Reactivity Indexes of Fullerene and Bismullene Mixed Clusters: How the Intruders Modify the Properties

In this investigation, the feasibility of functionalizing fullerene and bismullene with Bi and C as intruders is theoretically explored. The systems analyzed are C_{60–<i>x</i>}Bi_<i>x</i> (with x = 0–10, fullerene-like) and Bi_{60–<i>y</i>}C_<i>y</i> (with <i>y</i> = 0–10, bismullene-like). Optimized geometries, reactivity indexes, and highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO–LUMO) gaps (for analyzing the potential application of these molecules as materials for solar cells) are reported. The most stable structures of bismullene-like systems have cage geometries. The most stable fullerene-like geometries resemble a cup with bismuth atoms at the edge of the bowl. The presence of intruders increases the electron acceptor power and decreases the electron donor power in most cases. HOMO–LUMO gaps indicate that bismullene-like clusters represent better candidates for building solar cells than fullerene-like clusters. This information could be useful for future experiments

Is Silybin the Best Free Radical Scavenger Compound in Silymarin?

Silymarin is a natural mixture with beneficial properties for health, specifically due to its antiradical characteristics. The major components of this mixture are silybin (SIL), silychristin (SILYC), isosilybin (ISOSIL), silydianin (SILYD), and taxifolin (TAX). In this report, the electronic properties of these substances are investigated using density functional theory calculations, mainly in order to fully understand the free radical scavenger properties of these compounds. Optimized geometries and Raman spectra are reported. These results could be experimentally useful for identifying some of the major components of the mixture. The relative abundance of deprotonated species under physiological conditions is also included. The free radical scavenger capacity is studied in relation to three mechanisms: the single electron transfer (SET), the radical adduct formation (RAF), and the hydrogen atom transfer (HAT). According to this investigation, the HAT mechanism is the most efficient mechanism for scavenging free radicals for these compounds followed by the RAF mechanism where intramolecular hydrogen bonds are formed in order to stabilize the ^•OOH free radical. A particularly important factor is that none of the compounds being studied showed an outstanding antiradical capacity performance compared to the others. In this sense, silymarin is an interesting mixture with antiradical properties and we now know that one single component should be as effective as the mixture

Silybin and 2,3-Dehydrosilybin Flavonolignans as Free Radical Scavengers

The electronic properties of six derivatives of silybin (characterized by the absence of the 2,3 double bond) and six derivatives of 2,3-dehydrosilybin (characterized by the presence of the 2,3 double bond) have been studied by applying density functional theory to fully understand the free radical scavenger’s mechanism for action and the relationship between reactivity and chemical structure. Optimized geometries, Raman spectra, and λ_max values are reported, enabling us to characterize the systems. These spectra may be useful for monitoring the oxidation between silybin and 2,3-dehydrosilybin, thus providing important experimental information. The relative abundance of deprotonated species under physiological conditions is also reported. Under physiological conditions (pH 7.4), ∼70% of silybin is protonated, but 60% of 2,3-dehydrosilybin is deprotonated. The free radical scavenger capacity is analyzed in terms of two mechanisms: electron transfer and adduct formation. Deprotonated molecules are better electron donors and worse electron acceptors than non-deprotonated species. The conclusions derived from this investigation completely concur with previous experimental results. The free radical scavenging activity of 2,3-dehydrosilybin derivatives is higher than that for silybin derivatives. What was not previously considered was the importance of the deprotonated species, which is remarkable and may be important for future experiments

Carbohydrates and Their Free Radical Scavenging Capability: A Theoretical Study

A density functional theory (DFT) study on the free radical (OH^• and OOH^•) scavenging properties of some mono- and polysaccharides is presented. Two mechanisms, single electron transfer (SET) and hydrogen atom transfer (HAT), are considered. The former mechanism is studied by making use of the vertical ionization energy and vertical electron affinity of the radicals and carbohydrates. It is confirmed that the SET mechanism is not plausible to occur. With respect to the HAT, not only does the OH^• radical react preferably with one hydrogen atom bonded to one carbon atom, but also the reaction with a hydrogen atom bonded to an oxygen is possible. Finally, it is suggested that the carbohydrates are not able to directly scavenge OOH^•

<i>Cis</i> Carotenoids: Colorful Molecules and Free Radical Quenchers

We present a density functional theory (DFT) and time-dependent density functional theory (TD-DFT) study on the stability, antioxidant properties with respect to the single electron transfer mechanism, and electronic absorption spectra of some isomers (9-<i>cis</i>, 13-<i>cis</i>, and 15-<i>cis</i>) of carotenoids such as astaxanthin, lycopene, and those present in virgin olive oil (lutein, β-carotene, neoxanthin, antheraxanthin, violaxanthin, neochrome, luteoxanthin, mutatoxanthin, and violaxanthin). In general, the calculated relative stability of the <i>cis</i> isomers appears to be in line with experimental observations. It is predicted that the above-mentioned carotenoids (<i>cis</i> and <i>trans</i> isomers) will transfer one electron to the ^•OH radical. However, this transference is not plausible with radicals such as ^•OOH, ^•OC₂H₅, ^•OOC₂H₅, ^•NO₂, and ^•OOCH₂CHCH₂. On the other hand, some carotenoids (β-carotene, lycopene, lutein, astaxanthin, violaxanthin, and antheraxanthin) will likely accept, in a medium of low polarity, one electron from the radical ^•O₂^–. However, neoxanthin, auroxanthin, mutatoxanthin, luteoxanthin, and neochrome would not participate in such an electronic transfer mechanism. The TD-DFT studies show that neutral species of the <i>cis</i> and <i>trans</i> isomers maintain the same color. On the contrary, the ionic species undergo a “bleaching” process where the absorption wavelengths shift to longer values (>700 nm). Additionally, the formation of a complex between astaxanthin and Cu²⁺ is explored as well as the effect that the metal atom will have in the UV–vis spectrum

