Electrochemical characterization of bioactive hydroxyxanthones by cyclic voltammetry

The present study reports the electrochemical behavior of several phenolic and catecholic-substituted 2,3-diarylxanthones on a glassy carbon electrode, challenged by cyclic voltammetry at different pH values (4.0, 7.4, and 11.0). Higher pH values required lower anodic and cathodic peak voltages. The oxidation of catecholic groups occurred at lower peak potentials in a reversible and pH dependent manner. Anodic peak potentials appeared at higher pH values and were attributed to the electrochemically irreversible oxidation of the phenolic groups. The number and position of hydroxyl substituents were the determinants for the electrochemical behavior and found to correlate with the scavenging activity for reactive oxygen (ROS) and nitrogen species (RNS). A xanthone with two catechol units presented the lowest anodic potential voltage (Epa = 0.15 V) and proved to be the most effective ROS and RNS scavenger.Faculdade de Farmácia da Universidade do Porto, Universidade de Aveiro, Fundação para a Ciência e a Tecnologia (Portugal) and FEDER for funding the Organic Chemistry Research Unit (project PEst-C/QUI/UI0062/2011) and also to the Portuguese National NMR Network (RNRMN)

Structure–activity relationships in hydroxy-2,3-diarylxanthone antioxidants. Fast kinetics spectroscopy as a tool to evaluate the potential for antioxidant activity in biological systems

A structure–activity relationship has been established for eight hydroxy-2,3-diarylxanthones (XH) bearing hydroxy groups on the two aryl rings. One-electron oxidation by superoxide radical-anions (ΣO2 -) and ΣTrp radicals as well as reaction with ΣCCl3O2 and ΣCHCl2O2 radicals demonstrates that two OH groups are required for efficient antioxidant reactivity in cetyltrimethylammonium bromide micelles. Hydroxy groups at the meta and para positions on either of the two phenyl rings confer enhanced reactivity, but XH bearing an OH at the para position of either phenyl ring is unreactive. While oxidation is favoured by OH in both meta and para positions of 2-aryl xanthone substituents, addition of a third and/or fourth OH enhances electron-donating capacity. In Cu2+-induced lipid peroxidation of human LDL, the lag period preceding the commencement of lipid peroxidation in the presence of XH bearing OH at meta and para positions on the 3-phenyl ring is extended to twice that observed with a comparable concentration of quercetin, a reference antioxidant. These antioxidants are also superior to quercetin in protecting human skin keratinocytes against tert-butylhydroperoxideinduced oxidative stress. While XH antioxidant activity in model biological systems is consistent with the structure–activity relationship, their response is also modulated by the localization of XH and by structural factors.This is Document No. NDLR-4864 from the Notre Dame Radiation Laboratory which is supported by the Office of Basic Energy Sciences at the United States Department of Energy. This work was supported by the Franco-Portuguese exchange programs GRICES-INSERM 2005-2006 and Pessoa 07958NF. P. Filipe thanks the “Sociedade Portugesa de Dermatologia e Venerologia” for a travel grant. Thanks are due to the University of Aveiro, “Fundação para a Ciência e a Tecnologia” and FEDER for funding the Organic Chemistry Research Unit. Clementina M. M. Santos thanks the Calouste Gulbenkian Foundation for the award of a short period research grant (proc 87872)

Distribution and quantitation of skin iron in primary haemochromatosis: correlation with total body iron stores in patients undergoing phlebotomy

Measurement of the concentration of iron in the skin, if correlated with total body iron stores, may enable better informed decisions on when to initiate, change or stop therapy in hereditary heamochromatosis. Naïve haemochromatosis patients with iron overload and with C282Y and/or H63D HFE mutations were evaluated at the following time-points: disease diagnosis, end of the therapy programme, and 6 months after the end of therapy. The distribution and concentration of iron in the skin were assessed by quantitative nuclear microscopy methods, in parallel with serum and plasma iron concentration. Iron content in the liver was determined by nuclear magnetic resonance. Iron accumulated in the epidermis; its concentration increased from outer to inner layers, being maximal in the basal layer (7.33?±?0.98 µmol/g). At all 3 time-points, most of the iron was associated with the extracellular space. During the phlebotomy programme the iron content of the skin and the liver decreased by a factor of 2. These data suggest that measurements of iron concentration in the epidermis, which is a readily accessible tissue, reflect iron overload in the liver

Polyhydroxy-2,3-diarylxanthones as antioxidants

Xanthones are a class of oxygenated heterocyclic compounds widely occurring in nature.[1] The biological properties of these compounds have been extensively reported in the literature and one of the most promising is their potential application as antioxidant agents.[2] This fact led us to start a programme on the synthesis of 2,3-diarylxanthones bearing hydroxyl groups in certain positions of their skeleton for further structure- antioxidant activity studies. In this communication we will describe the synthesis of 2,3-diarylxanthones starting with 2’-hydroxyacetophenone and cinnamic acid derivatives (Scheme 1). The Heck reaction of 3-bromo-2-styrylchromones 3 with styrenes give the methoxyxanthones 4 which, after cleavage of the protecting groups, give the desired polyhydroxy-2,3-diarylxanthones 5.[3] We will also report the inhibitory effect of the synthesized xanthones 5 on Cu2+-induced oxidation of isolated human serum low-density lipoproteins (LDLs). The formation of conjugated dienes and the consumption of carotenoids were chosen as markers of LDL lipid peroxidation.[4] The induction of oxidative stress studies in NCTC 2544 cells will also be presented

