S-Se oxidative addition to auranofin derivatives: a DFT study.

Oxidative addition of the S-Se bond to Au(I) complexes is discussed for a series of 26 auranofin (AF) derivatives. AF and its analogues are Au(I) complexes with recognized anticancer activity that act by binding and inhibiting the thioredoxin reductase (TrxR) enzyme. Generally, the oxidative addition to Au(I) is a sluggish reaction under mild conditions (i.e., a high activation barrier - ΔH‡), which is also verified here for AF, ΔH‡ = 33.0 kcal mol-1. However, we predicted that subtle changes in the AF ligands can make the process feasible under standard conditions. For instance, the exchange of -PEt3 by -P(Et2)(OEt), which is a weaker electron σ-donor, reduced the activation barrier to 17.1 kcal mol-1. Furthermore, substitution of the -SAtg ligand by -Cl- leads to a ΔH‡ value of 22.5 kcal mol-1. Overall, the reaction is driven by the nucleophilic attack of the S-Se bond on the Au(I) center, attributed mainly to the charge transfer (4p)Se → (6p)Au, which characterizes the addition step. At the transition state (TS) point, the (5d)Au → σ*(S-Se) charge transfer becomes relevant, facilitating the S-Se bond breakage and the oxidation step. In addition to the electron transfers, the strain energy to deform the linear Au(I) geometry to the tetracoordinated Au(III) arrangement in the TS structure plays a primary role in explaining the trends in the activation barriers. Finally, the activation barrier (ΔH‡) and reaction energy (ΔH°) were correlated for most of the complexes studied, which suggests that the reaction passes through a late or product-like TS and, therefore, the steric and electronic factors affecting ΔH‡ also act on ΔH°. Overall, the results presented here might open up a new field of investigation for interactions between AF derivatives and TrxR, which contributes to a full understanding of the biological mechanism of action of these species.

Modeling the Cellular Uptake of Functionalized Carbon Nanohorns Loaded with Cisplatin through a Breast Cancer Cell Membrane.

The cisplatin encapsulation into carbon nanohorns (CNH) is a promising nanoformulation to circumvent the drug dissipation and to specifically accumulate it in tumor sites. Herein, biased molecular dynamics simulations were used to analyze the transmembrane transport of the CNH loaded with cisplatin through a breast cancer cell membrane prototype. The simulations revealed a four-stage mechanism: approach, insertion, permeation, and internalization. Despite the lowest structural disturbance of the membrane provided by the nanocarrier, the average free energy barrier for the translocation was 55.2 kcal mol-1, suggesting that the passive process is kinetically unfavorable. In contrast, the free energy profiles revealed potential wells of -6.8 kcal mol-1 along the insertion stage in the polar heads region of the membrane, which might enhance the retention of the drug in tumor sites; therefore, the most likely cisplatin delivery mechanism should involve the adsorption and retention of CNH on the surface of cancer cells, allowing the loaded cisplatin be slowly released and passively transported through the cell membrane.

Quantum chemical investigation of predominant conformation of the antibiotic azithromycin in water and DMSO solutions: thermodynamic and NMR analysis.

Azithromycin (AZM) is a macrolide-type antibiotic used to prevent and treat serious infections (mycobacteria or MAC) that significantly inhibit bacterial growth. Knowledge of the predominant conformation in solution is of fundamental importance for advancing our understanding of the intermolecular interactions of AZM with biological targets. We report an extensive density functional theory (DFT) study of plausible AZM structures in solution considering implicit and explicit solvent effects. The best match between the experimental and theoretical nuclear magnetic resonance (NMR) profiles was used to assign the preferred conformer in solution, which was supported by the thermodynamic analysis. Among the 15 distinct AZM structures, conformer M14, having a short intramolecular C6-OH … N H-bond, is predicted to be dominant in water and dimethyl sulfoxide (DMSO) solutions. The results indicated that the X-ray structure backbone is mostly conserved in solution, showing that large flexible molecules with several possible conformations may assume a preferential spatial orientation in solution, which is the molecular structure that ultimately interacts with biological targets.

