Solvent effects to compute UV–vis spectra for ionic metal complexes

Theoretical characterization and biological activity investigation of indirubins, cyclin dependent kinases inhibitors

Up to now, significant research efforts have been directed towards investigating indirubin and its derivatives as potential candidates for developing new compounds with multiple biological activities. In the present work, natural indirubin and numerous of its chemical derivatives referred to as indirubins have been investigated computationally using DFT method with the B3LYP/6-311 + G(d,p) level of theory, in order to reveal structure- biological activity relationship. We started with a structural properties description. Results analysis indicated that extra interaction sites were provided through the set of substitutions in compounds (1): Indirubin-3'-monoxime, (2): Indirubin-5-sulfonic acid, (3): 5-Nitro-indirubinoxime, (4): 5'-OH-5-nitro-indirubinoxime (AGM130), (5): 7-Bromo-5'-carboxyindirubin-3'-oxime, and (6): 7 BIO and consequently, extra hydrogen bonds may be formed with the active sites of molecular targets, such as GSK-3, CDKs, and Aurora kinases, as well as the aryl hydrocarbon receptor. Subsequently, to get more information on the electronic properties of indirubin and its analogues, HOMO, LUMO, Egap, and further electronic parameters were carried out. The indirubin derivatives showed an easier interaction with its environment than indirubin, the parent compound. The UV-Visible spectra of indirubin and compounds 1-6 were also produced using TD-DFT with B3LYP functional and 6-311 + G(2d,p) basis set. The relationship between absorption and chemical structure is discussed. Two phototoxic brominated compounds showed important absorption spectra modifications. It was also found that the main absorption bands of all compounds derived from π→π*(HOMO→LUMO) transitions.Communicated by Ramaswamy H. Sarma.

New insights for titanium(iv) speciation in acidic media based on UV-visible and 31P NMR spectroscopies and molecular modeling

Titanium chemistry in aqueous acidic media has been extensively investigated over the last decades. Hydrolyzed species such as Ti(OH)³⁺, TiO²⁺, Ti(OH)₂²⁺ or Ti(OH)₃⁺ have been identified and their equilibria have been studied in nitric and perchloric acid. A predominance of the divalent cations was found for low pH (i.e., pH <2). Nonetheless, recent literature reports the existence of small titanium oxo-clusters in aqueous acidic media for large titanium(iv) concentration (typically., >0.1 mol L⁻¹), as stable precursors for the formation of condensed titanium dioxide. The present paper reconsiders firstly previous knowledge about the speciation of titanium(iv) in non-complexing acidic media by giving evidence for the presence of polynuclear hydrolyzed species, even at very low Ti(iv) concentration (i.e., typically <0.1 mmol L⁻¹). UV-visible absorbance spectra recorded for diluted nitric acid solutions (a model of non-complexing acidic medium) containing titanium(iv) were compared to time-dependent density functional theory (TD-DFT) predicted excitation energies. Experimental and predicted maximal absorbance wavelengths showed significantly improved matches when polynuclear species were considered in TD-DFT calculation. Then, 0.1–12.7 mol L⁻¹ phosphoric acid solutions containing titanium(iv) were studied by means of spectroscopic techniques (UV-visible, NMR) in order to identify qualitatively the presence of titanium(iv) complexes and to link this speciation to the acid concentration. Two different titanium(iv) orthophosphate complexes, potentially polynuclear, were detected, and the presence of free titanium(iv) is also expected for low phosphoric acid concentration (i.e., <0.1 mol L⁻¹). A general complexation scheme for a large range of H₃PO₄ concentration was thus formulated.

A spectroscopic study of titanium(iv) speciation in diluted nitric acid (model of non-complexing medium) and 0.1–12.7 mol L⁻¹ phosphoric acid aqueous solutions. Evidence for the presence of polynuclear species is supported by molecular modeling.