The dependence of alpha-tocopheroxyl radical reduction by hydroxy-2,3-diarylxanthones on structure and micro-environment

The flavonoid quercetin is known to reduce the α-tocopheroxyl radical (˙TocO) and reconstitute α-tocopherol (TocOH). Structurally related polyphenolic compounds, hydroxy-2,3-diarylxanthones (XH), exhibit antioxidant activity which exceeds that of quercetin in biological systems. In the present study repair of ˙TocO by a series of these XH has been evaluated using pulse radiolysis. It has been shown that, among the studied XH, only 2,3-bis(3,4-dihydroxyphenyl)-9H-xanthen-9-one (XH9) reduces ˙TocO, though repair depends strongly on the micro-environment. In cationic cetyltrimethylammonium bromide (CTAB) micelles, 30% of ˙TocO radicals are repaired at a rate constant of ∼7.4 × 106 M−1 s−1 by XH9 compared to 1.7 × 107 M−1 s−1 by ascorbate. Water-soluble Trolox (TrOH) radicals (˙TrO) are restored by XH9 in CTAB (rate constant ∼3 × 104 M−1 s−1) but not in neutral TX100 micelles where only 15% of ˙TocO are repaired (rate constant ∼4.5 × 105 M−1 s−1). In basic aqueous solutions ˙TrO is readily reduced by deprotonated XH9 species leading to ionized XH9 radical species (radical pKa ∼10). An equilibrium is observed (K = 130) yielding an estimate of 130 mV for the reduction potential of the [˙X9,H+/XH9] couple at pH 11, lower than the 250 mV for the [˙TrO,H+/TrOH] couple. A comparable value (100 mV) has been determined by cyclic voltammetry measurements

Equilibrium and kinetic studies of the interactions of a porphyrin with low-density lipoproteins.

Low-density lipoproteins (LDL) play a key role in the delivery of photosensitizers to tumor cells in photodynamic therapy. The interaction of deuteroporphyrin, an amphiphilic porphyrin, with LDL is examined at equilibrium and the kinetics of association/dissociation are determined by stopped-flow. Changes in apoprotein and porphyrin fluorescence suggest two classes of bound porphyrins. The first class, characterized by tryptophan fluorescence quenching, involves four well-defined sites. The affinity constant per site is 8.75 x 10(7) M(-1) (cumulative affinity 3.5 x 10(8) M(-1)). The second class corresponds to the incorporation of up to 50 molecules into the outer lipidic layer of LDL with an affinity constant of 2 x 10(8) M(-1). Stopped-flow experiments involving direct LDL porphyrin mixing or porphyrin transfer from preloaded LDL to albumin provide kinetic characterization of the two classes. The rate constants for dissociation of the first and second classes are 5.8 and 15 s(-1); the association rate constants are 5 x 10(8) M(-1) s(-1) per site and 3 x 10(9) M(-1) s(-1), respectively. Both fluorescence and kinetic analysis indicate that the first class involves regions at the boundary between lipids and the apoprotein. The kinetics of porphyrin-LDL interactions indicates that changes in the distribution of photosensitizers among various carriers could be very sensitive to the specific tumor microenvironment

Plasma Lipoproteins as Mediators of the Oxidative Stress Induced by UV Light in Human Skin: A Review of Biochemical and Biophysical Studies on Mechanisms of Apolipoprotein Alteration, Lipid Peroxidation, and Associated Skin Cell Responses

There are numerous studies concerning the effect of UVB light on skin cells but fewer on other skin components such as the interstitial fluid. This review highlights high-density lipoprotein (HDL) and low-density lipoprotein (LDL) as important targets of UVB in interstitial fluid. Tryptophan residues are the sole apolipoprotein residues absorbing solar UVB. The UVB-induced one-electron oxidation of Trp produces •Trp and O2•- radicals which trigger lipid peroxidation. Immunoblots from buffered solutions or suction blister fluid reveal that propagation of photooxidative damage to other residues such as Tyr or disulfide bonds produces intra- and intermolecular bonds in apolipoproteins A-I, A-II, and B100. Partial repair of phenoxyl tyrosyl radicals (TyrO•) by α-tocopherol is observed with LDL and HDL on millisecond or second time scales, whereas limited repair of α-tocopherol by carotenoids occurs in only HDL. More effective repair of Tyr and α-tocopherol is observed with the flavonoid, quercetin, bound to serum albumin, but quercetin is less potent than new synthetic polyphenols in inhibiting LDL lipid peroxidation or restoring α-tocopherol. The systemic consequences of HDL and LDL oxidation and the activation and/or inhibition of signalling pathways by oxidized LDL and their ability to enhance transcription factor DNA binding activity are also reviewed