Elucidating the binding mechanisms of GABA and Muscimol as an avenue to discover novel GABA-mimetic small molecules.

Gamma-aminobutyric acid (GABA) signaling is the principal inhibitory pathway in the central nervous system. It is critical in neuronal cell proliferation and fate determination. Any aberration in GABA inhibition results in psychiatric and neurological diseases. Thus, modulating GABAergic neurotransmission has become the basis of drug therapy for psychiatric and several neurological diseases. Though GABA and muscimol are classical inhibitors of GABA receptors, the search for novel inhibitors continues unabated. In this study, the binding mechanism of GABA and muscimol was elucidated and applied in the search for small molecule GABAergic inhibitors using comprehensive computational techniques. It was revealed that a high-affinity binding of GABA and muscimol was mediated by a water molecule involving α1Thr129 and then stabilized by strong interactions including salt bridges with ß2Glu155 and α1Arg66 amidst hydrogen bonds, π-π stacking, and π -cation interactions with other residues. The binding of GABA and muscimol was also characterized by stability and deeper penetration into the hydrophobic core of the protein which resulted in conformational changes of the binding pocket and domain, by inducing correlated motions of the residues. Thermodynamics analysis showed GABA and muscimol exhibited total binding free energies of -19.85 ± 8.83 Kcal/mol and -26.55 ± 3.42 Kcal/mol, respectively. A pharmacophore model search, based on the energy contributions of implicating binding residues, resulted in the identification of ZINC68604167, ZINC19735138, ZINC04202466, ZINC00901626, and ZINC01532854 as potential GABA-mimetic compounds from metabolites and natural products libraries. This study has elucidated the binding mechanisms of GABA and muscimol and successfully applied in the identification of GABA-mimetic compounds.Communicated by Ramaswamy H. Sarma.

Rhenium(I) Tricarbonyl Complexes of 1,10-Phenanthroline Derivatives with Unexpectedly High Cytotoxicity.

Eight rhenium(I) tricarbonyl aqua complexes with the general formula fac-[Re(CO)3(N,N'-bid)(H2O)][NO3] (1-8), where N,N'-bid is (2,6-dimethoxypyridyl)imidazo[4,5-f]1,10-phenanthroline (L1), (indole)imidazo[4,5-f]1,10-phenanthroline (L2), (5-methoxyindole)-imidazo[4,5-f]1,10-phenanthroline (L3), (biphenyl)imidazo[4,5-f]1,10-phenanthroline (L4), (fluorene)imidazo[4,5-f]1,10-phenanthroline (L5), (benzo[b]thiophene)imidazo[4,5-f]1,10-phenanthroline (L6), (5-bromothiazole)imidazo[4,5-f]1,10-phenanthroline (L7), and (4,5-dimethylthiophene)imidazo[4,5-f]1,10-phenanthroline (L8), were synthesized and characterized using 1H and 13C{1H} NMR, FT-IR, UV/Vis absorption spectroscopy, and ESI-mass spectrometry, and their purity was confirmed by elemental analysis. The stability of the complexes in aqueous buffer solution (pH 7.4) was confirmed by UV/Vis spectroscopy. The cytotoxicity of the complexes (1-8) was then evaluated on prostate cancer cells (PC3), showing a low nanomolar to low micromolar in vitro cytotoxicity. Worthy of note, three of the Re(I) tricarbonyl complexes showed very low (IC50 = 30-50 nM) cytotoxic activity against PC3 cells and up to 26-fold selectivity over normal human retinal pigment epithelial-1 (RPE-1) cells. The cytotoxicity of both complexes 3 and 6 was lowered under hypoxic conditions in PC3 cells. However, the compounds were still 10 times more active than cisplatin in these conditions. Additional biological experiments were then performed on the most selective complexes (complexes 3 and 6). Cell fractioning experiments followed by ICP-MS studies revealed that 3 and 6 accumulate mostly in the mitochondria and nucleus, respectively. Despite the respective mitochondrial and nuclear localization of 3 and 6, 3 did not trigger the apoptosis pathways for cell killing, whereas 6 can trigger apoptosis but not as a major pathway. Complex 3 induced a paraptosis pathway for cell killing while 6 did not induce any of our other tested pathways, namely, necrosis, paraptosis, and autophagy. Both complexes 3 and 6 were found to be involved in mitochondrial dysfunction and downregulated the ATP production of PC3 cells. To the best of our knowledge, this report presents some of the most cytotoxic Re(I) carbonyl complexes with exceptionally low nanomolar cytotoxic activity toward prostate cancer cells, demonstrating further the future viability of utilizing rhenium in the fight against cancer.

