Absorption Spectra Of Complexes Research Articles

Methyl- versus ethyl-directed formation of square-planar Cobalt(II) and octahedral Cobalt(III) chelates of the in situ generated Schiff bases

Two mononuclear complexes [Co(LI)2] (1) and [Co(LII)3] (2) (HLI = N-cyclohexyl-3-methoxysalicylideneimine, HLII = N-cyclohexyl-3-ethoxysalicylideneimine) are reported. Although both compounds were obtained using the same synthetic procedure, 1 contains Co(II), while 2 is a Co(III) complex. The crystal structure of 1 was found to be the same as reported recently in the literature, while the crystal structure of 2 was established herein for the first time. The ligands in 1 are trans-1,5-N,O-coordinated, yielding a square planar CoN2O2 coordination core, while complex 2 adopts a meridinal isomeric form with both Δ and Λ configurations (Δ-mer-2 and Λ-mer-2). Optical properties of 1 and 2 were studied by the means of UV–vis spectroscopy in CH2Cl2 and MeOH. Both compounds absorb, mainly, in the UV region due to intraligand transitions and a ligand-to-metal charge transfer. The absorption spectra of both complexes also revealed two low intense bands at about 550 and 650 nm, which were attributed to d–d transitions. Molecular docking was applied to probe 1, Δ-mer-2 and Λ-mer-2 toward a series of the SARS-CoV-2 proteins. Both 1 and 2 are active against all the applied proteins; however, 1 is significantly more efficient with the docking scores being either comparable to or even higher of those of the initial ligands. The best binding affinity for 1 was established for PLpro, Mpro and Nsp3_range 207–379-MES. For both isomers of 2, the best binding scores were revealed with Nsp16_GTA site, PLpro, and Nsp3_range 207–379-MES (for Δ-mer-2) and Nsp16_SAM site (for Λ-mer-2). Ligand efficiency scores for the initial ligands as well as complexes 1 and 2 inside the binding sites of the applied proteins were also calculated. Of all complexes, the ligand efficiency scores for complexes of 1 with PLpro and Mpro are within the recommended ranges for a Hit, and the obtained ligand efficiency scores for complexes PLpro–1 and Mpro–1 are superior to those for the initial ligands.

Ab initio multireference calculation of electronic spectra of the osmium complexes, [Os(bpy) ] and [Os(phen) ] .

The spin-orbit coupling corrected absorption spectra of osmium complexes, [Os(bpy) ] and [Os(phen) ] , were calculated by using ab initio multireference perturbation method (NEVPT2) with relativistic effects taken into account throughout ZORA approximation and corresponding all-electron basis sets. For the same purpose, the time-dependent DFT techniques were used. A very good agreement between NEVPT2 and experimental spectra should be highlighted, especially for the MLCT transitions that occur in visible and near-UV regions ( cm ). Moreover, the present study offers description of excited states of titled osmium complexes and their spectra interpretation using molecular orbitals.

Construction of high-throughput magnetic circular dichroism measurement system and its application to research on magnetic and optical properties of phthalocyanine complexes.

Magnetic circular dichroism (MCD) spectroscopy is a powerful method for evaluating the electronic structure and magnetic and optical properties of molecules. In particular, MCD measurements have been performed on phthalocyanines and porphyrins with various central metal ions, axial ligands, and substituents to elucidate their properties. It is essential to develop a robust high-throughput technique to perform these measurements comprehensively and efficiently. However, MCD spectroscopy requires very high optical quality for each component of the instrument, and even slight cell distortions can impair the baseline flatness. Consequently, when versatility and data quality are important, an optical system designed for a microplate reader is not suitable for the MCD spectrometer. Therefore, in this study, we develop a new magnetic flow-through cell and combine it with an existing CD spectrometer and autosampler to construct a high-throughput system. The effectiveness and performance of this new system are then evaluated. In addition, based on the MCD and absorption spectra of various phthalocyanine complexes, the effects of substituents and solvents on their magnetic and optical properties and the causes of these effects are discussed. The results demonstrate that this system is effective for the evaluation of the physicochemical properties of various phthalocyanine complexes.

