Spectroscopic Changes Research Articles

Synthesis and spectroscopic characterization of multifunctional D-π-A benzimidazole derivatives as potential pH sensors

We report multifunctional D-π-A molecular systems having the N,N-diethylamino group as a pH sensitive donor unit connected to the electron accepting substituents such as the cyano moiety and the pH responsive benzimidazole fragment. For that purpose we have synthesized three types of derivatives, namely acrylonitrile, Schiff base and iminocoumarin derived benzimidazoles, whose photophysical characterization has been carried out in several polar and non-polar organic solvents as well as aqueous solution buffers with different pH. Computationally supported determination of species involved in prototrophic equilibria, including their respective pKa values, has been performed to aid in understanding structural and substituent effects on their UV–Vis and fluorescence spectral properties and pH sensing potentials. Both sets of results jointly indicate that all molecules are predominantly unionized under neutral conditions, while their transition towards monocationic forms, responsible for spectroscopic response changes, occurs at pH ≈ 1–4 for acrylonitriles 4–5, pH ≈ 4–5 for Schiff bases 11–12, and pH ≈ 4.5–6.5 for iminocoumarins 7–8, thus increasing their practical attractiveness for a broad range of applications in the same order.

The thioacetal-modified phenothiazine as a novel colorimetric and fluorescent chemosensor for mercury in aqueous media

A new probe, called PTE, has been developed for detecting Hg2+ using a phenothiazine (PTE) unit with a dithioacetal group as the response unit. The synthesis of PTE was conducted initially, followed by studying its interactions with a variety of ions using UV–Vis and fluorescence spectroscopy in real water samples. Based on the UV–Vis and fluorescence studies, it was discovered that PTE exhibited turn-on sensor properties against Hg2+ ions. Additionally, the LOD value for PTE/Hg2+ was determined to be 102 nM. Moreover, the study also examined the visual color changes of PTE on filter paper and TLC test strips. The results indicated that PTE can be effectively used in mercury-containing solutions, as it produces fast color and spectroscopic changes.

A Novel Strategy Based on Zn(II) Porphyrins and Silver Nanoparticles to Photoinactivate Candida albicans.

Photodynamic inactivation (PDI) is an attractive alternative to treat Candida albicans infections, especially considering the spread of resistant strains. The combination of the photophysical advantages of Zn(II) porphyrins (ZnPs) and the plasmonic effect of silver nanoparticles (AgNPs) has the potential to further improve PDI. Here, we propose the novel association of polyvinylpyrrolidone (PVP) coated AgNPs with the cationic ZnPs Zn(II) meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin or Zn(II) meso-tetrakis(N-n-hexylpyridinium-2-yl)porphyrin to photoinactivate C. albicans. AgNPs stabilized with PVP were chosen to allow for (i) overlap between the NP extinction and absorption spectra of ZnPs and (ii) favor AgNPs-ZnPs interaction; prerequisites for exploring the plasmonic effect. Optical and zeta potential (ζ) characterizations were performed, and reactive oxygen species (ROS) generation was also evaluated. Yeasts were incubated with individual ZnPs or their respective AgNPs-ZnPs systems, at various ZnP concentrations and two proportions of AgNPs, then irradiated with a blue LED. Interactions between yeasts and the systems (ZnP alone or AgNPs-ZnPs) were evaluated by fluorescence microscopy. Subtle spectroscopic changes were observed for ZnPs after association with AgNPs, and the ζ analyses confirmed AgNPs-ZnPs interaction. PDI using ZnP-hexyl (0.8 µM) and ZnP-ethyl (5.0 µM) promoted a 3 and 2 log10 reduction of yeasts, respectively. On the other hand, AgNPs-ZnP-hexyl (0.2 µM) and AgNPs-ZnP-ethyl (0.6 µM) systems led to complete fungal eradication under the same PDI parameters and lower porphyrin concentrations. Increased ROS levels and enhanced interaction of yeasts with AgNPs-ZnPs were observed, when compared with ZnPs alone. We applied a facile synthesis of AgNPs which boosted ZnP efficiency. We hypothesize that the plasmonic effect combined with the greater interaction between cells and AgNPs-ZnPs systems resulted in an efficient and improved fungal inactivation. This study provides insight into the application of AgNPs in PDI and helps diversify our antifungal arsenal, encouraging further developments toward inactivation of resistant Candida spp.

