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Experimental, biophysical, and molecular modelling studies between 25I-NBOH and 25I-NBOMe with human serum albumin (HSA) have indicated that these recreational drugs simultaneously bind to site I and II of the protein, with 25I-NBOMe showing a greater affinity for the transport protein, and that both compounds were able to perturb the native structure of HSA. The 25I-NBOH and 25I-NBOMe drugs spontaneously interact with protein (ΔG < 0), and with the complex formation in the ground state (static quenching). Preferred forces in the interaction process were determined as hydrogen bonding and van der Waals forces. From 1H NMR studies, it was possible to determine the epitope of the molecules, as these and other results agree with the theoretical molecular docking. Overall, our results suggest that the interaction between 25I-NBOH and 25I-NBOMe with HSA can affect its distribution in the body and cause harmful effects, resulting from conformational changes in the protein which can affect its function from the decreased availability of HSA to carry other essential compounds.

Synthesis of dinuclear oxadiazole-adamantane platinum(II) and palladium(II) complexes (PtO, PdO) and mononuclear thiazolidine derivative complexes (PtT, PdT) was described. Characterization was performed by elemental analysis, infrared, UV-visible, 1H, 13C, 195Pt NMR spectra, MS spectroscopy and single crystal X-ray diffraction. The cytotoxicity by MTT assay against tumor and normal cell lines with or without extracellular GSH was also investigated. In general, mononuclear complexes containing thiazolidine-adamantane ligands were more cytotoxic than oxadiazole-adamantane derivatives. PtT complex proved to be as active as cisplatin. Dinuclear compounds were considered inactive to cells in evaluated conditions, due to their high stability with ligands in a chelated and bridged way. Results suggest that GSH cannot be considered a target. DNA- and BSA-binding interactions were evaluated using UV-visible and fluorescence spectroscopy, intercalating dyes and molecular docking. Upon coordination to platinum(II), the cytotoxic effect was appreciably improved against tested cell lines, in comparison to free thiazolidine ligand. Comparing thiazolidine derivatives, it is noticeable that the less active compound (PdT) presents stronger interaction with BSA, while PtT has the weaker interaction with BSA and relatively strong binding to isolated DNA, resulting in the most cytotoxic complex. This work shows that the presence of metal is significant but it should be available for interaction. The high lability of palladium complex made this stay retainable in BSA and two metal atoms do not increase activity if it is not able to do any interaction.

A simple methodology to design metal-free porphyrin (meso-tetra-(1-naphthyl)porphyrin) and their corresponding transition metal complexes (Zn(II), Cu(II), Ni(II), Co(III) and Mn(III)) is described. These porphyrins were fully characterized by elemental analysis, HRMS-ESI, UV–vis absorption, steady state fluorescence emission, theoretical calculations and electrochemical analysis. Photo-stability and singlet oxygen generation species were also evaluated. Spectroscopic studies and molecular docking calculations evinced the ability of each porphyrin derivatives to establish interactions with calf-thymus DNA (ct-DNA).

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 Sudlow's site I in HSA, interacting with hydrophobic and hydrophilic amino acid residues.

Metformin hydrochloride is an oral hypoglycemic agent prescribed for the treatment of diabetes mellitus type II. Three fluorinated metformin derivatives (MTF1, MTF2 and MTF3) were synthesized and investigated for their human bloodstream – human serum albumin (HSA) interaction studies by multi-spectroscopic techniques (circular dichroism, steady state, time-resolved and synchronous fluorescence), combined with molecular docking and quantum chemical calculation. Steady state and time-resolved fluorescence indicate static fluorescence quenching as the main mechanism. Binding of HSA/metformin is moderate and there is just one main binding site in the protein structure for all samples. In addition, MTF1 can cause weak perturbations on the secondary structure of the albumin, while MTF2 and MTF3 cause moderate perturbations. Synchronous fluorescence spectroscopy showed that MTF1 and MTF3 can decrease the hydrophobicity around the Trp-214 amino acid residue. Molecular docking results suggest hydrogen bonding, van der Waals and hydrophobic interactions as the main binding forces for the association HSA:MTF1 and HSA:MTF2, while for HSA:MTF3 the main binding forces are hydrogen bonding and van der Waals interaction.