TD-DFT assessment of UV-vis spectra palladium and platinum complexes with thiols and disulfides

Time-dependent density functional theory (TD-DFT) and spectrophotometric methods were used for speciation analysis in systems disulfides (cystine, cystamine, homocystine, 3,3-dithiodipropionic acid) - [PdCl₄]^2- or [PtCl₄]^2-. We use the M06-2X and CAM-B3LYP density functionals with Def2-SVP basis set to reproduce the experimental UV-vis spectra; the polarized continuum solvation model (PCM) was fitted to take into account solvation effects of the medium (water). Used methods have shown the good agrees with the experiment - theoretical values of transition energies differ from real parameters within ±0.15 eV for functional CAM-B3LYP. Binuclear disulfide complexes of Pd(II) with cystine and cystamine have form S,N-coordination sites, instead of S,S-conformation. It was shown that Pd(II) thiolate complexes formed by cleavage of the disulfide bond exist as [PdCl₃L] and [Pd₂S₂L₂]. Pt(II)-disulfide systems have confirmed the presence of [Pt₂Cl₆(R-SS-R)] and [PtCl₄(S-R)] complex species. The DFT/CAM-B3LYP/Def2-SVP/SMD level can be recommended for theoretical estimations of absorption spectra of complexes of palladium or platinum and sulfur-containing ligands.

Study of interaction of metal ions with methylthymol blue by chemometrics and quantum chemical calculations

In this study, we determine the acidity constants of methylthymol blue (MTB) and association constants of its complexes with the ZnII, CuII, and FeII metal ions (MIs), through theoretical and experimental means. The complexes were characterized using UV-Visible absorption spectroscopy combined with soft/hard chemometrics methods and quantum chemical calculations. Quantum chemical calculations revealed that electronic transitions in the UV-Visible spectra of MTB have mixed n → π* and π → π* characters. The results of molar ratio and multivariate curve resolution alternating least squares (MCR-ALS) revealed the formation of successive 1:2 and 1:1 complexes (MI:MTB) for the ZnII and CuII systems. However, the formation of successive 1:1 and 2:1 complexes are suggested for FeII by the molar ratio and MCR-ALS. The majority of transitions observed in the UV-Visible spectra of the Zn(MTB) and Cu(MTB) complexes have ligand-to-ligand charge transfer (LLCT) characters. However, the transitions in the UV-Visible spectrum of the Fe(MTB) complex have LLCT and metal-to-ligand charge transfer (MLCT) characters. For the Fe2(MTB) complex, the lowest energy transition of has an LLCT character. However, its higher energy transitions are a mixture of LLCT, MLCT, and metal-to-metal charge transfer (MMCT) characters. The correlation between experimental and computed wavelengths revealed that the 1:1 complexes of ZnII and CuII prefer square pyramidal geometries. However, the FeII complexes always show octahedral geometry.

Spectroscopic and theoretical study of the pH effect on the optical properties of the calcium-morin system

Morin (2-(2,4-dihydroxyphenyl)-3,5,7-trihydroxychromen-4-one) is an abundant flavonoid with various pharmacological and biological activities. Considering the ubiquitous presence of calcium cations in biological systems, it seems relevant to study the interaction of this ion with morin and the influence of pH on this system. In a first step, among the four hypothetical chelation sites, the preferential fixing site, its protonation state and the Ca environment have been determined by combining electronic spectroscopies and density functional theory (DFT) and time-dependent DFT calculations. Then, using the same methodology, the fate of the formed complex with the variation of pH was studied. Calcium chelation occurs with the 3-hydroxy-4-keto site with deprotonation of the hydroxyl group. The coordination number of Ca^II does not seem to be a determining parameter insofar whatever the number of solvent molecules present in the coordination sphere of the metal, the calculation of the electronic transitions leads to the same results. With the increase in pH, a first deprotonation of the complex occurs at the level of a solvent molecule in the metal coordination sphere, followed by a deprotonation of the hydroxyl function in position 7.

pH dependency of the structural and photophysical properties of the atypical 2′,3-dihydroxyflavone