Salt Bridge Structure of Microhydrated Arginine Kinetically Trapped in the Gas Phase by Evaporative Cooling.

Amino acids and peptides generally exhibit zwitterionic forms with salt bridge (SB) structures in solution but charge-solvated (CS) motifs in the gas phase. Here, we report a study of non-covalent complexes of the protonated amino acid arginine, ArgH+(H2O)n (n = 1-5), produced in the gas phase from an aqueous solution with a controlled number of retained water molecules. These complexes were probed by cold ion spectroscopy and treated by quantum chemistry. The spectroscopic changes induced upon gradual dehydration of arginine were assigned by structural calculations to the transition from SB to CS geometries. SB conformers appear to be present for the complexes with as few as 3 retained water molecules, although energetically CS structures should become prevailing already for ArgH+ with 7-8 water molecules. We attribute the revealed kinetic trapping of arginine in native-like zwitterionic forms to evaporative cooling of the hydrated complexes to as low as below 200 K.

A New LBV Candidate in M33

The evolutionary relationships and mechanisms governing the behavior of the wide variety of luminous stars populating the upper H-R diagram are not well established. Luminous blue variables (LBVs) are particularly rare, with only a few dozen identified in the Milky Way and nearby galaxies. Since 2012, the Barber Observatory Luminous Stars Survey has monitored more than 100 luminous targets in M33, including M33C-4119 which has recently undergone photometric and spectroscopic changes consistent with an S Doradus eruption of an LBV.

Identification of 7-hydroxyindole as an alternative substrate of MauG by in silico and in vitro analysis

MauG catalyzes the six-electron oxidation of pre-tryptophan tryptophylquinone (preTTQ) cofactor in methylamine dehydrogenase (MADH) to form mature tryptophan tryptophylquinone (TTQ) via long-range electron transfer. To identify alternative substrates for MauG, docking models for 10 tryptophan-like compounds were constructed using Autodock Vina. These demonstrated spontaneous binding to the preTTQ binding site of MauG, with hydroxyindoles most frequently sharing the natural substrate binding site of MauG. To confirm the result of in silico analysis, 7-hydroxyindole was reacted with bis-FeIV of MauG. The spectroscopic change, representing the reactivity of MauG, revealed the highly increased reaction rate (k3) toward 7-hydroxyindole, suggesting that bis-FeIV MauG extracted an electron from the 7-hydroxyindole and then oxidized to di-ferric MauG.

Molecular dynamics simulation of ssDNA and cationic polythiophene

In this work, molecular dynamics simulations of complexes composed of single strand DNA (ssDNA) sequences and cationic oligothiophenes are performed to understand experimental findings and the sensing ability of polythiophene electrolytes toward ssDNA. The simulation results exhibit no significant structural effect for replacing the cationic amine moiety with imidazole derivative on the side group of the oligomer. Adding a homopurine strand elongates the oligomer backbone; on the contrary, mixing up the homopyrimidine strand causes compression. On the other hand, these ssDNAs do not notably affect the compactness of the oligomer backbones. The anion-cation interactions play an essential role in the structural and spectroscopic change of cationic polythiophenes (CPTs) upon complexation with ssDNAs. The red shift of CPTs in the UV-VIS spectra with the addition of homopurine strands might be explained by the strong anion-cation, weak π −cation interactions, and high binding affinities. Nonpolar interactions (vdW and SA) and complex solvation energies dominate binding free energies. Hydrogen interaction analyses show that oligomers most likely approach the ssDNAs from their backbone upon complexation except for the duplex containing homopyrimidine strand and oligothiophene possessing imidazole derivative side chain.