Translocation Processes of Pt(II)-Based Drugs through Human Breast Cancer Cell Membrane: In Silico Experiments.

Breast cancer is one of the most frequent modalities of cancer worldwide, with notable mortality. The medication based on platinum drugs (cisplatin (cddp), carboplatin (cpx), and oxaliplatin (oxa)) is a conventional chemotherapy despite severe side effects and the development of drug resistance. In order to provide a deeper molecular description of the influx and efflux processes of platinum drugs through breast cancer tissues, this study focuses on molecular dynamics (MD) simulations of the passive translocation process through a realistic plasma membrane prototype of human breast cancer cell (c_memb). The results showed that the permeation events were mainly mediated by neutral lipids (DOPC, DOPE, and cholesterol), producing a low and temporary membrane deformation. The drug insertion in the region of polar heads was the most favorable stage of the translocation mechanism, especially for cddp and oxa with potential wells of -8.6 and -9.8 kcal mol-1, respectively. However, the potentials of mean force (PMF) revealed unfavorable kinetics for the permeation of these drugs through lipid tails, with energy barriers of 28.3 (cddp), 32.2 (cpx), and 30.4 kcal mol-1 (oxa). The low permeability coefficients (P) of cpx and oxa, which were 3 and 1 orders of magnitude inferior than for cddp, resulted from the high energy barriers for their traslocation processes through the membrane. The obtained results provide a more accurate picture of the permeation of Pt(II)-based drugs through breast cancer cells, which may be relevant for the design and evaluation of new platinum complexes.

Analysis of Pleurotin binding to human thioredoxin reductase using docking and molecular dynamics simulation.

Thioredoxin reductase (TrxR) has been considered a potential target for cancer chemotherapy. It acts by controlling the redox homeostasis of human cells and, therefore, interfering in its function may trigger apoptosis, which is a crucial tumor suppression mechanism. Despite the great effort in the search for TrxR inhibitors, none was approved for human therapy. In the present study a virtual screening for natural organic compounds is discussed for a set of 72 compounds with known IC-50 for TrxR inhibition. The results suggest the Pleurotin, a naphthoquinone obtained from Hohenbuehelia grisea fungus, as a potential TrxR inhibitor, which acts by binding to the active site of the enzyme, between the N- and C-terminal domains. The presence of the ligand blocks the approximation of the C-terminal arm to the N-terminal, which is an essential step of the enzyme function. Besides, the two equivalent binding sites of TrxR were explored, by docking two ligands simultaneously. The results indicate that both sites have an allosteric correlation and, the presence of the ligand in one site may interfere, or even prevent, the binding of the second ligand at the other site. All these findings are quantitatively discussed based on the analysis of long molecular dynamics trajectories, which provides a full description of the ligand-receptor binding modes, average binding energies and conformational changes.Communicated by Ramaswamy H. Sarma.

The role of tridentate ligands on the redox stability of anticancer gold(III) complexes.

Gold(III) complexes are promising compounds for cancer chemotherapy, whose action depends on their redox stability. In this context, the choice of ligands is crucial to adjust their reactivity and biological response. The present study addressed the effect of the gold coordination sphere on the reduction potential (Eo) for ten gold(III) complexes containing five or six-membered rings tridentate ligands - [AuIII(trident)Cl]3+n (trident = N^N^N, C^N^N, C^C^N, C^N^C, and N^C^N). The calculated Eo covered a broad range of 2500 mV with the most stable complexes containing two AuC bonds (Eo = -1.85 V for [AuIII(C^C^N)Cl] - f). For complexes with one AuC bond, the N^C^N ligands stabilize the gold(III) complex more efficiently than N^N^C; however, the inclusion of the non-innocent ligand bipy (2,2'-bipyridine) in N^N portion provides an extra stabilization effect. Among the derivatives with one AuC bond, [AuIII(N^N^C)Cl]+ (N^N = bipy) (a) showed Eo = -1.20 V. For the complexes with N^N^N ligands, Eo was positive and almost constant (+0.60 V). Furthermore, the kinetics for ligand exchange reactions (Cl-/H2O, H2O/Cys and Cl-/Cys) were monitored for the most stable compounds and the energy profiles compared to the reduction pathways.

