Experimental and quantum-chemical study of differential absorbance spectra of environmentally relevant species: A study of quercetin deprotonation and its interactions with copper (II) ions

Metal-Flavonoid Interactions-From Simple Complexes to Advanced Systems

For many years, metal-flavonoid complexes have been widely studied as a part of drug discovery programs, but in the last decade their importance in materials science has increased significantly. A deeper understanding of the role of metal ions and flavonoids in constructing simple complexes and more advanced hybrid networks will facilitate the assembly of materials with tailored architecture and functionality. In this Review, we highlight the most essential data on metal-flavonoid systems, presenting a promising alternative in the design of hybrid inorganic-organic materials. We focus mainly on systems containing CuII/I and FeIII/II ions, which are necessary in natural and industrial catalysis. We discuss two kinds of interactions that typically ensure the formation of metal-flavonoid systems, namely coordination and redox reactions. Our intention is to cover the fundamentals of metal-flavonoid systems to show how this knowledge has been already transferred from small molecules to complex materials.

The potential biological effects of quercetin based on pharmacokinetics and multi-targeted mechanism in vivo

Quercetin is a plant-derived polyphenol flavonoid that has been proven to be effective for many diseases. However, the mechanism and in vivo metabolism of quercetin remains to be clarified. It achieves a wide range of biological effects through various metabolites, gut microbiota and its metabolites, systemic mediators produced by inflammation and oxidation, as well as by multiple mechanisms. The all-round disease treatment of quercetin is achieved through the organic combination of multiple channels. Therefore, this article clarifies the metabolic process of quercetin in the body, and explores the new pattern of action of quercetin in the treatment of diseases.

Applying synchronous fluorescence spectroscopy conjunct second derivative and two-dimensional correlation to analyze the interactions of copper (II) with dissolved organic matter from an urbanized river

Heavy metal speciation and distribution is significantly influenced by dissolved organic matter (DOM) exhibited in ecosystems, particularly in urbanized rivers. Synchronous fluorescence spectroscopy (SFS) conjunct second derivative and two-dimensional correlation spectroscopy (2D-COS) was devoted to characterizing interactions of DOM-copper (II). Three typical water samples were collected from Baitapu River. Only protein-like fluorescence (PLF) and fulvic-like (FLF) were identified from the SFS. Stability constant (log K) values of PLF complexes with copper (II) varied from 4.277 to 5.833, and proportion of binding fluorescent materials (f) were 0.054-2.640. The log K values of FLF complexes with copper (II) varied from 3.996 to 4.243, while the f values were 0.001-0.036. Obviously, PLF had much stronger complexing capacity than FLF. There were four obvious peaks in the principal component analysis and second derivative fluorescence spectroscopy (SDFS), i.e., tyrosine-like (TYLF), tryptophan-like (TRLF), microbial humus-like (MHLF) and FLF. The log K values of TYLF and TRLF complexes were 4.899-5.907 and 4.598-5.831, respectively, which were similar to those from PLF. The log K values of MHLF complexes varied from 4.311 to 5.760, and the f values were 0.261-8.688. The log K values of FLF complexes were ranged from 4.598 to 5.831, which were higher than those deduced from the SFS. Interestingly, by the SDSF, PLF was divided into TYLF and TRLF, which increased the parameters values from DOM-copper (II) complexes. 2D-SFS-COS revealed that the TRLF was more susceptive response to copper (II) appended than TYLF, MHLF, and FLF. Moreover, TYLF and TRLF could priorly interact with copper (II). The SDSF conjunct 2D-COS could be effective approaches for insight into the complexing heterogeneity of DOM with copper (II). The study could present a support to preventing heavy metals and organic pollution in urbanized rivers.