Kinetic study of the azo – Hydrazone photoinduced mechanism in complexes with a -Re(CO)3L0/+ core by flash photolysis

The photoprocesses of three rhenium(I) tricarbonyl complexes, fac-Re(NN)(CO)3L0/+, bearing an azo/hydrazone ligand, NN, and L = Cl, Br or CH3CN as axial ligand, were studied by time-resolved spectroscopic techniques. Flash irradiation of deaerated solutions of the complexes in acetonitrile photogenerated intermediates with intense optical absorptions in the UV–vis spectral region of the hydrazone ligand. The decay of the intermediates photogenerated with the Cl, Br and CH3CN ligands in the axial position proceeds by three different mechanisms. For the fac-Re(NN)(CO)3Cl, a three-step mechanism involving isomerization, tautomerization and rotation accounts for the spectroscopic transformations. In contrast to the flash photolysis result, the continuous photolysis of the complex in acetonitrile does not show spectroscopic changes associated with the generation of a stable product, i.e., the complex exhibits a photoreversible behavior. The first step observed in the flash irradiation of fac-Re(NN)(CO)3Cl, is missing in the decay of the photogenerated intermediates with fac-Re(NN)(CO)3Br and fac-[Re(NN)(CO)3CH3CN]+. A possible reason for this absence is that the process is too fast for the time-response of the flash photolysis set-up. Time-resolved spectra and associated reaction rate data for the Re(I) complexes are communicated and discussed. DFT calculations support a plausible E → Z mechanism with thermal back-isomerization mediated by tautomerization.

Crystal formation in Eu3+ - Doped oxyfluorophosphate glass-ceramics for luminescence thermometry

Eu3+ doped-Oxyfluorophosphate glasses with a general composition 75 NaPO3 - (25-x) CaO – x CaF2 (with x ranging from 0 to 25 mol%) were prepared and their spectroscopic changes induced by the addition of fluorine and when applying a thermal treatment in order to grow crystals in the glasses were investigated. The addition of CaF2 at the expense of CaO reduces Ω2 and Ω4 parameters indicating that the Eu3+ ions are located in less covalent and well separated sites in oxyfluorophosphate glasses. An increase in the CaF2 content at the expense of CaO also leads to an increase in the absolute thermometric sensitivity from (4.2 × 10−4) K−1 to (17.6 × 10−4) K−1 at 523 K while decreasing the relative thermometric sensitivity from 3.0% to 1.4%.The heat treatment leads to the precipitation of crystals, the composition of crystal phase depends on the glass composition. For the glasses with x = 0 and 10, the thermal treatment increases Ω6 while reducing Ω4, as a result of an enhanced stiffness and reduced covalency of dopant's environment due to the precipitation of various crystals. As Ω2 remains unchanged, the Eu3+ are suspected to remain mainly in the amorphous part of the glass-ceramics in agreement with the lack of sharp peaks in the emission spectrum. For the glasses with x = 20 and 25, Ω2 increases indicating a change in the site symmetry of Eu3+ resulting from the dopant entering the CaF2 crystalline phase. Due to the changes in the Eu3+ sites in the heat treated glasses, the heat treatment of the glass increases the absolute thermal sensitivity, but reduces the relative thermal sensitivity. The oxyfluorophosphate glass-ceramic with x = 25 exhibits high absolute thermometric sensitivity compared to oxyfluorophosphate glass, but also comparatively low relative sensitivity of 0.5% K−1.

Age-Related Fourier-Transform Infrared Spectroscopic Changes in Protein Conformation in an Aging Model of Human Dermal Fibroblasts

The loss of proteostasis, which results in the accumulation of misfolded proteins, is one of the hallmarks of aging and is frequently associated with the aging process. Fibroblasts are a cellular model widely used in the study of aging and can mimic the loss of proteostasis that occurs in the human body. When studying human aging using fibroblasts, two approaches can be used: fibroblasts from the same donor aged in vitro until senescence or fibroblasts from donors of different ages. A previous study by our group showed that the first approach can be used in the study of aging. Thus, this work aimed to study the spectroscopic profile of human dermal fibroblasts from four donors of different ages using Fourier-transform infrared spectroscopy to identify changes in protein conformation and to compare results with those obtained in the previous study. Partial least squares regression analysis and peak intensity analysis suggested that fibroblasts from older donors were characterized by an increase in the levels of antiparallel β-sheets and a decrease in intermolecular β-sheets, in agreement with our previous results.