An investigation of the predominant structure of antibiotic azithromycin in chloroform solution through NMR and thermodynamic analysis.

Azithromycin (AZM) is a well-known macrolide-type antibiotic that has been used in the treatment of infections and inflammations. Knowledge of the predominant molecular structure in solution is a prerequisite for an understanding of the interactions of the drug in biological media. Experimental structural determination can be carried out for samples in solid-state (X-ray diffraction technique) and gas phase (electron diffraction experiment). In solution, spectroscopic methods can be used to extract valuable information which combined with quantum chemical calculations can lead to the determination of the preferred molecular structures to be observed when a given solute is dissolved in each solvent. That is precisely the aim of this work. We used experimental NMR chemical shift data (in CDCl3) as a reference for comparison with Density Functional Theory (DFT) NMR calculations, with geometry optimized having as guess input two crystallographic structures available in the literature with the configuration of all chiral carbon atoms inverted, named here A and B. The Polarizable Continuum Model (PCM) was used to describe the solvent effects (chloroform) including five explicit CHCl3 solvent molecules, which we believe can account for short and long-range solute-solvent interactions. Analysis of calculated thermodynamic, NMR chemical shift, MAE (Mean Absolute Error), and spin-spin coupling constant values revealed that both supposable C3R-C5S (named M2-A) and C3S-C5R (named M2-B) structures are equally probable to exist in chloroform solution. In addition, we found that the heavy atoms' conformation is reasonably similar in the solid-state and chloroform solution; however, regarding the OH groups, the spatial orientations are rather different with intramolecular OH⋯N and OH⋯O hydrogen bonds present in solution and with some of them being absent in the X-ray structure probably due to crystal packing effects.

Computational Prediction of Tc-99 NMR Chemical Shifts in Technetium Complexes with Radiopharmaceutical Applications.

The Tc-99m nucleus is the most used nuclide in radiopharmaceuticals designed for imaging diagnosis. The metal can exist in nine distinct oxidation states and forms distinct coordination complexes with a variety of chelating agents and geometries. These complexes are usually characterized through Tc-99 NMR that is very sensitive to the Tc coordination sphere. Therefore, predicting Tc-99 NMR might be useful to assist experimentalists in structural characterization. In the present study, we propose three computational protocols for predicting Tc-99 NMR chemical shifts based on density functional theory calculations using relativistic and nonrelativistic Hamiltonians: the relativistic Model 1, the nonrelativistic Model 2, and the empirical nonrelativistic Model 3. In Models 2 and 3, the NMR-DKH basis set was used for all atoms, including the Tc, for which it was developed here. All models were applied for a set of 41 Tc-complexes with metal oxidation states 0, I, and V, for which the Tc-99 chemical shift was available experimentally. The mean absolute deviation and the mean relative deviation were 67 ppm and 4.8% (Model 1), 92 ppm and 6.2% (Model 2), and 65 ppm and 4.9% (Model 3), respectively. Last, the effect of the explicit solvent was evaluated for the [TcO2(en)2]+─Tc(V) complex. The calculated results for the Tc-99 NMR chemical shift at SO-ZORA-SSB-D/TZ2P-ZORA/COSMO//TPSS/def2-SVP/IEF-PCM(UFF) show that the inclusion of 14 water molecules (first solvation shell) together with the implicit solvation model leads to an absolute deviation of only 7 ppm (0.3%) from the experimental value, indicating that the solvent effects play a key role in predicting Tc-99 NMR.

Solvent Dependent Competitive Mechanisms for the Ugi Multicomponent Reaction: A Joint Theoretical and Experimental Study in the α-Acyl Aminocarboxamides vs α-Amino Amidines Formation.