Novel Insights into the Molecular-Level Mechanism Linking the Chemical Diversity and Copper Binding Heterogeneity of Biochar-Derived Dissolved Black Carbon and Dissolved Organic Matter

Biochar-derived dissolved black carbon (DBC) varies in chemical composition and significantly affects the environmental fate of metal ions. However, the intrinsic molecular composition of DBC fractions and their molecular interaction mechanisms with metal ions remain unclear. We propose a novel, molecular-level covariant binding mechanism to comparatively interpret the heterogeneities, active sites, and sequential responses of copper binding with molecular compounds in DBC and natural dissolved organic matter (DOM). Relatively large proportions of lipid/aliphatic/peptide-like compounds with low mass distributions and lignin-like compounds with oxidized/unsaturated groups existed in acidic- and alkaline-extracted DBC, respectively. A larger percentage of tannin-like/condensed aromatic compounds and higher average conditional stability constants (logK̅_Cu) of visible fluorescent components were found for DOM than for DBC. Overall, 200-320 Da and 320-480 Da molecular components contributed significantly to the logK̅_Cu values of UVA and visible fluorescent components, respectively, in DBC/DOM. Nitrogenous groups likely exhibited stronger binding affinities than phenolic/carboxylic groups. The sequential copper-binding responses of molecular compounds in DBC/DOM generally followed the order lipid/aliphatic/peptide-like compounds → tannin-like compounds → condensed aromatic compounds. These insights will improve the prediction of the potential effects of DBC on various contaminants and the risks of biochar application to ecosystems.

Interpretation of the differential UV-visible absorbance spectra of metal-NOM complexes based on the quantum chemical simulations for the model compound esculetin

In this study, the model compound esculetin that has functional groups typical for natural organic matter (NOM) was used to ascertain the nature of the characteristic bands in the differential UV-visible absorbance spectra (DAS) associated with the formation of metal-NOM complexes. The binding of ten different metal ions (Cu(II), Ni(II), Co(II), Fe(III), Cr(III), Al(III), Zn(II), Ca(II), Mg(II) and Pb(II)) with esculetin generate four bands in the DAS. These bands are similar to those present in the DAS of metal-NOM complexes. The UV-visible absorbance spectra of the metal-esculetin systems were calculated using time-dependent density functional theory (TD-DFT). The TD-DFT results demonstrate that the prominent features of the DAS of esculetin are primarily associated with the electron transitions between the molecular orbitals near the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the metal-esculetin complex. Charge decomposition analysis (CDA) results demonstrated that these electron transitions originate from the esculetin fragment to the Zn(II) fragment in the complex. Covalent indexes [(χ_m)²r_c] of the metal ions were found to be correlated with the metal-specific features of the DAS of metal-esculetin systems. The strength of the linear correlations between the quantitative parameters of the electron density of the bond critical points (BCP) is indicative of the strength of the metal-esculetin interactions.

A Review on Coordination Properties of Al(III) and Fe(III) toward Natural Antioxidant Molecules: Experimental and Theoretical Insights

This review focuses on the ability of some natural antioxidant molecules (i.e., hydroxycinnamic acids, coumarin-3-carboxylic acid, quercetin, luteolin and curcumin) to form Al(III)- and Fe(III)-complexes with the aim of evaluating the coordination properties from a combined experimental and theoretical point of view. Despite the contributions of previous studies on the chemical properties and biological activity of these metal complexes involving such natural antioxidants, further detailed relationships between the structure and properties are still required. In this context, the investigation on the coordination properties of Al(III) and Fe(III) toward these natural antioxidant molecules might deserve high interest to design water soluble molecule-based metal carriers that can improve the metal’s intake and/or its removal in living organisms.

Human small intestine cancer cell membrane-camouflaged quercetin-melanin for antibacterial and antitumor activity

E. coli has become an important factor that can lead to cancer because of its ability to cause diverse intestinal changes. Nano-polymer materials provide ideal drug delivery systems for preparing antibacterial and anti-cancer drugs because of their unique structure, easy modification, and high drug loading. The modified natural melanin has the potential to be an excellent nano-carrier. By improving the water-solubility and biocompatibility of the loaded natural drug quercetin, the antibacterial effect of quercetin can be fully played. Here, natural melanin was extracted from frozen squid to synthesize carrier polydopamine (PDA) nanoparticles, and the natural drug quercetin (Q) was modified on the surface of PDA by π-π bond and covalent bond action to produce melanin-quercetin (PDA-Q). We also developed human small intestinal cancer cells (HIC) membrane-camouflaged melanin-Quercetin (PDA-Q) nanoparticles as an anti-cancer platform in vivo. The potential bacteriostatic mechanism was likely driven by the penetration of PDA-Q in E. coli cells, damaging the integrity of the membranes of E. coli and inducing cell death. The mice wound experiment and bacteremia model experiment revealed that C@PDA-Q had a strong inhibitory effect on E. coli in vivo. In addition, the results of the in vitro tumor test also revealed that C@PDA-Q had strong anti-tumor activity against HIC cells of human small intestinal cancer, and the IC50 value was 12.3 ± 0.7 μg/ml, which was slightly better than that for cisplatin. As both melanin nanoparticles and HIC membrane are natural biomaterials, the synthesized C@PDA-Q nano-polymer material shows great potential for use in anti-cancer nano-drug loading.