Solvatochromic, Halochromic and Thermochromic Effects of Copper(II) Complexes Containing N-tert-butyl 2-Picolylamine, and Halide Ion; a Computational Study

The dihalido N-tert-butyl 2-picolylamine copper(II) complexes, [CuLX2], where X= Cl, Br were synthesized and characterized by elemental analysis, UV–Vis, FT-IR, and EPR spectra, and molar conductance measurements as well as TG and DTA. The crystal structures of the complexes were identified by single-crystal X-ray diffraction analysis, and the density functional theory (DFT) level calculations that reveal both complexes adopt distorted square-planar geometries. The complexes are thermochromic, solvatochromic, and halochromic. The chromotropism properties were investigated using visible electronic absorption spectroscopy. Their solvatochromism properties were investigated in solvents with different polarities and it was found that the observed solvatochromism originates from structural changes of the complexes in different solvents. The aqueous solutions of the complexes exhibit color changes in the pH range of 2.68 to 9.84due to protonation ad deprotonation of ligand. The compounds are also thermochromic and show reversible color changes in different temperatures in high boiling point solvents. Both compounds showed the same identity in an aqueous solution due to the substitution of the water molecules with the halide ligands. These spectroscopic changes were supported by density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations.

Improved Analytical Performance of an Amphiphilic Probe upon Protein Encapsulation: Spectroscopic Investigation along with Computational Rationalization.

An easily synthesizable pyrene-based amphiphilic probe (Pybpa) has been developed, which exhibited no responses with metal ions in the pure aqueous medium despite possessing a metal ion-chelating bispicolyl unit. We believe that spontaneous aggregation of Pybpa in aqueous medium makes the ion binding unit not accessible to the metal ions. However, the sensitivity and selectivity of Pybpa toward Zn2+ ions drastically improve in the presence of serum albumin protein, HSA. The differences in the microenvironment inside the protein cavity, in terms of local polarity, and conformational rigidity might be attributing factors for that. The mechanistic investigations also suggest that there might be the involvement of polar amino acid residues that take part in coordination with Zn2+ ions. Pybpa shows no detectable spectroscopic changes with Zn2+ ions in aqueous medium in the absence of HSA. However, it can effectively recognize Zn2+ ions in the protein-bound form. Moreover, the photophysical behavior of Pybpa and its zinc complex have been investigated with DFT and docking studies. Noteworthy, such an unusual sensing aspect of Zn2+ exclusively in the protein-bound state and particularly in aqueous medium is truly rare and innovative.

The binding of nitrogen-donor ligands to the ferric and ferrous forms of cytochrome P450 enzymes

The cytochrome P450 superfamily of heme-thiolate monooxygenase enzymes can catalyse various oxidation reactions. The addition of a substrate or an inhibitor ligand induces changes in the absorption spectrum of these enzymes and UV–visible (UV–vis) absorbance spectroscopy is the most common and readily available technique used to interrogate their heme and active site environment. Nitrogen-containing ligands can inhibit the catalytic cycle of heme enzymes by interacting with the heme. Here we evaluate the binding of imidazole and pyridine-based ligands to the ferric and ferrous forms of a selection of bacterial cytochrome P450 enzymes using UV–visible absorbance spectroscopy. The majority of these ligands interact with the heme as one would expect for type II nitrogen directly coordinated to a ferric heme-thiolate species. However, the spectroscopic changes observed in the ligand-bound ferrous forms indicated differences in the heme environment across these P450 enzyme/ligand combinations. Multiple species were observed in the UV–vis spectra of the ferrous ligand-bound P450s. None of the enzymes gave rise to the isolation of a single species with a Soret band at ∼442–447 nm, indicative of a 6-coordinate ferrous thiolate species with a nitrogen-donor ligand. A ferrous species with Soret band at ∼427 nm coupled with an α-band of increased intensity was observed with the imidazole ligands. With some enzyme-ligand combinations reduction resulted in breaking of the iron‑nitrogen bond yielding a 5-coordinate high-spin ferrous species. In other instances, the ferrous form was readily oxidised back to the ferric form on addition of the ligand.

Spectroscopic/colorimetric dual-mode rapid and ultrasensitive detection of reactive oxygen species based on shape-dependent silver nanostructures.