A synthetic protocol for the preparation of α-acyl aminocarboxamides and α-amino amidines is proposed. The selectivity toward each of these two possible products was tuned by simple modifications of the reaction conditions. A broad scope is presented, allowing access to the desired products in up to 87% (Ugi adduct) and 93% (α-amino amidine). Theoretical calculations were carried out, and the analysis led to the proposal of a new mechanistic pathway for the Ugi reaction, in which methanol acts not only as the solvent but also as a reagent. High-resolution (tandem) mass spectrometry experiments allowed the detection and characterization of the key intermediate associated with this new and alternative reaction pathway, thus supporting the theoretical proposal.

Unveiling the Releasing Processes of Pt(II)-Based Anticancer Drugs from Oxidized Carbon Nanohorn: An In Silico Study.

About half of all cancer chemotherapies currently applied involve medication with the three worldwide approved Pt(II)-based drugs, cisplatin (cddp), carboplatin (cpx), and oxaliplatin (oxa), due to their notable antitumor activity for several cancers. However, this wide application is accompanied by severe side effects, such as nephrotoxicity, myelosuppression, and neurotoxicity, as a result of their low bioavailability and selectivity for cancer cells. To mitigate these drawbacks, the use of chemically functionalized carbon nanohorns (CNH) as nanocarriers represents a potential formulation since CNH has been noted for their biodegradability, biocompatibility, low toxicity, and cavities dimensionally compatible with small drugs. This work reports energetic and dynamic analyses of complexes formed by oxidized CNH (CNHox) and the cddp, cpx, and oxa drugs. Using unbiased molecular dynamics (MD) simulations, we show that the encapsulated formulations (cddp@CNHox, cpx@CNHox, and oxa@CNHox) were more stable by ∼11.0 kcal mol-1 than the adsorbed ones (cddp > CNHox, cpx > CNHox, and oxa > CNHox). This high stability, mainly governed by van der Waals interactions, was responsible for the drug confinement during the entire simulation time (200 ns). The biased MD simulations of the inclusion complexes confirmed the nonspontaneity of the drug release since the potentials of mean force (PMF) indicated the endergonic character of this process. Additionally, the releasing energy profiles pointed out that the free energy barrier (ΔΔG≠) for the escape from CNHox cavity follows the order oxa > cpx ∼ cddp, with the value for the oxa complex (21-26 kcal mol-1) found to be about 36 and 30% larger than those for cpx and cddp, respectively. While the approximate residence time (tres) of the oxa drug inside the CNHox cavity was 5.45 × 108 s, the same measure for the cddp and cpx drugs was 5.3 × 105 and 1.60 × 103 s. Simulations also revealed that the escape of oxa with the oxalate group facing the nanowindow was the most unfavorable process, giving tres = 1.09 × 109 s. Besides reinforcing and extending the nanovectorization of cddp, cpx, and oxa in CNHox for cancer chemotherapies, all features considered may provide interpretations for experimental data and encourage new investigations aiming to propose less aggressive treatments for oncological diseases.

Records and altitudinal assessment of Amblyomma aureolatum and Amblyomma ovale (Acari: Ixodidae) in the State of Rio de Janeiro, southeast Brazil.

Information on the altitudinal distribution of the hard ticks Amblyomma aureolatum and Amblyomma ovale in Brazil is scarce and mainly limited to occasional records. In this study we report our evaluation of records on the altitudinal distribution of A. aureolatum and A. ovale collected from dogs and humans and directly from the environment (host-questing ticks), based on active or passive procedures. The collections were conducted in rural areas of municipalities in the state of Rio de Janeiro, Brazil between 2013 and 2017. Active procedures consisted of dragging or flagging, visual examination of vegetation and removal of ticks present on the authors' clothing or on infested dogs. Overall, 222 ticks were collected. The altitudes at the collection sites ranged from 98 to 1220 m a.s.l. We noted a significant difference in the altitudinal distribution of A. aureolatum and A. ovale (Mann-Whitney U-test, U = 518.5, P < 0.001). The overlap of these two species occurred at altitudes of between 650 and 900 m a.s.l. The results indicated that the higher the altitude, the greater the probability for the occurrence of A. aureolatum and, conversely, the lower the likelihood for the occurrence of A. ovale. The findings of this study improve currrent knowledge on the bioecology of these tick species and have implications for studies on the epidemiology of spotted fever in Brazil.