Interpreting main features of the differential absorbance spectra of chlorinated natural organic matter: Comparison of the experimental and theoretical spectra of model compounds

This study compared chlorination-induced changes of the properties of natural organic matter (NOM) represented by standard humic substances and NOM present in pristine and anthropogenically-affect reservoirs, rivers, groundwater and seawater. The chlorination-induced changes of NOM properties were quantified using the differential absorbance spectra (DAS) which were processed via numeric deconvolution. Six Gaussian bands were found to comprise the DAS of all examined waters. These bands (denoted as A0, A1, A2, A3, A4 and A5, respectively) have maxima located at ca. 200, 240, 276, 316, 385 and 547 nm. The bands A1-A4 were observed in the DAS of representative model chlorinated compounds. Quantum chemical (QC) calculations were carried out to examine the intrinsic nature of these bands and electronic transitions associated with them. QC data demonstrate that bands A1 and A2 are present in almost all aromatic organic species, A3 is likely to be associated with acetophenone- and/or styrene-like groups. A4 can be attributed to the engagement of m-hydroxyaromatic and flavone-type groups typical for the polyphenolic moiety in NOM and known to be the key precursors of disinfection by-product (DBP) formation. Thus, the intensity of band A4 is predicted to be an especially strong predictor of DBP formation.

Investigation of the deprotonation of tetracycline using differential absorbance spectra: A comparative experimental and DFT/TD-DFT study

Tetracycline is a type of broad-spectrum, naturally occurring antibiotic that leads to several side effects, such as affecting intestinal flora and increase in bacterial resistance. The affinity of tetracycline for abiotic and biotic surfaces and metal ions is closely related to its deprotonation state and charge distribution; however, its deprotonation-protonation property remains unclear. In this study, the hydrolysis of tetracycline was investigated by combining experiments with quantum-chemical calculations. The molecular structure of the probable deprotonation states were optimized by quantum-chemical calculations, and the corresponding absorbance spectra were predicted based on frontier molecular orbital (FMO) theory. The absorbance spectra showed structure-specific features at the different deprotonation states. In addition, changes in tetracycline absorbance spectra in the pH range of 2.00-12.00 was examined by spectroscopic titration. The deprotonation was found to proceed in the order of site 3, 4, 12, and 10, which was identified by comparing the quantum-chemical calculations and experimental differential absorbance spectra (DAS). The results in this study are of great significance for further studies of the transport and fate of tetracycline in the environment.

Characterizing the interactions between copper ions and dissolved organic matter using fluorescence excitation-emission matrices with two-dimensional Savitzky-Golay second-order differentiation

Monitoring dissolved organic matter (DOM) content in aquatic environments is crucial for not only understanding the dynamics of heavy metals but also predicting their bioavailability. Fluorescence spectroscopy is typically employed to characterise DOM. Here, the interaction between DOM and trace metals was investigated by combining excitation-emission matrix (EEM) quenching with two-dimensional Savitzky-Golay second-order differentiation (2D-SG-2nd-df) analysis. The 2D-SG-2nd-df analysis decomposed the EEM spectra of commercial humic acid (HA) samples into six separate fluorescence peaks, which agreed with the results obtained through conventional parallel factor (PARAFAC) analysis. Compared with PARAFAC modeling, the 2D-SG-2nd-df approach provided more valid and reliable results when the dataset contained distinct samples. Moreover, since the results obtained from 2D-SG-2nd-df for each sample are independent, shifts in the peak wavelength can be reproduced more efficiently using this method. Triplicate titration experiments showed clear differences in HA-copper interactions for samples with different HA composition and molecular weight. The binding strength between copper and low-molecular-weight DOM in water was weaker than that observed for high-molecular-weight DOM. The results obtained in this study will serve as a basis for applying 2D-SG-2nd-df not only to DOM but also to other samples studied using EEM measurements.