Scaling-up VPT2: A feasible route to include anharmonic correction on large molecules

Vibrational analysis plays a crucial role in the investigation of molecular systems. Though methodologies like second-order vibrational perturbation theory (VPT2) have paved the way to more accurate simulations, the computational cost remains a difficult barrier to overcome when the molecular size increases. Building upon recent advances in the identification of resonances, we propose an approach making anharmonic simulations possible for large-size systems, typically unreachable by standard means. This relies on the fact that, often, only portions of the whole spectra are of actual interest. Therefore, the anharmonic corrections can be included selectively on subsets of normal modes directly related to the regions of interest. Starting from the VPT2 equations, we evaluate rigorously and systematically the impact of the truncated anharmonic treatment onto simulations. The limit and feasibility of the reduced-dimensionality approach are detailed, starting on a smaller model system. The methodology is then challenged on the IR absorption and vibrational circular dichroism spectra of an organometallic complex in three different spectral ranges.

Promising sensors for pharmaceutical pollutant adsorption using Clar’s goblet-based 2D membranes

This study focuses on the design of new 2D membranes from connected Clar’s Goblet as a potential sensor for pharmaceutical pollutants, specifically the painkiller drugs aspirin, paracetamol, ibuprofen, and diclofenac. The electronic, optical, and interaction properties are investigated using density functional theory calculations. The Clar’s Goblet membranes (CGMs) that were chosen are semiconductors with an energy gap of around 1.5 eV, according to energy gap calculations and density of states. Molecular electrostatic potential (ESP) analysis shows that CGMs have electrophilic and nucleophilic sites, suggesting their suitability for interacting with pharmaceutical pollutants. The adsorption energies confirm the chemical adsorption of pharmaceutical pollutants with diclofenac showing the strongest adsorption. The UV–Vis absorption spectra of CGMs-drug complexes are analyzed, revealing a redshift compared to the absorption spectrum of CGMs alone, confirming the adsorption of these drugs. Further analysis using hole/electron examinations indicates that the type of excitation is local excitation rather than charge transfer excitation. This study quantitatively characterized hole and electron distribution in excited states using various indices. The analysis revealed local excitation transitions and significant charge transfer between the CGMs molecule and pharmaceutical pollutants. Additionally, non-covalent interaction analysis indicates the presence of van der Waals interactions, highlighting the adsorption behavior of the drugs. These results demonstrate the potential of CGMs as a highly sensitive sensor for pharmaceutical pollutants.

One- and two-photon absorption spectra of organoboron complexes: vibronic and environmental effects.

We synthesized a series of four parent aza-β-ketoiminate organoboron complexes and performed spectroscopic studies using both experimental and computational techniques. We studied how benzannulation influences the vibronic structure of the UV/Vis absorption bands with a focus on the bright lowest-energy π → π* electronic excitation. Theoretical simulations, accounting for inhomogeneous broadening effects using different embedding schemes, allowed gaining in-depth insights into the observed differences in band shapes induced by structural modifications. We observed huge variations in the distributions of vibronic transitions depending on the position of benzannulation. By and large, the harmonic approximation combined with the adiabatic hessian model delivers qualitatively correct band shapes for the one-photon absorption spectra, except in one case. We also assessed the importance of non-Condon effects (accounted for by the linear term in Herzberg-Teller expansion of the dipole moment) for S0 → S1 band shapes. It turned out that non-Condon contributions have no effect on the band shape in one-photon absorption spectra. In contrast, these effects significantly change the Franck-Condon band shapes of the two-photon absorption spectra. For one of the studied organoboron complexes we also performed a preliminary exploration of mechanical anharmonicity, resulting in an increase of the intensity of the 0-0 transition, which improves the agreement with the experimental data compared to the harmonic model.