Raman spectra-based structural classification analysis of quinoidal and derived molecular systems.

This work reports a classification analysis method based on the vibrational Raman spectra of 38 quinones and related structures, spectrally ordering and classifying the compounds. The molecular systems are relevant for chemical and biological processes, with applications in pharmacology, toxicology and medicine. The classification strategy uses a combination of principal component analysis with K-means clustering methods. Both theoretical simulations and experimental data are analysed, thus establishing their spectral characteristics, as related to their chemical structures and properties. The protocol introduced here should be broadly applicable in other molecular and solid state systems.

Antivirals virtual screening to SARS-CoV-2 non-structural proteins.

In March 2020, the World Health Organization (WHO) declared coronavirus disease-19 (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a pandemic. Since then, the search for a vaccine or drug for COVID-19 treatment has started worldwide. In this regard, a fast approach is the repurposing of drugs, primarily antiviral drugs. Herein, we performed a virtual screening using 22 antiviral drugs retrieved from the DrugBank repository, azithromycin (antibiotic), ivermectin (antinematode), and seven non-structural proteins (Nsps) of SARS-CoV-2, which are considered important targets for drugs, via molecular docking and molecular dynamics simulations. Drug-receptor binding energy was employed as the main descriptor. Based on the results, paritaprevir was predicted as a promising multi-target drug that favorably bound to all tested Nsps, mainly adipose differentiation-related protein (ADRP) (-36.2 kcal mol-1) and coronavirus main proteinase (Mpro) (-32.2 kcal mol-1). Moreover, the results suggest that simeprevir is a strong inhibitor of Mpro (-37.2 kcal mol-1), which is an interesting finding because Mpro plays an important role in viral replication. In addition to drug-receptor affinity, hot spot residues were characterized to facilitate the design of new drug derivatives with improved biological responses.

Vibrational frequencies and intramolecular force constants for cisplatin: assessing the role of the platinum basis set and relativistic effects.

The role of platinum basis set (PTBS) and relativistic effects for predicting the vibrational frequencies and intramolecular force constants for cisplatin are discussed. Nonrelativistic and relativistic computational protocols were built at B3LYP/PTBS/jorge-DZP/C-PCM and B3LYP-DKH2/PTBS/jorge-DZP-DKH/C-PCM levels, respectively, where 19 distinct PTBS were tested. As expected, the structural parameters were not very sensitive to the PTBS, however, the inclusion of relativistic effects improves the description of the cisplatin structure. When it comes to the vibrational frequencies, the results show that the PTBS, and mainly the relativistic effects, are both important. Moreover, the PBE0 functional led to better results than B3LYP in the protocols PBE0/LANL2TZ(f)/jorge-DZP/C-PCM (P20) and PBE0-DKH2/Sapporo-DKH3-DZP-2012/jorge-DZP-DKH/C-PCM (P22), which provided a mean absolute deviation (MAD) of only 10.8 cm-1 and 9.5 cm-1, respectively, for vibrational frequencies, which are excellent choices to study Pt complexes. Finally, a discussion of the intramolecular force constants for cisplatin is carried out, with the calculated bond and angles force constants with P20 and P22 protocols being recommended for the parameterization of the force field of cisplatin.

Origin of Enantioselectivity in Chiral Phosphoric-Acid-Catalyzed Azlactone Dynamic Kinetic Resolution.

Theoretical calculations, associated with control experiments, were carried out to gain insights into the mechanism and origin of enantioselectivity in the phosphoric-acid-catalyzed dynamic kinetic resolution of azlactones. The results revealed a Münchnone intermediate as the key species involved in the isomerization of azlactone rings. The developed model was successfully employed in the comprehension and prediction of enantioselectivity under diverse of reaction conditions, including alcoholysis and aminolysis protocols.