Excessive production of reactive oxygen species (ROS) from endogenous and exogenous pathways is linked to oxidative stress and various diseases. Although a variety of ROS probes have been developed, their multistep synthesis strategies and complicated instrumental operating procedures limit their frequent use. In this work, different shaped silver nanostructures including nanoparticles, nanoprisms, and nanocubes were utilized to demonstrate simple spectroscopic and colorimetric techniques for sensitive ROS detection. The nanostructures displayed different sensing behaviours recorded via plasmon tuning with morphological changes upon exposure to ROS. Among the nanostructures, silver nanocubes were found to be extremely efficient in recognising a particular ROS, namely hypochlorite ions. The detection limits of this ROS were calculated to be 23.76 nM, 85.71 nM, and 36.37 nM for silver nanoparticles, nanoprisms, and nanocubes, respectively. A time-dependent microscopic examination was carried out and revealed that the presence of hypochlorite ions deteriorates structural morphologies. The formation of highly reactive chlorite, chlorate, and chloride ions in hypochlorite ion solution was ascribed to the significant spectroscopic and microscopic changes in all the nanostructures. The attenuation of plasmonic peaks and etching of nanostructures by ROS were supported by the increment of the oxidation state of silver. In addition, silver nanocubes were successfully applied to recognize ROS in Spinacia oleracea and real water samples. The results confirm the potentiality of silver nanostructures for sensitive detection of ROS in biological and environmental systems.

Reversible and irreversible stimuli-responsive chromism of a square-planar platinum(ii) salt.

A new simple Pt(ii) terpyridyl salt that shows reversible response towards acetonitrile and irreversible response towards methanol has been reported, accompanied with the colorimetric/luminescent changing from red to yellow. Experimentally and theoretically, the spectroscopic change derives from the hydrogen bonds between crystal water in the Pt(ii) terpyridyl salt and external organic molecules, and the different strength of hydrogen bond leads either reversible or irreversible stimuli-response. Furthermore, this Pt(ii) terpyridyl salt has been on one hand applied as a probe for sensing acetonitrile in water solution, with high selectivity, good reversibility, proper sensitivity and fast response rate, and on the other hand as advanced anticounterfeiting materials. The current study provides a new approach to acquire and design either reversible or irreversible stimuli-responsive luminescent materials.

Judd-Ofelt Analysis and Emission Properties of Dy3+ Ions in Borogermanate Glasses.

Borogermanate glasses singly doped with Dy3+ ions were synthesized and then studied using the absorption and luminescence spectra. Spectroscopic changes of Dy3+ ions have been examined for compositional-dependent glasses with various molar ratios GeO2:B2O3. In this work, several spectroscopic parameters of Dy3+ ions were obtained experimentally and compared to the calculated values from the Judd-Ofelt theory. Luminescence spectra measured for borogermanate glasses consist of blue, yellow and red bands, which correspond to 4F9/2 → 6H15/2, 4F9/2 → 6H13/2 and 4F9/2 → 6H11/2 transitions of Dy3+, respectively. Luminescence lifetimes for the 4F9/2 excited state are reduced, whereas the stimulated emission cross-sections for the most intense 4F9/2 → 6H13/2 yellow transition of Dy3+ increase with increasing GeO2 and decreasing B2O3 concentrations in glass-hosts. Quantum efficiency of the 4F9/2 (Dy3+) excited state is nearly independent on molar ratios GeO2:B2O3. Attractive spectroscopic properties related to the 4F9/2 → 6H13/2 transition of Dy3+ ions are found for borogermanate glasses implying their potential utility for yellow laser action and solid-state lighting technology.

Azo-hydrazone tautomerism in a simple coumarin azo dye and its contribution to the naked-eye detection of Cu2+ and other potential applications