Absorption spectra of the purple nonsulfur bacteria light-harvesting complex: A DFT study of the B800 part

We’ve studied the B800 part of Rhodoblastus acidophilus light-harvesting complex (LH2) by several quantum chemical techniques based on the density functional theory (DFT) and determined the specific method and a minimal reliable model suitable for further studies of the LH2. In addition to bacteriochlorophyll a molecules, the minimal model includes two α and one β chain amino acids. Within the model, we are able to reproduce the contribution of the B800 ring of nine bacteriochlorophyll a molecules to the near infrared Qy absorption band. We also discuss the use of hybrid DFT calculations for precise energy and optical estimations and DFT-based tight binding (DFTB) method for the large-scale calculations. Crucial importance of Hartree-Fock exchange interaction for the correct description of B800 peak position was shown.

Synergistic anti-fungal action of azole compounds with mixed ligand complexes and their applications

Selenium (Se (II)) and Zinc (Zn (II)) mixed ligand complexes containing glycine (gly) and pyroglutamic acid (l-PCA) as primary ligands, and salicylic acid (HL1) and glycolic acid (HL2) as secondary ligands have been prepared. These novel complexes were characterized using elemental analysis, 1H NMR, 13CNMR, UV–Vis., FTIR, X-ray diffraction, mass spectra and thermal analysis. The elemental analysis revealed the formation of [1:1:1] zinc or selenium amino acid-hydroxy acid complexes.The bonds interactions between Se (II), Zn (II), and ligands had been investigated using IR spectrophotometry revealing that the ligands exhibited neutral bidentate behavior; the amino acid (primary ligand) coordinates through the nitrogen atom and the carbonyl group's oxygen atom, whereas the hydroxy acids HL1 and HL2 (secondary ligands) coordinate through the OH group and the carbonyl group's oxygen atom.Electronic absorption spectra of the mixed-ligand complexes indicate a square planar shape. The prepared mixed ligand complexes were encapsulated with another powerful antifungal agent, climbazole, in nanostructured lipid carriers (NLCs) for topical application to improve the therapeutic activity. The NLCs were prepared by a hot homogenization method and characterized by particle size, drug entrapment, in vitro drug release, and zeta potential to guarantee the stability of the desired nanostructured lipid carriers (NLCs). The microbiological activity of climbazole-mixed ligand complexes and their encapsulated forms were assessed against Malassezia furfur, Staphylococcus aureus, Pseudomonas aeruginosa, Candida albicans and Escherichia coli. In comparison to Se (II) and Zn (II) mixed ligand complexes alone, the study revealed that the combination of Se (II) and Zn (II) mixed ligand complexes with climbazole in NLCs considerably increased antibacterial activity against the tested microbial strains. P ≤ 0.05 indicates statistical significance for the difference.

Structural and optical properties of the Agn-tyrosine complexes (n = 3-12): a density functional theory study.

We study the optical properties of Agn (n = 3-12) neutral clusters and their coordination with a tyrosine (Tyr) molecule. A global search strategy coupled with density functional theory (DFT) computations explored the potential energy surface. Adsorption energy calculations predicted that Tyr coordination stabilizes the metal clusters, favouring the Agn-Tyr complexes with an even number of silver atoms. For the Agn low-lying isomers, the general shape and the major transitions of the calculated time dependent-DFT (TD-DFT) absorption spectra align with those of previous reports measured in an argon environment. We use the analysis of non-covalent interactions to identify the specific interactions between each silver cluster and functional groups of Tyr. The TD-DFT absorption spectra for the Agn-Tyr complexes showed that Tyr significantly modifies the optical properties of the coordinated silver clusters and affects the smaller systems to a greater extent. The optical absorption results of the bare Agn clusters and the Agn-Tyr complexes are compared and discussed in detail.

HOMO- AND HETERONUCLEAR Yb(III) AND Lu(III) COMPLEXES WITH CALIX[4]ARENE MODIFIED WITH PORPHYRIN SUBSTITUENTS.