The novel β-enamino thiosemicarbazide derivatives (Z)-2-(3-(phenethylamino)-but-2-enoyl)-hydrazine carbothioamide (ETS1) and (Z)-N-methyl-2-(3-(phenethylamino)-but-2-enoyl)-hydrazine carbothioamide (ETS2) as well as their β-enamino ester precursor (Z)-ethyl-3-(phenethylamino)-but-2-enoate (ENE) were synthesized and characterized. The synthetic compounds were tested against tyrosinase activity and ETS1 was the only compound showing high inhibition percentage (89%) through an uncompetitive inhibitory mechanism and with IC50 = 49 μM. Quantum chemical calculations showed a clear relationship between the HOMO and sulfur electrostatic values of ETS1 and its efficiency as tyrosinase inhibitor. Molecular docking results suggest that ETS1 can be better accommodated than ENE and ETS2 inside the active tyrosinase site. ETS1 is able to interact with the amino acid residues His-47, Asn-152, His-155 and Phe-156. The transportation behavior of human serum albumin (HSA) towards ETS1 was evaluated by spectroscopic methods (steady state, time-resolved, synchronous and 3D fluorescence, and circular dichroism), zeta potential and theoretical calculations (molecular docking). Studies on the interaction HSA:ETS1 revealed the existence of a moderate and spontaneous association between them in the ground state. Besides, it has been shown that hydrogen bonding played an important role in the transportation of ETS1. Competitive binding studies indicated Sudlow's site I as the main binding site for ETS1 and molecular docking results suggested that the amino acid residues Trp-214, Val-343, Asp-450, Leu-452, Ser-453, Leu-456 and Leu-480 can interact via hydrogen bond and hydrophobic forces with ETS1. Overall, the experimental parameters indicated that ETS1 could be effectively stored and carried by HSA.

The evaluation of tyrosinase activity of CH, CH3F, CH4F, CH23F, CH25F, CH35F and CH2356F as well as of the interactions of CH and fluorinated chalcones CH3F, CH4F and CH2356F with bovine serum albumin (BSA) in a PBS buffer solution (pH = 7.4), at 288 K, 293 K and 298 K, were investigated by using spectroscopic techniques, zeta potential and computational methods. Tyrosinase enzymatic activity was measured by UV–vis spectrometry of the oxidation of l -DOPA in the presence of the enzyme and chalcones: the unsubstituted chalcone inhibits tyrosinase activity, whereas its fluorinated derivatives showed the opposite effect, whose sample CH2356F showed the highest percentage activation. Studies of quenching of the BSA fluorescence by CH, CH3F, CH4F and CH2356F show a combination of static and dynamic quenching mechanisms. In addition FRET mechanism can occurs with high probability. For CH the binding constant (Ka) is in the range of 105 M−1 indicating a moderate association between chalcone and BSA. On the other hand, for the fluorinated derivatives CH3F, CH4F and CH2356F Ka = 104 M−1, which indicates that the presence of fluorine atoms on the chalcone structure decreases the binding ability. The thermodynamic parameters ΔH° and ΔS° show that the BSA:CH association is enthalpically and entropically driven, whereas for fluorinated derivatives it is only entropically driven. The negative ΔG° values found in all cases are consistent with a spontaneous albumin:chalcone association. Circular dichroism and zeta potential data show that the binding does not affect significantly the albumin structure. The binding site study clearly indicates that the main binding pocket for the samples is the Trp-212-containing binding site. The largest docking score values also suggest the same binding site. Molecular docking results suggested that the presence of fluorine atoms decrease the number of amino acid residues that can stabilize the interaction of the fluorinated chalcones with BSA. For CH3F and CH2356F the type of interaction between the amino acid residues and ligand structure did not change significantly.

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.

