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Chile - Places in the News | Library of Congress
Insert image from URL. Printable version. Join this workspace. To join this workspace, request access. Recent Activity. Morin is structurally similar to quercetin, differing only in the position of a hydroxyl group in the B ring. Neglecting ionization of the hydroxyl groups or differences in electron delocalization dependent on the planarity of the molecule, this increased reactivity may be possibly explained by the direct activation of the double bond in position C2-C3 of C ring.
However, at physiological pH, ionization of polyphenols is an important factor than must be heed. Different pKa values for these flavonoids have been reported, which depend on the method used for their determination [ 74 — 78 ]. Rafols et al. Musialik et al. In general, the pKa of 7-OH group is similar for all flavonoids 7.
However, morin has a pKa of 5. In order to explain this strong acidity these authors suggest the existence of a tautomeric form involving a hydrogen bond with the hydroxyl at position 3, stabilizing this deprotonated form observed. The study of the influence of hydroxyl deprotonation on the photo-oxidation of phenols must be focused on the balance between physical and chemical quenching contributions at each pH. The reaction rate constant for each ArO- species are between one and two orders of magnitude higher than that for the undissociated species [ 79 ].
The value of k T is strongly dependent on the structural properties of the phenol, on the solvent, and on the phenol ionization degree.
Also, it has been reported that photooxidation reaction rate of enolic tautomers of dicarbonyl compounds is greatly enhanced by the presence of fluoride ion or tetrabutilammonium hydroxide [ 81 ]. This effect has been attributed to hydrogen bond formation between the fluoride ion and the enol hydrogen which enhances its nucleophilicity [ 82 ].
The HOMO of morin shows localized electron density over the C2-C3 double bond Fig 6B ; therefore affecting the chemical reaction rate with singlet oxygen. On the other hand, kaempferol and quercetin neutral optimized geometry are planar data not shown and even though their HOMO also show electron density on the C2-C3 double bond, this is delocalized due to resonance. Furthermore, absence of an hydroxyl group in ring B avoids the formation of hydrogen bond with 3OH. The increased acidity besides the enhanced nucleophilicity of enolic bond could explain to the higher chemical reactivity observed for morin.
White, cyan and red represent Hydrogen, Carbon and Oxygen atoms respectively. In this work, the total rate constants k T for the reaction of singlet oxygen with flavonoids in erythrocyte membranes dispersed in D 2 O pD 7. Fig 7 shows Stern-Volmer plot for singlet oxygen deactivation by kaempferol in erythrocyte ghost membranes.
The determination of k T was restricted to a narrow range of flavonoid concentration, due to low solubility. All compounds show similar total rate constants being for kaempferol the lower one 2. The total rate constants agree to those reported previously [ 31 ]. According to literature singlet oxygen deactivation is not influenced by ring A, but the catechol group ring B is involved in physical quenching [ 29 — 31 ].
In heterogeneous media vesicles, biological membranes or cells , the evaluation of the antioxidant capacity is more complex than in homogeneous media, because there are multiple factors involved, such as lipophilicity, antioxidant location, generation, lifetime and diffusion of ROS, among others. The thickness of the lipid bilayer is frequently compared with a polymer film, where degradation processes have been shown to depend on the site of generation of singlet oxygen and its diffusion.
Ogilby et al, studied poly methyl methacrylate films containing cinnamic acid, and observed dependence of oxygen diffusion coefficients with the extent of cross-linking in polymer [ 83 , 84 ]. Also, Ogilby et al. Oxygenation is pronounced and extensive when singlet oxygen is generated within the polymeric film, but when it is produced at the polymer surface the reactions are restricted to the surface [ 85 , 86 ].
In thin superficial layer, the kinetics of local reactions can be diffusion-controlled or not diffusion-controlled depending on heterogeneity of reactants distribution [ 87 ]. The deactivation of singlet oxygen by different quenchers in solid organic polymers and in liquid analogs of polymers shows that bimolecular quenching rate constants kq for efficient quenchers are smaller than for homogeneous solvents because quenching process in polymers is mainly controlled by solute diffusion to yield the singlet oxygen-quencher encounter pair.
The solid media has a leveling effect on the magnitude of quenching rate constants, minimizing the marked differences which can be observed in liquids [ 88 ].
Lipid membranes are less rigid than solid polymer providing less control for diffusion of singlet oxygen-flavonoid encounter pair. Laurdan GP these three compounds promote similar alterations in liposomes and erythrocyte membranes. DPH behavior allows to discard the location of these antioxidants in deeper regions of the lipid bilayer. Localization and reactivity towards singlet oxygen of quercetin, morin and kaempferol in liposomes POPC, DPPC and mixtures and in erythrocyte membranes is highly influenced by the presence of ionized specie. Browse Subject Areas?
Click through the PLOS taxonomy to find articles in your field. Abstract In this work, the relationship between the molecular structure of three flavonoids kaempferol, quercetin and morin , their relative location in microheterogeneous media liposomes and erythrocyte membranes and their reactivity against singlet oxygen was studied. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited Data Availability: All relevant data are within the paper.
Introduction For many years, flavonoids have been studied for their role in a broad variety of beneficial properties on human health [ 1 — 5 ]. Download: PPT. Small unilamellar liposomes SUVs preparation. Large unilamellar liposomes LUVs and extruded liposomes preparation. Erythrocyte membranes preparation. Size and zeta potential measurements. Preparation of flavonoid loaded liposome and erythrocyte ghost membranes samples for HPLC analysis. High-performance liquid chromatography HPLC. Fluorescent probes incorporation. Fluorescence spectroscopy measurements.
Determination of distribution and partition coefficients To determine distribution coefficients of flavonoids in homogeneous medium, 2. Kinetic measurements Experimental chemical reaction rate constants were determined in microheterogeous media using a 10 mL double wall cell, light-protected by black paint. Table 1. Fig 3. Relative location in micro-heterogeneous media In this work we studied the relative location of flavonoids in the micro-heterogeneous medium, using fluorescent probes Laurdan and DPH.
Modeling of stress transfer in the Coquimbo region of central Chile
Laurdan generalized polarization GP as a function of temperature. Fig 5. Table 2. Experimental rate constants k EXP results at different microheterogeneous media.