In this work we describe the approaches for the synthesis of lanthanide-containing homo- and heteronuclear complexes using 5,11,17,23-tetra-tert-butyl-25,27-dihydroxy-26,28-bis[(N-meso-­(p-aminophenyl)-meso-triphenylporphyrincarbonyl)methoxy]-calix[4]arene (1). Such spatially preorganizedcalix[4]arene macrocycle facilitates lanthanide cation coordination, impacting the structure and analytical signal of porphyrin substituents. The study employs two synthesis stra­tegies resulting in mononuclear and homobinuclear complexes, influenced by steric effects and changes in hydrogen bonding. The complexes exhibit changes in fluorescence spectra due to intramolecular dimerization and hydrogen bonding during complex formation. Mononuclear complexes (Ln-1) are synthesized under mild conditions, while homobinuclear complexes (Ln2-1) require high-boiling solvents and specific reaction conditions, detailed synthesis procedures are described. The structural changes during complex formation were analyzed with the use of NMR analysis. The absorption spectra of lanthanide complexes demonstrate shifts in maxima, reflecting coordination changes, with notable variations between mononuclear and homobinuclear complexes. Additionally, heteronuclear compounds with copper(II), zinc(II), and palladium(II) exhibit distinctive absorption patterns, emphasizing the role of d-metal ions. Luminescence stu­dies reveal the sensitization of 4f-luminescence by both calix[4]arene and porphyrin fragments. The use of deuterated solvents enhances 4f-luminescence intensity, highlighting solvent effects. Furthermore, 4f-luminescence characteristics were investigated in both solid and solution states for heterometallic compounds, emphasizing differences between copper-containing and zinc/palladium-containing complexes. The results offer valuable insights into the design, synthesis, and spectral properties of these complexes, showcasing their potential applications in various fields, and the findings contribute to the understanding of lanthanide-porphyrin systems and their coordination behavior.

Platinum(II) Complexes of Nonsymmetrical NCN-Coordinating Ligands: Unimolecular and Excimeric Luminescence Properties and Comparison with Symmetrical Analogues.

A series of seven new platinum(II) complexes PtLnCl have been prepared, where Ln is an NCN-coordinating ligand comprising a benzene ring 1,3-disubstituted with two different azaheterocycles. In PtL1-5Cl, one heterocycle is a simple pyridine ring, while the other is an isoquinoline, a quinoline, a pyrimidine (L1, L2, L3), or a p-CF3- or p-OMe-substituted pyridine (L4 and L5). PtL6Cl incorporates both a p-CF3 and a p-OMe-substituted pyridine. The synthesis of the requisite proligands HLn is achieved using Pd-catalyzed cross-coupling methodology. The molecular structures of six of the Pt(II) complexes have been determined by X-ray diffraction. All the complexes are brightly luminescent in deoxygenated solution at room temperature. The absorption and emission properties are compared with those of the corresponding symmetrical complexes featuring two identical heterocycles, PtLnsymCl, and of the parent Pt(dpyb)Cl containing two unsubstituted pyridines [dpybH = 1,3-di(2-pyridyl)benzene]. While the absorption spectra of the nonsymmetrical complexes show features of both PtLnsymCl and Pt(dpyb)Cl, the emission generally resembles that of whichever of the corresponding symmetrical complexes has the lower-energy emission. PtL1Cl differs in that─at room temperature but not at 77 K─it displays emission bands that can be attributed to excited states involving both the pyridine and the isoquinoline rings, despite the latter being unequivocally lower in energy. This unusual behavior is attributed to thermally activated repopulation of the former excited state from the latter, facilitated by the very long-lived nature of the isoquinoline-based excited state. At elevated concentrations, all the complexes show an additional red-shifted emission band attributable to excimers. For PtL1Cl, the excimer strikingly dominates the emission spectra at all but the lowest concentrations (<10-5 M). Trends in the energies of the excimers and their propensity to form are compared with those of the symmetrical analogues.

A Combined Wave Function and Density Functional Approach for K-Edge X-ray Absorption Near-Edge Spectroscopy: A Case Study of Hydrated First-Row Transition Metal Ions.

The prediction of X-ray absorption spectra (XAS) of transition metal complexes has important and broad application areas in chemistry and biology. In this letter, we have investigated the predictive ability of multiconfiguration pair-density functional theory (MC-PDFT) for X-ray absorption spectra by calculating the metal K pre-edge features of aquated 3d transition metal ions in common oxidation states. MC-PDFT results were compared with experimentally measured spectra as well as analyzed against results from restricted active-space second-order perturbation theory (RASPT2) and time-dependent density functional theory (TDDFT). As expected, TDDFT performs well for excited states that can be accurately represented by singly excited configurations but fails for excited states where higher order excitations become important. On the other hand, both RASPT2 and MC-PDFT provide quantitatively accurate results for all excited states irrespective of their character. While core-level spectroscopy with RASPT2 is accurate, it is computationally expensive. Our results show that MC-PDFT performs equally well with significantly lower computational cost and is an encouraging alternate approach for X-ray spectroscopies.