Serum albumin is the most abundant protein in the circulatory system. It plays an important role in the transport and deposition of many molecules in the body. The 7- O - β - d -glucopyranosyl-6-(3-methylbut-2-enyl)-5,4′-dihydroxyflavonol (PF) is a glycosylated flavonoid isolated from Ouratea hexasperma (Ochnaceae) branches. It shows important biological activities, such as cytotoxic, antimicrobial and antitumor. The aim of this study was to examine the interaction of bovine serum albumin (BSA) with PF in a PBS buffer solution (pH = 7.4) at 296 K, 303 K and 310 K, by spectroscopic techniques (UV–vis, circular dichroism, FTIR-ATR, steady-state, time-resolved, synchronous and 3D fluorescence) and molecular docking. The experimental results showed that the intrinsic fluorescence quenching of BSA induced by PF resulted in an association BSA:PF in the ground state (k q 10 12 M −1 s −1 ). The association is moderate with K a ≈ 10 4 M −1 and K b ≈ 10 5 M −1 . The CD, 3D and synchronous fluorescence, as well as FTIR-ATR results indicate that the association BSA:PF does not result in a significant perturbation on the secondary structure of BSA. Forster resonance energy transfer (FRET) is not operating in the present case. The negative values for ΔG° are in accord with the spontaneity of the interaction. ΔH° 0 indicate an enthalpically and also entropically driven association and the main interaction forces between BSA:PF are hydrogen bonding and/or electrostatic forces. The number of binding sites ( n ≈ 1) indicate only one main binding site for the interaction BSA:PF. The competitive binding studies indicate Trp-212-containing binding site as the main pocket for the interaction BSA:PF and molecular docking results also suggest the same cavity. Inside this site, the amino acid residues Glu-152, Arg-194, Arg-198, Arg-217, Ser-343, Asp-450 and Ser-453 interact by hydrogen bond with PF, whereas Ala-341 and Val-342 residues interact by hydrophobic interactions.

The interaction between the transport protein bovine serum albumin (BSA) and the natural product lophirone B, was investigated by spectroscopic techniques combined with a computational method (molecular docking). From the KSV and kq values it was concluded that lophirone B quenches the fluorescence of BSA by dynamic and static mechanisms. The Ka values, of the order of 104 M−1, and the number of binding sites (n ≈ 1), indicate that the binding is moderate and there is just one main binding site in BSA for lophirone B. The negative ΔG° values are in accordance with the spontaneity of the process and the positive ΔH° and ΔS° values indicate that the binding is entropically driven; the main binding forces for the association BSA:lophirone B are probably lipophilic interactions. Circular dichroism (CD) studies show there is not a significant perturbation on the secondary structure of the albumin upon the binding process. In order to better understand the spectroscopic results, a computational method was applied: molecular docking suggests Trp-213 site, as the main binding site for the ligand. Lophirone B seems to be exposed to the aqueous media as well as accommodated inside the protein cavity, resulting in a moderate affinity for the albumin. The Arg-198, His-287, Lys-294 and Lys-439 residues are interacting via hydrogen bonding with lophirone B, whereas the interaction with Trp-213 residue occurs through a lipophilic interaction.

This article deals with the synthesis of Schiff-based bis-azomethine-based ligands derived from pyridoxal and aliphatic dihydrazides and the synthesis of nickel(II) complexes C1-C4. The synthesized complexes had their structures elucidated by monocrystal X-ray diffraction and were characterized by vibrational and absorption spectroscopy. The synthesized ligands have characteristics that allow the formation of self-assembly processes, thus, the flexibility or rigidity of the coordination of organic molecules added to the orbitals of the NiII cation leads to the formation of helical complexes with double helix and a dinucler nickel(II) complex. Moreover, compounds was their interactions with CT-DNA and HSA absorption and emission analysis and molecular docking calculations.