Crystal and molecular structures of fac-[Re(Bid)(PPh3)(CO)3] [Bid is tropolone (TropH) and tribromotropolone (TropBr3H)].

Two rhenium complexes, namely, fac-tricarbonyl(triphenylphosphane-κP)(tropolonato-κ2O,O')rhenium(I), [Re(C7H5O2)(C18H15P)(CO)3] or fac-[Re(Trop)(PPh3)(CO)3] (1), and fac-tricarbonyl(3,5,7-tribromotropolonato-κ2O,O')(triphenylphosphane-κP)rhenium(I), [Re(C7H2Br3O2)(C18H15P)(CO)3] or fac-[Re(TropBr3)(PPh3)(CO)3] (2) (TropH is tropolone and and TropBr3H is tribromotropolone), were synthesized and their crystal and molecular structures confirmed by single-crystal X-ray diffraction. Both crystallized in the space group P-1 and display an array of inter- and intramolecular interactions which were confirmed by solid-state 13C NMR spectroscopy using cross polarization magic angle spinning (CPMAS) techniques, as well as Hirshfeld surface analysis. The slightly longer Re-P distance of 1 [2.4987 (5) versus 2.4799 (11) Šfor 1 and 2, respectively] suggests stronger back donation from the carbonyl groups in the former case, possibly due to the stronger electron-donating ability of the unsubstituted tropolonate ring system. However, this is not supported in the Re-CO bond distances of 1 and 2.

Aquation and Anation Kinetics of Rhenium(I) Dicarbonyl Complexes: Relation to Cell Toxicity and Bioavailability.

The aquation reactions of four rhenium(I) dicarbonyl complexes, [Re(CO)2(NN)(PR3)(Cl)], where NN = 1,10-phenanthroline (Phen) and 2,9-dimethyl-1,10-phenanthroline (DMPhen) and PR3 = 1,3,5-triaza-7-phosphaadamantane (PTA) and 1,4-diacetyl-1,3,7-triaza-5-phosphabicylco[3.3.1]nonane (DAPTA). Additionally, the anation reactions of the corresponding aqua complexes with Cl- were investigated. Single crystals of [Re(CO)2(DMPhen)(PTA)(Cl)]·DMF and [Re(CO)2(DMPhen)(DAPTA)(Cl)] were obtained, and their structures were determined using X-ray diffraction. The Re-Cl interatomic distances are 2.4991(13) and 2.4922(6) Å, respectively, indicating a mild trans influence effect of the phosphine ligands. The rate constants, kaq, for the aquation reactions of these complexes spanned a range of (3.7 ± 0.3) × 10-4 to (15.7 ± 0.3) × 10-4 s-1 with the two Phen complexes having rate constants that are 2.5 times greater than those of the DMPhen complexes at 298 K. Similarly, the second-order anation rate constants (kCl) of the resulting aqua complexes, [Re(CO)2(NN)(PR3)(H2O)]+, with Cl- ions at 298 K varied between (2.99 ± 0.05) × 10-3 and (6.79 ± 0.09) × 10-3 M-1 s-1. Likewise, these rate constants for the Phen complexes were almost 2 times faster than those of the DMPhen complexes. The pKa values of the four aqua complexes were determined to be greater than 9.0 for all of the complexes with [Re(CO)2(Phen)(PTA)(H2O)]+ having the highest pKa value of 9.28 ± 0.03. From the pKa values and the ratios of the aquation and anation rate contants, which give thermodynamic Cl- binding constants, the speciation of the rhenium(I) complexes in blood plasma, the cytoplasm, and the cell nucleus were estimated. The data suggest that the aqua complexes would be the dominant species in all three environments. This result may have important implications on the potential biological activity of these complexes.

