Analytical approximation to the local softness and hypersoftness and to their applications as reactivity indicators.

Local density functional theory derivatives of the electron density have been calculated analytically for the set of canonical hydrogenic orbitals; original solutions have been obtained using the novel density gradient theorem. Results for the first and second derivatives of electron density over N (number of electrons) and over µ (chemical potential) have been demonstrated. Calculations of the state functions ΔN, ΔE, and Δµ disturbed by an external potential Δv(r) have been obtained via the concept of alchemical derivatives. The local softness s(r) and local hypersoftness [ds(r)/dN]v have been proved to provide crucial chemical information on the sensitivity of orbital density to the disturbance of the external potential Δv(r), leading to electron exchange ΔN and the corresponding changes of the state functions ΔE, Δµ. The results are fully compatible with the well-understood character of atomic orbitals in chemistry and open a perspective to applications to atoms, free or bonded.

From the Electron Density Gradient to the Quantitative Reactivity Indicators: Local Softness and the Fukui Function.

Important reactivity measures such as the local softness, the Fukui function, and the global hardness have been calculated directly from first principles with the use of the electron density function, beyond the finite difference approximation. Our recently derived density gradient theorem and the principle of nearsightedness of the electronic matter have been instrumental in obtaining the original, albeit approximate, result on the local softness of an atom. By integration of the local softness s(r), we obtain the global softness S and the Fukui function f(r) = s(r)/S. Local and global softness values have also been calculated analytically for the basic hydrogenic orbitals; the general relation to the atomic number S = σZ -2 has been demonstrated, with constants σ characteristic for each orbital type. Global hardness η = 1/S calculated for atoms and ions has been favorably tested against its conventional measure given by the finite difference approximation: (I - A). Calculated test results for atoms and ions in rows 1-4 of the periodic table have been presented.

The antiradical activity of some selected flavones and flavonols. Experimental and quantum mechanical study.

The aim of the study was to examine the antiradical and antioxidant activity of some flavones and flavonols with different models of hydroxylation and methoxylation. Antiradical activity was measured using ABTS and DPPH radicals and ferric ions (FRAP test). The reduction potential of the compounds was also investigated by determination of minimal hydrogen abstraction energy for each of the hydroxyl hydrogens of all compounds using quantum chemistry methods. Quercetin appeared to be a strong antioxidant when the FRAP test was performed and the strongest for ABTS and DPPH tests whereas genkwanin was the weakest antioxidant for three tests (FRAP, ABTS, and DPPH). Flavonols appeared to have much stronger antiradical activity than flavones. An exception was luteolin, which belongs to flavones but exhibited antiradical activity comparable to that of flavonols, probably due to the presence of a hydroxyl group in the B ring at the 3' position next to another hydroxyl group at position 4'. The study using UB3LYP/6-31G(d,p) model chemistry of density functional theory (DFT) showed the lowest hydrogen abstraction energy (HAE) for the hydroxyl group situated at 3' or 5' of myricetin. Based on the experimental results and computational studies, we conclude that the hydroxyl group situated at 4' in the B ring in flavonoids, and to a lesser at the 3' and 3 position in flavonols is the most important for antioxidant activity of flavonoids. We observe strong negative Spearman's rank order correlations between minimal HAE and antiradical activity of flavonoids in all three tests and double-tailed rejection P values are less than 0.001.

Stabilization of anthocyanin and skullcap flavone complexes--investigations with computer simulation and experimental methods.

We examined the stabilization of anthocyanins with flavones from the practical and theoretical perspective. The influence of addition of skullcap flavones, heating to 50 °C, and 12 day storage time (in the presence and absence of light) on the stability of anthocyanins in honeysuckle concentrates was investigated experimentally. Theoretical study was conducted with molecular dynamics methods in a model system, preceded by simulated annealing and thermalization. By the methods of the computer simulation of the copigmentation process we determined the sites responsible for the stabilization of a cyanidin quinoidal base-baicalin complex. We revealed both direct and water-mediated hydrogen bondings that keep the lamellar stacking structure of these molecules in the bounded form in water medium. The stacking occurs also due to hydrophobic interactions of the rings of both molecules. The experimental part of the study confirmed the effectiveness of anthocyanins stabilization in a concentrate of honeysuckle with the use of skullcap flavones.

Modifications of erythrocyte membrane hydration induced by organic tin compounds.

A study on the effects of selected organic chlorides of tin on the extent of hydration of the lipid bilayer of erythrocyte ghosts from pig blood is presented. The following compounds were used, dibutyltin dichloride (DBT), tributyltin chloride (TBT), diphenyltin dichloride (DPhT) and triphenyltin chloride (TPhT). The degree of membrane hydration was measured by the ATR FTIR technique, which makes it possible to estimate the level of carbonyl and phosphate group hydration in lipids of membranes. Other measurements were made with a fluorescence technique involving a laurdan probe. Tin organic compounds caused dehydration of the lipid bilayer of ghosts in the region of the carbonyl groups. DBT and TBT produced weak dehydration in the region of the phosphate group, whereas DPhT and TPhT increased hydration. The results allow one to determine the location of organotin compounds within a membrane, and show that TBT penetrates the membrane the deepest and DBT the shallowest. Phenyl tin compounds penetrate membranes to an intermediate depth. The results obtained indicate that the destructive properties of the organometallic compounds depend mostly on their effect on hydration of the membrane.

Antioxidative effect of quercetin and its equimolar mixtures with phenyltin compounds on liposome membranes.

Our earlier studies have shown that the compounds diphenyltin dichloride (DPhT) and triphenyltin chloride (TPhT) in the presence of UVC radiation enhanced the degree of phosphatidylcholine liposome membrane oxidation (J. Agric. Food Chem. 2005, 53, 76-83). The prooxidative behavior of the compounds has now been confirmed with the electron paramagnetic resonance method, which proved the possibility that the studied compounds can exist in free radical forms. The present work investigates the possibility of the protective action of quercetin on phosphatidylcholine liposome membranes exposed to the prooxidative action of DPhT and TPhT induced by UV radiation (lambda = 253.7 nm). The concentrations of quercetin and its equimolar mixtures with DPhT and TPhT were determined (and compared with well-known antioxidants as standards-trolox and butylated hydroxytoluene, also in the presence of phenyltins) as those that induce 50% inhibition in oxidation of liposomes radiated with UV. They are 5.1 +/- 0.10, 2.9 +/- 0.12, and 1.9 +/- 0.08 microM (differences between the values are statistically significant), constituting the following sequence of antioxidative activity: quercetin:TPhT > quercetin:DPhT > quercetin. This relation is confirmed by the results on the antiradical ability of quercetin and its mixtures with DPhT and TPhT toward the free radical 1,1-diphenyl-2-pricrylhydrazil. Similar sequences obtained in both studies suggest a possible mechanism of the antiradical action of the mixtures as free radical scavengers. We suggested that (i) quercetin's ability, documented by spectrophotometric, infrared attenuated total reflectance spectroscopy, (1)H NMR, and molecular modeling methods, to form complexes with phenyltins indicates a possible way of protection against the peroxidation caused by the free radical forms of phenyltins and (ii) the differentiation in the action of the quercetin/TPhT and quercetin/DPhT associates (statisticaly significant) may result from a different localization in the liposome membrane, which is indicated by the results of the fluorimetric studies.