By using spectroscopic methods (fluorescence and circular dichroism), the interactions of three di-fluorinated chalcones (CH23F, CH25F and CH35F) with bovine serum albumin (BSA) in a PBS buffer solution (pH = 7.4), at 288 K, 293 K and 298 K, were probed. Molecular docking was performed to evaluate the main protein cavity for the BSA/chalcone interactions. Fluorescence quenching of the albumin by the di-fluorinated chalcones follow a combination of static and dynamic quenching mechanisms. The Stern-Volmer binding constant (Ka) values are in the range of 104 M−1 indicating a moderate association between chalcones and BSA. Besides, circular dichroism data show that this association does not affect significantly the secondary structure of the albumin. The Forster resonance energy transfer (FRET) theory suggests that non-radiative energy transfer can occur from BSA to di-fluorinated chalcones (r ≈ 3 nm). The thermodynamic parameters ΔH° and ΔS° indicate that hydrogen bonding and/or hydrophobic interactions play a major role in the association, which is entropically driven. The enthalpy and entropy change values for CH25F and CH35F are similar, but very different for CH23F. The negative ΔG° values are consistent with a spontaneous association. The highest docking score suggests the Trp-212 site as the most probable binding site for the interaction between BSA with the di-fluorinated chalcones. Trp-212 residue interacts via hydrogen bonding with CH25F and CH35F. On the other hand, CH23F interacts via T-stacking with the Trp-212 residue and via hydrogen bonding with the Ser-343 residue. CH23F has the highest dipole moment value of all evaluated chalcones, having its fluorinated aromatic ring more exposed to the aqueous medium than inside the protein cavity. In addition to these results theoretical methods aimed to study the interaction BSA:CH2′3′F, BSA:CH2′5′F and BSA:CH3′5′F were also employed.

Serum albumin is the most abundant protein in blood plasma; among its functions is the transport of a high variety of drugs in the body. Quinones show several biological and pharmacological activities, such as anti-malarial, antitumor, anti-microbial, anti-inflammatory and anti-parasitic. We report fluorescence and circular dichroism (CD) spectroscopic studies to try to understand the interaction process between α-lapachone (α-LAP) and bovine serum albumin (BSA). Studies using computational methods, such as molecular docking, were performed to identify the main cavity in which this interaction occurs as well as the type of intermolecular interactions between the amino acid residues from albumin and the ligand. The BSA fluorescence quenching by added α-LAP is a static process, indicating an initial association BSA: α-LAP. The Ka and Kb values for the interaction BSA: α-LAP are in the range 105-104 L∙mol-1, indicating a strong binding between these two species. CD data show that there is no significant perturbation on the secondary structure of the protein with binding. The negative ΔGo values are consistent with spontaneous binding occurring endothermically (ΔHo = 127 kJ∙mol-1), and possibly driven by hydrophobic factors (ΔSo = 0.526 kJ∙mol-1∙s-1). The number of binding sites (n) indicates the existence of just one main binding site in BSA for α-LAP, with molecular docking results showing that it binds preferentially to the albumin in the domain IIA, where the Trp-212 residue is located. The ligand interacts via hydrogen bond with Arg-259 and Tyr-149 residues and via T-stacking with the fluorophore Trp-212 residue.

The interaction between human serum albumin (HSA), the most abundant protein in the human circulatory system, and the potential anti-parasitic drug (E)-2-((6-iodobenzo[d][1,3]dioxol-5-yl)methylene)hydrazinecarbothioamide (6-IPTSC) was evaluated in vitro by spectroscopic techniques (circular dichroism, steady-state, time-resolved and synchronous fluorescence) combined with molecular docking and dynamics simulation. The interaction HSA:6-IPTSC can occur via a static process, which involves a ground-state association. The molecule 6-IPTSC binds moderately (Kb ~ 104 M−1) and spontaneously (ΔG° 0 resulted in negative values for ΔG° at 296, 303 and 310 K, indicating that the binding HSA:6-IPTSC is entropically and enthalpically driven. There is just one binding site in HSA for 6-IPTSC, namely Sudlow's site I. The binding process does not perturb significantly the secondary structure of the protein, as well as the microenvironment around the Tyr residue. Molecular docking results suggest that inside Sudlow's site I the potential anti-parasitic drug can interact with Lys(K)-197, Trp(W)-213, Gln(Q)-219, Arg(R)-220, Glu(E)-448, and Ser(S)-453 residues via hydrogen bonding and hydrophobic forces. Overall, the dynamics simulation results complemented the molecular docking discussion, mainly showing that Gln(Q)-219, Arg(R)-220, and Glu(E)-448 residues are not the primary amino acids involving in the binding ability HSA:6-IPTSC. Besides, these results confirm that 6-IPTSC does not perturb the secondary structure of the protein significantly.