Synthesis, biological evaluation and molecular docking studies of a new magnesium (II) porphyrin complex

A new bioactive material of magnesium porphyrin complex [Mg(TPP)](H2O)2].2/3(2.2.2).1/3(Bz) (I), (TPP = meso-‑tetraphenylporphyrinato and (2.2.2) = cryptand, and Bz is benzene (C6H6)) has been synthesized and characterized by Single-crystal X-ray diffraction (SCXRD). The title compound crystallizes in trigonal crystal system in space group P-3. The magnesium ion is coordinated to four nitrogen atoms of the porphyrin core and two oxygen atoms of the axial ligands. The crystal packing is stabilized by inter-and intramolecular hydrogen bonds, and by weak C–H…Cg π interactions leading to a bi-dimensional network. In order to confirm the crystal structure, the photophysical properties have been evaluated by infrared (IR), proton nuclear magnetic resonance 1H NMR and UV–Vis spectroscopy. UV–Visible absorption spectrum of complex (I) displayed a red shifted Soret (429 nm) and Q (567 and 609 nm) bands due to the extended conjugation. The IR, and 1H NMR confirm the crystal structure. Finally, bioactivity investigations revealed that free porphyrin H2TPP, [Mg(TPP)] and complex (I) could be used as potential novel significant antibacterial agents. The docking of the free base porphyrins, metallated and the complex (I), in the active sites of bacterial proteins EEscherichia coli (1ECL) and Aspergillus Niger (3K4P) was carried out to detect the degree of antibacterial activity of recognition.

Cd(II) complexes derived from thiazoline, hydrazide and carbodithioate ligands: synthesis, crystal structures and electrochemical sensing of uric acid

Three novel Cd(II) metal complexes 1–3 have been synthesized with different derivatives of thiazoline, hydrazide and carbodithioate. These metal complexes are abbreviated as [Cd (tht)2]n, [Cd (Hpph)2]n and [Cd (mpps)2]n along with their respective ligands Htht, H2pph and mpps. The developed metal complexes are characterized using X‐ray diffraction (XRD) data and other spectroscopic techniques (Infrared [IR], nuclear magnetic resonance [NMR], ultraviolet–visible [UV–vis] and fluorescence). The polymeric nature of these cadmium complexes have been ascertained by single XRD data. Several significant interactions were also revealed, which aid in the stabilization of these complexes' supramolecular architecture. Comparing complex 2 with complexes 1 and 3, the absorption spectra of complex 2 exhibits a greater λmax. On comparing the fluorescence study of these complexes, complex 2 has a higher fluorescence than complexes 1 and 3. The cyclic voltammetry (CV) approach was employed to detect the electrochemical behaviour of these complexes, as well as the sensing of uric acid (UA) by these complexes via modified glassy carbon electrodes (GCEs). According to the conclusions received by CV study, the modified electrode containing complex 3 has admirable UA electro catalytic activity. This UA electrochemical sensing device offers a low detection limit (0.3 μM), fine linear ranges (30–1500 μM), reasonable sensitivity, and a fast reaction time.

