Otávio Augusto Chaves

Fluorescence and Docking Studies of the Interaction between Human Serum Albumin and Pheophytin.

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.

A Study of the Interaction Between trans-Dehydrocrotonin, a Bioactive Natural 19-nor-Clerodane, and Serum Albumin

The interaction between 19-nor-clerodane trans-dehydrocrotonin (from Croton cajucara Benth.) and bovine serum albumin was studied, applying spectroscopic techniques (fluorescence and circular dichroism), combined with molecular modeling. Fluorescence quenching of albumin by the nor-clerodane (kq ca. 1011 mol L-1 s-1 and Stern-Volmer, KSV, increase with temperature) indicates a combination of static and dynamic quenching mechanism. The binding constant (Kb ca. 103 mol L-1) and circular dichroism data suggest that this association is weak and causes only a moderate change in the α-helix content of the protein. Thermodynamic parameters indicate a spontaneous (Gibbs free energy, ΔGo, ca. -21.28 kJ mol-1 at 310 K) and probably entropy-driven (ΔSo = 0.072 kJ mol-1 K-1) association, typical of hydrophobic interactions. The number of binding sites (n ca. 1) indicates one main binding site and molecular modeling suggests subdomain IIIA (Sudlows site II) as the main binding site to the nor-clerodane, which is able to make hydrophobic interactions with leucine (Leu)-24, phenylalanine (Phe)-36, valine (Val)-40 and tryptophan (Trp)-134 residues.

Thiosemicarbazones as inhibitors of tyrosinase enzyme

In the search for compounds which may inhibit the development of melanomas, a series of thiosemicarbazones has been investigated as possible inhibitors of the tyrosinase enzyme. The results showed that all the thiosemicarbazones tested exhibited significant inhibitory effects on the enzyme. Thiosemicarbazones Thio-1, Thio-2, Thio-3 and Thio-4 substituted with oxygenate moieties, were better inhibitors (IC50 0.42, 0.35, 0.36 and 0.44mM, respectively) than Thio-5, Thio-6, Thio-7 and Thio-8. For the better inhibitors, molecular docking results suggested that the oxygen present in the para position of the aromatic ring is essential for the tyrosinase inhibition, due its high ability for complexation with Cu2+ ions. Inside the active protein pocket, Thio-2 - the best studied inhibitor - is able to interact with the amino acid residues His-155, Gly-170 and Val-172 via hydrogen bonding and hydrophobic force. Thio-2, containing a substituent on the aromatic ring similar to the substrate l-DOPA, showed a competitive inhibition mechanism as viewed in a Lineweaver-Burk plot. The same results were observed in the UV-Vis curves.

Studies of the Interaction between BSA and a Plumeran Indole Alkaloid Isolated from the Stem Bark of Aspidosperma cylindrocarpon (Apocynaceae)

Binding between bovine serum albumin (BSA) and a plumeran indole alkaloid (PIA) isolated from the stem bark of Aspidosperma cylindrocarpon (Apocynaceae) was studied by spectroscopic techniques (UV-Vis absorption, circular dichroism, steady state and time-resolved fluorescence), combined with molecular docking. Steady state and time resolved fluorescence data revealed that PIA can quench the BSA fluorescence via a static mechanism: energy transfer from BSA to PIA occurs with high probability. The binding is strong (Kb ca. 10-10 L mol), spontaneous (ΔG° ca. –35.7 kJ mol at 310 K) and entropy-driven (ΔS° = 0.146 kJ mol K). There is just one main binding site (n ca. 1) for the BSA:PIA interaction and the α-helix content of the albumin does not suffer significant perturbation upon PIA binding. Molecular docking results suggest site I as the main binding site to PIA, which is able to interact with the Trp-212, Arg-217, Val-342 and Pro-446 residues.