Herein we report the synthesis of a series of amino acid-derived triazoles by an organocatalytic cycloaddition reaction between azides and carbonyl compounds, catalyzed by a simple amine. These compounds present absorption maxima located in the UV-B ascribed to fully spin and symmetry allowed electronic transitions and a main fluorescence emission in the UV-A (~380 nm) with a relatively large Stokes shift (5700 cm−1). No significant solvatochromism was observed in both ground and excited states. Unexpectedly, the benzoxazole derivatives presented much higher fluorescence quantum yield values (40–80%) of compared to the sulfur analogues (3–6%). In addition, the DNA binding assays indicated that these compounds presented strong interaction with CT-DNA, which could be attributed to π-stacking and intermolecular hydrogen-bonding. The interaction of the benzazoles with bovine serum albumin (BSA) was also investigated, where a suppression mechanism was observed. In each case, docking was performed to better understand the observed interactions.

This work presents the synthesis, characterization of copper(II) complexes (C1-C6) and the potential of these compounds to mimic the catalytic activity of the enzyme superoxide dismutase (SOD). The copper(II)complexes were obtained by reaction between the aldol condensation between substituted aromatic hydrazides and aromatic aldehydes (salicylic aldehyde and pyridoxal hydrochloride), forming two new ligands (L1 to L6), resulting in new dimeric dicopper (II) complexes (C1 and C2), new three monomeric CuII derivatives (C3, C4 and C6) and a polymeric complex (C5). The CuII complexes were fully characterized by X-ray diffraction, spectroscopic and electrochemical analysis. Subsequently, CuII derivatives were evaluated for their antioxidant activities, using the NBT (Nitro blue tetrazolium chloride) photoreduction methodology. After evaluating the antioxidant activity in vitro, it was observed that the best inhibition rates of the superoxide ion are associated to the C4 and C5 complexes. Computational analysis via molecular docking and quantum chemical calculation (Fukui map) offered a molecular level explanation on the biological activity of CuII complexes. Additionally, cytotoxicity of C1-C6 was tested in the first time in vivo in nematodes Caenorhabditis elegans, corroborating with the results identified for C4 and C5.

The interaction between human serum albumin (HSA) and two porphyrin-based tetra-cationic photosensitizers were investigated via spectroscopic techniques (steady-state, time-resolved fluorescence and circular dichroism) combined to theoretical calculations (molecular docking). Similarities and differences in the binding behavior of a porphyrin bearing peripheral chloro(2,2′bipyridine)platinum(II) complexes (4-PtTPyP) versus methyl groups (4-MeTPyP) attached in the N-pyridyl atoms were evaluated. There is a ground state association between HSA-porphyrins (static fluorescence quenching mechanism), with Ka values for 4-PtTPyP slightly higher than 4-MeTPyP. While the binding 4-MeTPyP is primarily enthalpically driven, 4-PtTPyP binds mainly due to hydrophobic forces in an entropically driven association. Far-UV CD showed that 4-PtTPyP perturbs the thermal stability of the protein, probably due to high volume of the porphyrin. Molecular docking suggested that 4-PtTPyP stablish a larger number of interactions with the neighboring amino acid residues. The likely location for the binding pocket is in subdomain IB, located in an external region of the protein structure. The bulky peripheral platinum(II)-complexes does not have a deleterious effect upon serum albumin binding spontaneity and extension, although modifies the nature of the complex formed.