Synthesis, characterization, kinetic investigation and biological evaluation of Re(i) di- and tricarbonyl complexes with tertiary phosphine ligands.

Rhenium(i) di- and tri-carbonyl complexes of the form fac-[Re(CO)3(L,L'-Bid)X] and [Re(CO)2(L,L'-Bid)X2], where X = aqua (H2O), methanol (CH3OH), triphenylphosphine (PPh3), 1,3,5-triaza-7-phosphaadamantane (PTA), tricyclohexylphosphine (PCy3) and L,L'-Bid = O,O' bidentate ligands (tropolone = TropH and 3-hydroxyflavone = FlavH) and N,O bidentate ligands (8-hydroxyquinoline = QuinH, 5,7-chloro-8-hydroxyquinoline = diCl-QuinH and quinoline-2,4-dicarboxylic acid = QuinH2), were synthesized and unambiguously characterized by 1H-, 13C-and 31P-NMR, IR, UV/Vis and micro-analysis. The crystal structures of four complexes, namely fac-[Re(CO)3(QuinH)(H2O)]·H2O (5), fac-[Re(CO)3(Quin)(PPh3)] (11), fac-[Re(CO)3(diCl-Quin)(PPh3)] (12) and [Re(CO)2(Trop)(PPh3)2]·2C6H5CH3 (20) were obtained. Re-P bonding distances for 11 and 12 are 2.4948(8) and 2.4908(8) Å, respectively, indicating the effect of the electron-withdrawing substituents of the diCl-Quin- ligand. The second-order rate constants for the substitutions of methanol at 25.1 °C in fac-[Re(CO)3(L,L'-Bid)(CH3OH)] (L,L'-Bid = Trop, Flav and QuinH) type complexes by different entering phosphine ligands (PPh3, PCy3, and PTA) varied between 7.23(7) × 10-5 and 1.32(3) × 10-3 M-1 s-1 and were found to depend on the coordinated bidentate ligand (in general k1 (QuinH) < k1 (Trop) < k1 (Flav)). The toxicity of fac-[Re(CO)3(QuinH)(PTA)], fac-[Re(CO)3(Trop)(PTA)], fac-[Re(CO)3(Trop)(PPh3)] and fac-[Re(CO)3(Flav)(PPh3)] on the cervical cancer HeLa and epithelial RPE-1 cell lines was then evaluated. Complex fac-[Re(CO)3(Flav)(PPh3)] (16) and fac-[Re(CO)3(Trop)(PPh3)] (13) displayed the highest cytotoxicity with IC50 values of 12.21 ± 0.17 µM and 13.35 ± 0.94 µM, respectively in HeLa cells. Interestingly, a small selectivity towards cancer over non-cancerous cells was observed for these compounds (IC50 = 18.41 ± 3.16 µM and >25 µM in RPE-1 cells).

Crystal structure of 2-(methyl-amino)-tropone.

The title compound, 2-(methyl-amino)-cyclo-hepta-2,4,6-trien-1-one, C8H9NO, crystallizes in the monoclinic space group P21/c, with three independent mol-ecules in the asymmetric unit. The planarity of the mol-ecules is indicated by planes fitted through the seven ring carbon atoms. Small deviations from the planes, with an extremal r.m.s. deviation of 0.0345 Å, are present. In complexes of transition metals with similar ligands, the large planar seven-membered aromatic rings have shown to improve the stability of the complex. Two types of hydrogen-bonding inter-actions, C-H⋯O and N-H⋯O, are observed, as well as bifurcation of these inter-actions. The N-H⋯O inter-actions link mol-ecules to form infinite chains. The packing of mol-ecules in the unit cell shows a pattern of overlapping aromatic rings, forming column-like formations. π-π inter-actions are observed between the overlapping aromatic rings at 3.4462 (19) Šfrom each other.

