscopic techniques (circular dichroism, steady-state, time-resolved, and synchronous fluorescence), zeta potential, and computational method (molecular docking). Steady-state and time-resolved fluorescence experiments indicated an association in the ground state between HSA:RPF101. The interaction is moderate, spontaneous (ΔG° < 0), and entropically driven (ΔS° = 0.573 ± 0.069 kJ/molK). This association does not perturb significantly the potential surface of the protein, as well as the secondary structure of the albumin and the microenvironment around tyrosine and tryptophan residues. Competitive binding studies indicated Sudlow’s site I as the main protein pocket and molecular docking results suggested hydrogen bonding and hydrophobic interactions as the main binding forces.