2′,3-Dihydroxyflavone (2′3HF) is a natural flavonol that has barely ever been studied, however the scarce studies of its physico-chemical properties have highlighted its atypical behaviour. We present a structural and spectral study of 2′3HF, performed using UV-visible absorption and fluorescence spectroscopies, coupled with DFT and TD-DFT calculations. Although its structure is close to that of 3-hydroxyflavone, 2′3HF shows a much lower pK_a value. We show that the origin of this particularity is the substitution by a hydroxyl group on position 2′, that induces a stronger inter-ring interaction weakening the bonding of the proton at position 3. The main absorption band of the is red-shifted upon deprotonation. The remaining proton is highly bonded in between oxygen atoms 3 and 2′, making the second deprotonation unattainable in methanol. The neutral form can undergo an excited-state intramolecular proton transfer to emit dual fluorescence by the normal and tautomer forms. We suggested five geometries to be the sources of the emission bands, and showed that the energy barriers to interconversions were almost null. The anion is also fluorescent. The Stokes shifts for the neutral normal and anion species are extremely high, that can be explained by the conformational rearrangement, as the species go from twisted in the ground-state, to planar in the excited-state. Finally, another emission band is evidenced when exciting in the vicinity of the absorption maximum of the anion species in acidic medium. We suggest an aggregate with the solvent to be the origin of the emission.

The assignment of the multiple fluorescence emission wavelengths of 2′,3-dihydroxyflavone highlights its particular properties compared to analogues.

Electronic Spectroscopies Combined with Quantum Chemistry Calculations: Study of the Interactions of 3-Hydroxyflavone with Copper Ions

The current study aims at obtaining a better understanding of the mechanisms involved in the complexation of copper ions by 3-hydroxyflavone (3HF), which is one of the most studied compounds of the flavonoid family. To achieve this goal, quantum chemistry calculations combined with electronic spectroscopies, including absorption, fluorescence emission, and excitation, have been used. The formation of successive complexes of stoichiometry (metal/ligand) 1:2, 1:1, and 3:2 has been highlighted. Even under acidic conditions (pH = 4.0), the α-hydroxy-keto function of the molecule presents a high complexing power with regard to copper ions, insofar as a stable complex of 1:2 stoichiometry is obtained with a large conditional stability constant (log β = 8.7). The formation of this predominant species induces a quenching of the dual fluorescence of 3HF, whereas the second complex of stoichiometry 1:1 presents a fluorescence emission.

The crucial role of the inter-ring hydrogen bond to explain the properties of morin

The acid-base properties of morin and its complexation with the Zn^II cation are investigated by experimental and theoretical approaches.

Sorption of 3-hydroxyflavone within channel type zeolites: the effect of confinement on copper(ii) complexation

The confinement effect on the complexation process of Cu(ii) by 3-hydroxyflavone (3HF) was investigated by studying 3HF incorporation in channel-type copper-containing ZSM-5 and mordenite (MOR) zeolites characterized by different pore diameters. Complementary electronic and vibrational spectroscopy techniques point out two distinct behaviors upon 3HF sorption and subsequent complexation depending on the channel diameter in CuZSM-5 and CuMOR. To determine the influence of the internal environment on the interaction between the copper cation and the guest molecule, and to predict the structure of the complexes formed within the narrow-pore ZSM-5 and in the larger pore mordenite, the vibrational spectra of the complexes were calculated using quantum chemical calculations at the DFT level. From the calculations, it is derived that the Cu(3HF)⁺ chelate is formed in CuMOR indicating a weak interaction with the pore walls. In contrast, due to high confinement in CuZSM-5, interactions between copper cations and the narrower pore walls are assumed to take place in addition to 3HF metal complexation. To emphasize the fact that zeolites act as a solid solvent, 3HF complexation was also investigated in methanol solution. In such liquid media, a stable complex Cu(3HF)₂ of 1 : 2 stoichiometry resulting in a double chelation with the metal cation was found to coexist with a minor species [Cu(3HF)(MeOH)₂]⁺ of 1 : 1 stoichiometry. These two complexes show striking analogy with those observed in CuZSM-5 and CuMOR, respectively. Thus, it appears clearly that zeolites can constitute an ideal tool to control and orientate molecular reactivity for the guest in the isolated state.