Ambient and high-pressure kinetic investigation of methanol substitution in fac-[Re(Trop)(CO)3(MeOH)] by different monodentate nucleophiles.

Methanol substitution in the fac-[Re(CO)3(Trop)(MeOH)] complex (Trop- = tropolonate) was studied with a range of seven nucleophiles, namely pyridine (Py), 4-dimethylaminopyridine (DMAP), imidazole (Im), thiourea (TU), 1-methyl-2-thiourea (MeTU), bromide (Br-) and iodide (I-) at variable temperature, and at ambient and high pressure. The substitution products were characterized by NMR, IR and UV/vis spectroscopy, and by chemical analysis, and the crystal structures of two of these, namely fac-[Re(Trop)(CO)3(Im)] and fac-[Re(Trop)(CO)3(DMAP)], are reported. High-pressure kinetic studies with four of these entering nucleophiles in methanol at 25 °C on fac-[Re(Trop)(CO)3(MeOH)] yielded the following activation volumes, ΔV≠(kL), for the ligation by four nucleophiles as defined by kL (cm3 mol-1): Im: 9.0 ± 0.2; Py: 10.1 ± 0.2; TU: 10.0 ± 0.3 and MeTU: 14.5 ± 0.3. Since these experimental ΔV≠(kL) values were positive but smaller than expected, it was interpreted that these indicated a dissociative/dissociative interchange pathway for these substitution reactions. Kinetic studies at ambient pressure and variable temperature in methanol on fac-[Re(Trop)(CO)3(MeOH)] with a range of eight entering nucleophiles pointed more clearly to a dissociative pathway and yielded the following results, wherein a clear linear free-energy relationship (LFER) was established for the entering nucleophiles Py, DMAP, Im, TU, MeTU, NCS-, Br- and I-, within the following ranges: kL (ligation; M-1 s-1), 0.263 ± 0.001 to 0.765 ± 0.002; k-L (solvolysis; s-1), (0.07 ± 0.01) × 10-3 to 0.674 ± 0.001; KL (equilibrium; M-1); 1.06 ± 0.01 to 2000 ± 500; ΔH≠(kL) (kJ mol-1), 58.0 ± 0.7 to 76.1 ± 0.6, and ΔS≠(kL) (J K-1 mol-1); -55 ± 2 to 6 ± 3.

Structures of rhenium(I) complexes with 3-hydroxyflavone and benzhydroxamic acid as O,O'-bidentate ligands and confirmation of π-stacking by solid-state NMR spectroscopy.

The synthesis and crystal structures of two new rhenium(I) complexes obtained utilizing benzhydroxamic acid (BHAH) and 3-hydroxyflavone (2-phenylchromen-4-one, FlavH) as bidentate ligands, namely tetraethylammonium fac-(benzhydroxamato-κ2O,O')bromidotricarbonylrhenate(I), (C8H20N)[ReBr(C7H6NO2)(CO)3], 1, and fac-aquatricarbonyl(4-oxo-2-phenylchromen-3-olato-κ2O,O')rhenium(I)-3-hydroxyflavone (1/1), [Re(C15H9O3)(CO)3(H2O)]·C15H10O3, 3, are reported. Furthermore, the crystal structure of free 3-hydroxyflavone, C15H10O3, 4, was redetermined at 100 K in order to compare the packing trends and solid-state NMR spectroscopy with that of the solvate flavone molecule in 3. The compounds were characterized in solution by 1H and 13C NMR spectroscopy, and in the solid state by 13C NMR spectroscopy using the cross-polarization magic angle spinning (CP/MAS) technique. Compounds 1 and 3 both crystallize in the triclinic space group P-1 with one molecule in the asymmetric unit, while 4 crystallizes in the orthorhombic space group P212121. Molecules of 1 and 3 generate one-dimensional chains formed through intermolecular interactions. A comparison of the coordinated 3-hydroxyflavone ligand with the uncoordinated solvate molecule and free molecule 4 shows that the last two are virtually completely planar due to hydrogen-bonding interactions, as opposed to the former, which is able to rotate more freely. The differences between the solid- and solution-state 13C NMR spectra of 3 and 4 are ascribed to inter- and intramolecular interactions. The study also investigated the potential labelling of both bidentate ligands with the corresponding fac-99mTc-tricarbonyl synthon. All attempts were unsuccessful and reasons for this are provided.