Carbon nanohorns as nanocontainers for cisplatin: insight into their interaction with the plasma membranes of normal and breast cancer cells.

Cisplatin (cddp)-based chemotherapy is one of the most effective therapeutic alternatives for breast cancer treatment, the most common form of cancer, despite the severe side effects related to the high toxicity and low selectivity of cddp. To circumvent these drawbacks, the encapsulation of cddp into oxidized carbon nanohorns (CNHoxs) has been shown as a promising formulation with biocompatibility and low toxicity. However, there is still a lack of studies regarding the behavior of this cddp@CNHox nanovector on the cell membranes. This study presents an in silico description of the interactions between cddp@CNHox and membrane models of cancer (C_memb) and normal (N_memb) cells referring to a typical human breast. The results revealed the interaction mechanism of the inclusion complex 3cddp@CNHox (three cddp molecules are included in the CNHox cavity) with these biomembranes, which is a multistep process including approach, landing, insertion, and penetration. The 3cddp@CNHox stability was monitored over time, and demonstrated the trapping of cddp molecules inside the CNHox cavity over all simulations. The van der Waals contribution played a primary role (∼74%) for the complex stability. Moreover, the binding free energy calculations indicated that the interaction of the 3cddp@CNHox complex with the C_memb model was slightly more favorable, on average, than with the N_memb model. Analysis of the hydrogen bonds (HBs) formed over simulations of 800 ns explains the selectivity for the C_memb model, since the total number of HBs established between the inclusion complex and the C_memb model was about three times greater than that with the N_memb model. By reinforcing the potentiality of oxidized CNHox as a nanovector of cddp, the results presented in this study may assist and drive new experimental studies with this nanomaterial, focusing on the development of less aggressive formulations for breast cancer treatment.

Predicting the structure and NMR coupling constant 1J(129Xe-19F) of XeF6 using quantum mechanics methods.

The XeF6 molecule exists as a monomer in the gas phase and as the (XeF6)4 tetramer in solution. Herein we used distinct quantum mechanics methods to study the conformational equilibrium for the XeF6 monomer, which is represented mainly by Oh and C3v symmetric geometries, and for the (XeF6)4 structure found in condensate phases. The NMR 1J(129Xe-19F) coupling constant is predicted using our own NMR-DKH basis set, designed for NMR properties. The C3v conformer of XeF6 was stable only with HF, CCSD, and hybrid DFT functionals with at least 28% exact HF exchange. Increasing the % of HF exchange improves the description of the geometry and the Oh→C3v equilibrium. The BMK, BHandHLYP and LC-ωPBE functionals produce results in excellent agreement with experiments and high-level calculations for the XeF6 molecule. When it comes to the 1J(129Xe-19F) coupling constant, the (XeF6)4 structure must be considered. For that compound, BHandHLYP leads to the best structure, and BMK leads to the best coupling constant; therefore, the generalized protocol BMK/NMR-DKH//BHandHLYP/def2-SVP is recommended to study the XeF6 molecule in the gas phase and solution.

Unveiling the Molecular Structure of Antimalarial Drugs Chloroquine and Hydroxychloroquine in Solution through Analysis of 1H NMR Chemical Shifts.