EXTRACTION OF LIGHT GROUP LANTHANIDE NITRATES BY A BINARY EXTRACTANT BASED ON DI-(2-ETHYLHEXYL)PHOSPHORIC ACID AND TRIOCTYLAMINE

The process of extracting the light group lanthanide nitrates from neutral aqueous solutions with a binary extractant based on di-(2-ethylhexyl)phosphoric acid (HA), tri-n-octylamine (NR3) and solvating additive of tri-n-octylamine nitrate in a non-polar organic diluent was investigated in this study. Using neodymium nitrate as an example, it was shown that this extractant (NR3HA+NR3HNO3) extracts Ln(NO3)3 according to the laws of binary extraction. The cation and anion of the inorganic salt pass into the organic phase in a stoichiometric ratio. The extraction isotherms of lanthanide nitrates are S shape, and only slight variation of pH is observed during the extraction. Based on the study of the absorption spectra of Nd complexes in the organic phase, it was found that at low concentration the Nd form a complex (NdA3)•NR3HNO3, and at a high concentration of Nd (when the extraction is saturated), a compound of the composition (NdA2NO3)•NR3HNO3 is formed. The introduction of salting-out agents in the aqueous phase or the organic phase makes it possible to vary the distribution coefficients (DNd). With an increase in the concentration of sodium nitrate in the aqueous phase, DNd increases, and with an increase in the concentration of NR3HNO3, it correspondingly decreases. The introduction of the salting-out and solvating agent NR3HNO3 into the organic phase eliminates the formation of precipitation in the organic phase during the extraction of lanthanide nitrates. Quantitatively, the Nd extraction process in a wide range of Nd(NO3)3 concentrations and extractant compositions can be described on the basis of two coupled binary extraction equations and the corresponding concentration constants. It has been established that the proposed extractant provides high separation factors for neodymium and other light group lanthanides.

Pd(II), Ni(II), and Cu(II) complexes of α,α'-ditolylmethanone dipyrroethene.

Dipyrromethenes containing two pyrrole rings connected by one meso-carbon are versatile monoanionic bidentate ligands and form coordination complexes with many metals/nonmetals/metalloids. Dipyrroethenes containing one additional meso-carbon compared to dipyrromethenes have more space between coordinating pyrrole nitrogens and provide a good coordination environment but have not been explored as ligands in coordination chemistry. Dipyrroethenes are dianionic bidentate ligands and by suitable modifications, the coordination environment of dipyrroethenes can be changed further. Herein, we successfully synthesized α,α'-ditolylmethanone dipyrroethene which is a bipyrrolic tetradentate ligand with an ONNO ligand core and used it for the synthesis of novel Pd(II), Ni(II), and Cu(II) metal complexes by treating it with respective metal salts in CH2Cl2/CH3OH at room temperature. The X-ray crystallographic structure of the metal complexes showed that the M(II) ion was coordinated to the ONNO atoms of the ligand in a perfect square planar geometry. The NMR studies of Pd(II) and Ni(II) complexes also supported the highly symmetric nature of the metal complexes. The absorption spectra of the metal complexes showed strong bands in the region of 300-550 nm. The electrochemical studies of metal complexes revealed that only ligand-based oxidation and reduction were observed. The DFT and TD-DFT studies were in agreement with the experimental observations. Our preliminary studies indicated that the Pd(II) complex can be used as a catalyst for the Fujiwara-Moritani olefination reaction.

Spectroscopic Investigations of Porphyrin-TiO2 Nanoparticles Complexes

This study presents the spectral characterization of TiO2 nanoparticles (NPs) functionalized with three porphyrin derivatives: 5,10,15,20-(Tetra-4-aminophenyl) porphyrin (TAPP), 5,10,15,20-(Tetra-4-methoxyphenyl) porphyrin (TMPP), and 5,10,15,20-(Tetra-4-carboxyphenyl) porphyrin (TCPP). UV-Vis absorption and Fourier transform infrared spectroscopy-attenuated total reflection (FTIR-ATR) spectroscopic studies of these porphyrins and their complexes with TiO2 NPs were performed. In addition, the efficiency of singlet oxygen generation, the key species in photodynamic therapy, was investigated. UV-Vis absorption spectra of the NPs complexes showed the characteristic bands of porphyrins. These allowed us to determine the loaded porphyrins on TiO2 NPs functionalized with porphyrins. FTIR-ATR revealed the formation of porphyrin-TiO2 complexes, suggesting that porphyrin adsorption on TiO2 may involve the pyrroles in the porphyrin ring, or the radicals of the porphyrin derivative. The quantum yield for singlet oxygen generation by the studied porphyrin complexes with TiO2 was higher compared to bare porphyrins for TAPP and TMPP, while for the TCPP-TiO2 NPs complex, a decrease was observed, but still maintained a good efficiency. The TiO2 NPs conjugates can be promising candidates to be tested in photodynamic therapy in vitro assays.