Synthesis of ReI tricarbonyl complexes with various sulfur- and oxygen-donating ligands: crystal structures of two ReI dinuclear structures bridged by S atoms.

The synthesis and characterization of two dinuclear complexes, namely fac-hexacarbonyl-1κ3C,2κ3C-(pyridine-1κN)[µ-2,2'-sulfanediyldi(ethanethiolato)-1κ2S1,S3:2κ3S1,S2,S3]dirhenium(I), [Re2(C4H8S3)(C5H5N)(CO)6], (1), and tetraethylammonium fac-tris(µ-2-methoxybenzenethiolato-κ2S:S)bis[tricarbonylrhenium(I)], (C8H20N)[Re2(C7H7OS)3(CO)6], (2), together with two mononuclear complexes, namely (2,2'-bithiophene-5-carboxylic acid-κ2S,S')bromidotricarbonylrhenium(I), (3), and bromidotricarbonyl(methyl benzo[b]thiophene-2-carboxylate-κ2O,S)rhenium(I), (4), are reported. Crystals of (1) and (2) were characterized by X-ray diffraction. The crystal structure of (1) revealed two Re-S-Re bridges. The thioether S atom only bonds to one of the ReI metal centres, while the geometry of the second ReI metal centre is completed by a pyridine ligand. The structure of (2) is characterized by three S-atom bridges and an Re...Re nonbonding distance of 3.4879 (5) Å, which is shorter than the distance found for (1) [3.7996 (6)/3.7963 (6) Å], but still clearly a nonbonding distance. Complex (1) is stabilized by six intermolecular hydrogen-bond interactions and an O...O interaction, while (2) is stabilized by two intermolecular hydrogen-bond interactions and two O...π interactions.

Distorted octahedral environments in tricarbonylrhenium(I) complexes of 5-[2-(2,4,6-trimethylphenyl)diazen-1-yl]quinolin-8-olate and 5,7-bis[2-(2-methylphenyl)diazen-1-yl]quinolin-8-olate.

The Re(I) centres of two Re(I)-tricarbonyl complexes, viz. tricarbonyl(pyridine-κN){5-[2-(2,4,6-trimethylphenyl)diazen-1-yl]quinolin-8-olato-κ(2)N(1),O}rhenium(I), [Re(C23H21N4O)(CO)3], (I), and {5,7-bis[2-(2-methylphenyl)diazen-1-yl]quinolin-8-olato-κ(2)N(1),O}tricarbonyl(pyridine-κN)rhenium(I), [Re(C28H23N6O)(CO)3], (II), are facially surrounded by three carbonyl ligands, a pyridine ligand and either a 5-[2-(2,4,6-trimethylphenyl)diazen-1-yl]quinolin-8-olate [in (I)] or a 5,7-bis[2-(2-methylphenyl)diazen-1-yl]quinolin-8-olate [in (II)] ligand, in a slightly distorted octahedral environment. The crystal structure of (I) is stabilized by two intermolecular C-H···O interactions and that of (II) is stabilized by three intermolecular C-H···O hydrogen-bonding interactions.