A new tailor-made azo dye of coumarin connected to phenolic derivative is presented herein. Azo-hydrazone tautomerism in aqueous solution of the dye was observed and studied using spectroscopic assays such as 1H NMR, absorption and emission assays, and theoretical studies. Tautomerism was attributed to the presence of a labile phenolic hydrogen in the ortho position to the azo functionality and the hydrazone was found to be the more dominant tautomer. Influence of metal ions on the azo-hydrazone chemical equilibrium and how the accompanying colour and spectroscopic changes can be exploited for various functions, especially the detection and quantification of Cu2+ in aqueous environments was explored. The presence of Cu2+ affects the azo-hydrazone equilibrium resulting in visual appearance and spectroscopic changes and the likely binding sites for Cu2+ were evaluated. Cu2+ pushes the azo-hydrazone equilibrium towards the more conjugated form and the presence of other metal ions does not have any perceivable impact on this mechanism. The dye showed potential applications as a sensor in colorimetric and spectroscopic detection and quantification of Cu2+ in domestic and environmental water samples, photo-imprinting and as a logic gate. The limits of detection (LOD) and quantification (LOQ) for Cu2+ were found to be 0.0779 mg/L and 0.236 mg/L, respectively, much lower than the World Health Organization (WHO) guideline limit for Cu2+ levels in drinking water.

Enhanced Visible Light Absorption and Photophysical Features of Novel Isomeric Magnesium Phthalocyaninates with Cyanophenoxy Substitution

Novel metal-free and Mg(II) [3/4-(3,4-dicyanophenoxy)phenoxy]-substituted phthalocyanines were obtained and characterized using NMR, IR, UV-vis spectroscopy, and mass spectrometry. This substitution provided compounds with good solubility in organic media, and thus their spectroscopic and fluorescent properties were studied. The formation of the investigated phthalocyanine complexes with central magnesium ions led to the stabilization of macrocyclic molecules in solution, preventing aggregation through specific and universal solvation. From meta-substitution to para-substitution, insignificant spectroscopic changes were observed. The complexes exhibited higher values of molar light absorption coefficients and fluorescence quantum yields (up to 55%) compared to ligands. The efficiency of both the fluorescence-quenching and singlet oxygen generation processes in the case of magnesium [3/4-(3,4-dicyanophenoxy)phenoxy]phthalocyaninates was found to exceed that of the unsubstituted zinc phthalocyaninate, which suggests the potential applicability of these compounds as PDT sensitizers.

In situ investigation of the oxidation of a phospholipid monolayer by reactive oxygen species

The oxidation of membrane lipids has been widely studied for several decades owing to its significance in biological systems. However, despite its damaging physiological impact and its known role in many diseases, relatively little is understood about the specific structural consequences of oxidative action, particularly in vivo. In this work, a combination of sum-frequency generation spectroscopy, surface tensiometry, and surface-selective infrared spectroscopies are used to gain deeper insight into the oxidation of phospholipids by reactive oxygen species generated in situ. Oxidation is achieved by employing the Fenton reaction to convert physiological levels of H2O2 into OH and HO2 radicals in proximity to the headgroups of lipid monolayers at the air-water interface. By temporally monitoring the surface tension and spectroscopic changes at the interface as the oxidation proceeds, the impact of oxidation on the structure, conformation, and intermolecular interactions within the membrane has been revealed.

Influence of CO2 on Nanoconfined Water in a Clay Mineral

Developing new technologies for carbon sequestration and long-term carbon storage is important. Clay minerals are interesting in this context as they are low-cost, naturally abundant, can adsorb considerable amounts of CO2, and are present in storage sites for anthropogenic carbon. Here, to better understand the intercalation mechanisms of CO2 in dehydrated and hydrated synthetic Na-fluorohectorite clay, we have combined powder X-ray diffraction, inelastic and quasi-elastic neutron scattering, and density functional theory calculations. For dehydrated Na-fluorohectorite, we observe no crystalline swelling or spectroscopic changes in response to CO2, whereas for the hydrated case, damping of the librational modes related to the intercalated water was clearly observed. These findings suggest the formation of a more disordered water coordination in the interlayer associated with highly confined water molecules. From the simulations, we conclude that intercalated water molecules decrease the layer–layer cohesion energy and create physical space for CO2 intercalation. Furthermore, we confirm that interlayer confinement reduces the Na+ hydration number when compared to that in bulk aqueous water, which may allow for proton transfer and hydroxide formation followed by CO2 adsorption in the form of carbonates. The experimental results are discussed in the context of previous and present observations on, a similar smectite, Ni-fluorohectorite, for which it is established that CO2 attaches to the edge of nickel hydroxide islands present in the interlayer.