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 (&Delta, G&deg, &lt, 0), and entropically driven (&Delta, S&deg, = 0.573 &plusmn, 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&rsquo, s site I as the main protein pocket and molecular docking results suggested hydrogen bonding and hydrophobic interactions as the main binding forces.

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 Sudlow’s 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.

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. 105 M-1), 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.

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 Leu-480 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 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.

The present work reports the interaction between human serum albumin (HSA) – the main bio-distributor of exogenous and endogenous compounds in the human bloodstream -and two synthetic Ru(II)-porphyrins (4-RuTPyP and 4-ZnRuTPyP), by multiple spectroscopic techniques (steady-state, time-resolved, circular dichroism, synchronous and 3D fluorescence) combined with molecular docking calculations. The interaction between HSA and each Ru(II) derivatives is spontaneous and moderate, being enthalpically and entropically driven. Steady-state and time-resolved fluorescence analysis showed static process as the main fluorescence quenching mechanism (ground-state association). The binding of tetra-ruthenated derivative containing Zn(II) ion caused more perturbation on the secondary structure of the albumin than the free-base porphyrin. Each Ru(II)-porphyrin interacts preferentially in the site III (subdomain IB), mainly via electrostatic and hydrophobic forces, as well as via van der Waals forces for the sample which contain Zn(II) ion.

Synthesis of three novel inorganic complexes (Zn(II), Pd(II), and Pt(II) – [Zn(H2L)](NO3)2, [Pd(L)], and [Pt(L)], respectively) based on 2,6-diformylpyridine-bis-(4-phenyl) thiosemicarbazone (H2L) was obtained under reflux medium, yielded in the range of 67% and 86%. Characterization of each compound was done via X-ray diffraction and spectroscopic methods. Structural analysis revealed the formation of complexes with pentagonal and quadratic bipyramidal geometry. Electronic absorption spectra for the ligand and inorganic complexes exhibited π → π* and n → π* transitions in the UV region, however, [Pd(L)] and [Pt(L)] showed other less energetic bands attributed to ligand-to-metal charge transfer (LMCT). The interaction between each compound and two macromolecules: calf-thymus DNA (CT-DNA) and bovine serum albumin (BSA) was conducted by spectroscopic techniques combined to molecular docking calculation. For CT-DNA interaction, [Zn(H2L)](NO3)2 is the weakest intercalate compound and the increasing order of Kb values follows [Zn(H2L)](NO3)2

MnTPPS is a metallic water soluble porphyrin with high potential to be used as a contrast agent in photoacoustic tomography. In order to fully understand the interaction between MnTPPS and serum albumin and to investigate the effect of the light induced fast in situ heat deposition by MnTPPS in the protein, we performed several experimental studies using fluorescence and circular dichroism spectroscopies, as well as photoacoustic calorimetry. To identify the possible binding site(s) of the metalloporphyrin in serum albumin and to help interpret the spectroscopic results, a molecular docking exercise was also carried out. The fluorescence data indicate a 1 : 1 stoichiometry for the complex BSA : MnTPPS. The molecular docking results suggest one binding site at the subdomain IB of albumin, where Trp-134 is found, as the main binding site for MnTPPS. The CD data indicate no significant conformational changes of the BSA secondary structure upon MnTPPS binding and even after several minutes of laser excitation of MnTPPS. TR-PAC results show that the in situ heat deposition from MnTPPS does not cause any significant transient conformational change to the BSA structure. In conclusion, this work demonstrates that MnTPPS, in addition to the necessary physical and chemical properties to be used as a contrast agent in photoacoustic tomography, can be effectively carried by albumin and that in situ heat release following light absorption does not cause any significant damage to the protein structure.

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. 105-106 L mol-1), spontaneous (ΔG° ca. -35.7 kJ mol-1 at 310 K) and entropy-driven (ΔS° = 0.146 kJ mol-1 K-1). 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.

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 (Sudlow's 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.