Multiple Spectroscopic and Theoretical Approaches to Study the Interaction between HSA and the Antiparasitic Drugs: Benznidazole, Metronidazole, Nifurtimox and Megazol

The interaction between four antiparasitic drugs (benznidazole (BZL), metronidazole (MTZ), nifurtimox (NFX) and megazol (MZ)) with human serum albumin (HSA), the main vehicle of biodistribution of xenobiotics, hydrophobic, small and endogenous molecules in the bloodstream, was evaluated by multiple spectroscopic techniques and theoretical calculations. In all cases quenching of the fluorescence of HSA by these drugs involve a static mechanism, due to ground state association. There is just one main binding site in HSA for these four ligands (Sudlow’s site I); binding is spontaneous, moderate, does not have any effect on the polarity around the Tyr and Trp residues and does not perturb significantly the secondary structure of the protein. Molecular docking studies suggest hydrogen bonding and hydrophobic interactions as the main binding forces, i.e., BZL associates with the Trp-214 residue via hydrophobic interactions and with Gln-220, Arg-221 and Glu-449 residues via hydrogen bonding; whereas MTZ associates with Leu-197 and Leu480 residues via hydrophobic interactions and with Trp-214, Glu-449 and Ser-453 via hydrogen bonding. Furthermore, electrostatic interactions were also suggested for HSA:MZ and HSA:NFX.

In vitro Analysis of the Interaction between Human Serum Albumin and Semi-Synthetic Clerodanes

The interaction between HSA and two semi-synthetic potential anti-cancer agents derived from trans-dehydrocrotonin-methyl-hydrazone (MHDCTN) and phenyl-hydrazone (PHDCTN) was evaluated under physiological conditions at 296, 303 and 310 K by multi-spectroscopic techniques and molecular docking calculations. Steady state fluorescence quenching indicated a ground state association (static quenching) for both samples; however, the quenching induced by PHDCTN was not essentially static and can be accompanied by a dynamic quenching mechanism. The binding is strong (modified Stern-Volmer binding constant (Ka) ca. 10 M), causing a very weak perturbation on the secondary structure of the protein and there is just one main binding site for both samples (Sudlow’s site I). Molecular docking results suggested hydrogen bonding and hydrophobic interactions as the main binding forces for both samples.

Novel piperonal 1,3,4-thiadiazolium-2-phenylamines mesoionic derivatives: Synthesis, tyrosinase inhibition evaluation and HSA binding study

A novel series of piperonal mesoionic derivatives (PMI 1-6) was synthesized. Tyrosinase inhibition in the presence of PMI-1, -2, -3, -4, -5 and -6 as well as human serum albumin (HSA) binding studies with PMI-5 and PMI-6 were done by spectroscopic and theoretical methods. The mesoionic compound PMI-5 is the most promising tyrosinase inhibitor with a noncompetitive inhibitory mechanism and an IC50=124μmolL-1. In accordance with the kinetic profile, molecular docking results show that PMI-5 is able to interact favorably with the tyrosinase active site containing the substrate molecule, L-DOPA, interacting with Val-247, Phe-263 and Val-282 residues. The spectroscopic results for the interaction HSA:PMI-5 and HSA:PMI-6 indicated that these mesoionic compounds can associate with HSA in the ground state and energy transfer can occur with high probability. The binding was moderate, spontaneous and can perturb significantly the secondary structure of the albumin. The molecular docking results suggest that PMI-5 and PMI-6 are able to be accommodated inside the Sudlows site I in HSA, interacting with hydrophobic and hydrophilic amino acid residues.

Synthesis and biological evaluation of N-aryl-2-phenyl-hydrazinecarbothioamides: Experimental and theoretical analysis on tyrosinase inhibition and interaction with HSA

A series of N-aryl-2-phenyl-hydrazinecarbothioamides have been investigated as possible inhibitors of tyrosinase, an enzyme involved in the development of melanomas. The hydrazinecarbothioamides 1-6 were synthesized from the reaction between phenylhydrazine and isothiocyanates, for which three different methods have been employed, namely stirring at room temperature, by microwave irradiation or by mechanochemical grinding. Quantitative yields were obtained for the later technique. Compound 4 showed the best value for tyrosinase inhibition (IC50 = 22.6 µM), which occurs through an uncompetitive mechanism. Molecular docking results suggested that 4 can interact via T-stacking with the substrate L-DOPA and via hydrogen bonding and hydrophobic forces with the amino acid residues Ala-79, His-243, Val-247, Phe-263, Val-282, and Glu-321. The interaction between human serum albumin (HSA) and compound 4 occurs through a ground state association and does not perturb the secondary structure of the albumin as well as the microenvironment around Tyr and Trp residues. The binding is spontaneous, moderate and occurs mainly in the Sudlows site I. Molecular docking results suggested hydrogen bonding, hydrophobic and electrostatic interactions as the main binding forces between the compound 4 and the amino acid residues Lys-198, Trp-214, Glu-449, Leu-452, and Leu-480.