A biophysical approach of cytarabine anticancer drug insights into human serum albumin and checkpoint kinase 1

The present studies used biophysical techniques to examine the binding interaction mechanism of cytarabine anticancer drugs with human serum albumin (HSA) and checkpoint kinase 1. The UV–vis absorption spectrum of the cytarabine-HSA complex reveals a blue shift due to the presence of the cytarabine drug. The results of the fluorescence spectroscopy indicate that static quenching happened as the cytarabine drug concentration in HSA was increased and thermodynamical parameters, binding constant (K) were calculated at different temperatures (293 K, 298 K, 303 K) results have good agreement with molecular docking studies. Molecular docking and molecular dynamics studies were carried out for both HSA and Chk1 protein complexes. In continuing with docking, we have also carried out DFT analysis at the cytarabine drug binding active site of HSA and Chk1 molecule to understand the internal stability at the atomic level.

A Study on the Luminescent Terbium(III) and Pyridine 2,6 Dicarboxylate Complexes by Experimental and TD-DFT Approaches.

Tb3+ luminescence is enhanced by complex formation in aqueous phases as its pyridine 2,6 dicarboxylate (dpa2-) complexes by using experimental spectroscopic techniques and theoretical time-dependent density functional theory (TD-DFT) calculations. The fluorimetric titration of Tb3+ ion with dpa2- ion is followed at λext/λems = 310/490 nm and 310/545 nm, the emission intensities of which are graphed against the mol ratios of the ligand to metal ion [moles of dpa2-/mol of Tb3+]. Experimental results denote that the tris complex; [Tb(dpa)3]3- is the most stable form at pH > 5.3. Molecular absorption spectra of tris complex shows a batho-chromic shift of 222nm of dpa2- band to 232nm accompanied by the hyper-chromic effect at 272nm band. The luminescence intensities at 490, 545, 592 and 620nm are enhanced over 100 times in tris complex. The coordination of complexes calculated by thermodynamic cycles and with supporting the experimental result, the most stable form was found to be nine coordinated tris complex; [Tb(dpa)]3-. The theoretical TD-DFT calculations perfectly matched the experimental absorption and emission bands of tris-complex. The novelty of this study is to present the first theoretical calculation of the phosphorescence results and energy transfer process for emission path of Tb3+ and pda2- aqua complexes.

Neutron crystallography and quantum chemical analysis of bilin reductase PcyA mutants reveal substrate and catalytic residue protonation states

PcyA, a ferredoxin-dependent bilin pigment reductase, catalyzes the site-specific reduction of the two vinyl groups of biliverdin (BV), producing phycocyanobilin. Previous neutron crystallography detected both the neutral BV and its protonated form (BVH+) in the wildtype (WT) PcyA-BV complex, and a nearby catalytic residue Asp105 was found to have two conformations (protonated and deprotonated). Semiempirical calculations have suggested that the protonation states of BV are reflected in the absorption spectrum of the WT PcyA-BV complex. In the previously determined absorption spectra of the PcyA D105N and I86D mutants, complexed with BV, a peak at 730nm, observed in the WT, disappeared and increased, respectively. Here, we performed neutron crystallography and quantum chemical analysis of the D105N-BV and I86D-BV complexes to determine the protonation states of BV and the surrounding residues and study the correlation between the absorption spectra and protonation states around BV. Neutron structures elucidated that BV in the D105N mutant is in a neutral state, whereas that in the I86D mutant is dominantly in a protonated state. Glu76 and His88 showed different hydrogen bonding with surrounding residues compared with WT PcyA, further explaining why D105N and I86D have much lower activities for phycocyanobilin synthesis than the WT PcyA. Our quantum mechanics/molecular mechanics calculations of the absorption spectra showed that the spectral change in D105N arises from Glu76 deprotonation, consistent with the neutron structure. Collectively, our findings reveal more mechanistic details of bilin pigment biosynthesis.