New Free Radicals to Measure Antiradical Capacity: A Theoretical Study

A new family of free radicals, that are soluble in water and stable at all pH values, were recently synthesized and used to assess the antiradical capacity of several polyphenols. In the present work, density functional calculations were used to investigate the single electron transfer reactions between these new free radicals and polyphenols in aqueous solution. The quantification of the antiradical capacity is a challenge, particularly for polyphenols, since they become unstable under experimental conditions. It was found that the electron transfer from polyphenols to the newly developed free radicals can be used to assess the efficiency of this kind of compound for preventing oxidative stress. Since one of the free radicals can be deprotonated under experimental conditions, this newly synthesized radical can help distinguish more clearly between different antiradical compounds with similar antioxidant capacity by modifying the pH in the experiments. The results reported here are in good agreement with the available experimental data and allowed making recommendations about possible experimental conditions in the design of antioxidant assays using the investigated radicals

Which Is The Best Sandwich Compound? Hexaphenylbenzene Substituted By Sandwich Compounds Bearing Sc, Cr, and Fe

The electronic properties of nine different hexaarylbenzene molecules substituted by sandwich compounds have been studied by applying density functional theory. Different structures and the particular electron donor power of these systems have been considered in order to analyze their oxidant capacity, using bis­(ciclopentadienyl) scandium, ferrocene, and bis­(benzene)chromium as sandwich compounds. Both monometallic and bimetallic combinations are investigated. According to the ionization energies and electron affinities, compounds with Cr are nucleophiles and represent the best electron donors, whereas compounds with Sc are electrophiles and represent the best electron acceptors. The worse electron donor or acceptor is hexakis­(4-ferrocenyl phenyl) benzene. This is very significant, as it implies that the very well-known electronic properties of hexakis­(4-ferrocenyl phenyl) benzene can be improved by substituting with other metals, such as Sc and Cr. This suggests several possible applications for these compounds

Theoretical Study of Novel Azo-Tetraphenylporphyrins: Potential Photovoltaic Materials

A density functional theory study was performed to analyze the electron donor–acceptor properties of the cis and trans isomers of a novel azobenzene-containing tetraphenylporphyrin (TPPN₂PhC₁₄H₂₉) with different substituents (Br or TMS). In general, the trans isomers are better electron acceptors than the correspondent cis homologues. Their UV–vis spectra were also obtained and a comparison with available experimental results is included. According to these results, the azo compounds reported here are promising materials for the elaboration of dye-sensitized solar cells because their HOMO–LUMO gaps are close to 2 eV. Moreover, the energy of the high intensity absorption bands also fulfills the requirements needed for the operation of a solar cell built with TiO₂ and the I^–/I₃^– pair

Dinuclear Copper Complexes with Imidazole Derivative Ligands: A Theoretical Study Related to Catechol Oxidase Activity

Catechol oxidase is a very important and interesting metalloprotein. In spite of the efforts to understand the reaction mechanism of this protein, there are important questions that remain unanswered concerning the catalytic mechanism of this enzyme. In this article, dinuclear copper compounds are used as biomimetic models of catechol oxidase to study plausible reaction paths. These dinuclear copper­(II) complexes have distant metal centers (of 7.5 Å approximately) and superior catalytic activity to that of many dicopper complexes with shorter Cu–Cu distances. One mononuclear copper­(II) complex is also analyzed in this investigation in order to see the influence of the two metal centers in the catalytic activity. Density functional theory calculations were performed to obtain optimized structures, vertical ionization energies, vertical electron affinities, the electrodonating power (ω^–), the electroaccepting power (ω⁺) and the energy difference of several reaction paths. The <i>K</i>_M experimental results that were previously reported compare well with the electroaccepting power (ω⁺) of the copper compounds that are included in this article, indicating that this index is useful for the interpretation of the electron transfer capacity and therefore the catalytic activity. The catechol moiety coordinates to only one Cu ion, but two metal atoms are needed in order to have a good electron acceptor capacity of the biomimetic models

Development of Blood–Brain Barrier Permeable Nitrocatechol-Based Catechol <i>O</i>‑Methyltransferase Inhibitors with Reduced Potential for Hepatotoxicity

Recent efforts have been focused on the development of centrally active COMT inhibitors, which can be valuable assets for neurological disorders such as Parkinson’s disease, due to the severe hepatotoxicity risk associated with tolcapone. New nitrocatechol COMT inhibitors based on naturally occurring caffeic acid and caffeic acid phenethyl ester were developed. All nitrocatechol derivatives displayed potent inhibition of peripheral and cerebral COMT within the nanomolar range. Druglike derivatives <b>13</b>, <b>15</b>, and <b>16</b> were predicted to cross the blood–brain barrier in vitro and were significantly less toxic than tolcapone and entacapone when incubated at 50 μM with rat primary hepatocytes. Moreover, their unique acidity and electrochemical properties decreased the chances of formation of reactive quinone-imines and, as such, the potential for hepatotoxicity. The binding mode of <b>16</b> confirmed that the major interactions with COMT were established via the nitrocatechol ring, allowing derivatization of the side chain for future lead optimization efforts