Chloroquine (CQ) and hydroxychloroquine (HCQ) have been standard antimalarial drugs since the early 1950s, and very recently, the possibility of their use for the treatment of COVID-19 patients has been considered. To understand the drug mode of action at the submicroscopic level (atoms and molecules), molecular modeling studies with the aid of computational chemistry methods have been of great help. A fundamental step in such theoretical investigations is the knowledge of the predominant drug molecular structure in solution, which is the real environment for the interaction with biological targets. Our strategy to access this valuable information is to perform density functional theory (DFT) calculations of 1H NMR chemical shifts for several plausible molecular conformers and then find the best match with experimental NMR profile in solution (since it is extremely sensitive to conformational changes). Through this procedure, after optimizing 30 trial distinct molecular structures (ωB97x-D/6-31G(d,p)-PCM level of calculation), which may be considered representative conformations, we concluded that the global minimum (named M24), stabilized by an intramolecular N-H hydrogen bond, is not likely to be observed in water, chloroform, and dimethyl sulfoxide (DMSO) solution. Among fully optimized conformations (named M1 to M30, and MD1 and MD2), we found M12 (having no intramolecular H-bond) as the most probable structure of CQ and HCQ in water solution, which is a good approximate starting geometry in drug-receptor interaction simulations. On the other hand, the preferred CQ and HCQ structure in chloroform (and CQ in DMSO-d6) solution was assigned as M8, showing the solvent effects on conformational preferences. We believe that the analysis of 1H NMR data in solution can establish the connection between the macro level (experimental) and the sub-micro level (theoretical), which is not so apparent to us and appears to be more appropriate than the thermodynamic stability criterion in conformational analysis studies.

Role of the Enzymatic Environment in the Reactivity of the AuIII-C

The action mechanism of anticancer gold(III) complexes is a multi-step process and depends on their redox stability. First, the gold(III) complex undergoes a ligand exchange reaction in the presence of cellular thiols, such as those available in the active site of the enzyme TrxR, and then, the AuIII → AuI reduction occurs. Most experimental and theoretical studies describe these processes under chemical conditions without considering the enzyme structure effect. In the present study, molecular models are proposed for the [AuIII(C^N^C)(SHCys-R)]+ adduct, with the [AuIII(C^N^C)]+ moiety bonded to the Cys498 residue in the C-terminal arm of the TrxR. This one represents the product of the first ligand exchange reaction. Overall, our results suggest that the exchange of the auxiliary ligand (for instance, Cl- to S-R) plays a primary role in increasing the reduction potential, with the enzyme structure having a small effect. The parent compound [AuIII(C^N^C)Cl] has E° = -1.20 V, which enlarges to -0.72 V for [AuIII(C^N^C)CH3SH]+ and to -0.65 V for the largest model studied, Au-trx. In addition to the effect of the enzyme structure on the redox stability, we also analyze the Au transfer to the enzyme using a small peptide model (a tetramer). This reaction is dependent on the Cys497 protonation state. Thermodynamics and kinetic analysis suggests that the C^N^C ligand substitution by Cys497 is an exergonic process, with an energy barrier estimated at 20.2 kcal mol-1. The complete transfer of the Au ion to the enzyme's active site would lead to a total loss of enzyme activity, generating oxidative damage and, consequently, cancer cell death.

A Novel Hybrid of Chloroquine and Primaquine Linked by Gold(I): Multitarget and Multiphase Antiplasmodial Agent.

Plasmodium parasites kill 435 000 people around the world every year due to unavailable vaccines, a limited arsenal of antimalarial drugs, delayed treatment, and the reduced clinical effectiveness of current practices caused by drug resistance. Therefore, there is an urgent need to discover and develop new antiplasmodial candidates. In this work, we present a novel strategy to develop a multitarget metallic hybrid antimalarial agent with possible dual efficacy in both sexual and asexual erythrocytic stages. A hybrid of antimalarial drugs (chloroquine and primaquine) linked by gold(I) was synthesized and characterized by spectroscopic and analytical techniques. The CQPQ-gold(I) hybrid molecule affects essential parasite targets, it inhibits ß-hematin formation and interacts moderately with the DNA minor groove. Its interaction with PfTrxR was also examined in computational modeling studies. The CQPQ-gold(I) hybrid displayed an excellent in vitro antimalarial activity against the blood-stage of Plasmodium falciparum and liver-stage of Plasmodium berghei and efficacy in vivo against P. berghei, thereby demonstrating its multiple-stage antiplasmodial activity. This metallic hybrid is a promising chemotherapeutic agent that could act in the treatment, prevention, and transmission of malaria.