- An Electron Spin Resonance Investigation of Radical Intermediates in Cholesterol and Related Compounds: Relation to Solid-State Autoxidation
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This work reports an investigation of the free-radical chemistry of cholesterol and a number of cholesterol derivatives and analogues.The initial radicals formed in solid oxygen-free cholesterol samples after γ-irradiation at 77 K are found to be a tertiary side-chain radical and an allylic radical in the cholesterol A and B rings.At 300 K only the allylic radical is found.The structure of the allylic radical is confirmed by experiments on two analogues of cholesterol, 7-OH cholesterol, and the selectively deuterated 7-D-7-OH cholesterol, both of which produce the allylic radical after γ-irradiation by loss of the hydroxyl group. Cholesterol samples with oxygen present provide evidence for formation of two distinct peroxy radicals originating with the two carbon radicals found in the oxygen-free samples.These peroxy radicals are suggested to have different reactivities resulting from the different motional freedom each possesses.We suggest that the products found after radiation-induced autoxidation at or near room temperature are consistent with the different reactivities of the peroxy radicals.It is found for four cholesterol esters, one cholesterol derivative, and one sterol that the only radical stable at room temperatures in oxygen-free samples is the allylic radical.In only one ester, cholesterol chloroacetate, is the allylic radical not the stable room temperature radical.
- Sevilla, Cynthia L.,Becker, David,Sevilla, Michael D.
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- CHEMISTRY OF SINGLET OXYGEN. 47. 9,10-DICYANOANTHRACENE-SENSITIZED PHOTOOXYGENATION OF ALKYL-SUBSTITUTED OLEFINS.
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9,10-Dicyanoanthracene (DCA) sensitizes the photooxygenation of 1-methylcyclohexene, 1,2-dimethylcyclohexene, and cholesterol in acetonitrile and benzene. For all three olefins, the products are the same as those formed by reaction with singlet oxygen. Th
- Araki,Dobrowolski,Goyne,Hanson,Zhi Qui Jiang,Lee,Foote
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- Photochemistry of bacteriochlorophylls in human blood plasma: 2. Reaction mechanism investigated by product analysis and deuterium isotope effect
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Transmetalated (Pd) bacteriochlorophyll derivatives are currently being clinically tested as sensitizers for photodynamic therapy. Protocols using short delay times between injection and irradiation generate interest in the photochemistry of these pigments in the blood. Using near-infrared irradiation where these pigments absorb strongly, we have studied the mechanism of photo-oxidation in two lipoprotein fractions, low- and high-density lipoproteins, derived from human blood plasma that preferentially accumulate these pigments (Dandler et al. [2009] Photochem. Photobiol., 85, in press). Using quenchers of reactive oxygen species, and chemical reporters, in particular peroxides generated from cholesterol as an inherent component of the lipoproteins, a Type II mechanism generating singlet oxygen has been demonstrated for Pd- and Zn-bacteriopheophorbides. In homogeneous systems, accelerated bleaching in D2O, compared with H2O, supports this mechanism. An unusual deuterium isotope effect was observed, by contrast, in heterogeneous amphiphilic-water systems. In the early phase, and under high oxygen concentrations, again a positive D-isotope effect is observed which later, in a second phase, is reversed to a negative D-isotope effect. The latter cannot be explained by heterogeneous pigment populations in the amphiphilic system; we, therefore, conclude a mechanistic switch, and discuss a possible mechanism.
- Dandler, Joerg,Wilhelm, Brigitte,Scheer, Hugo
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- Cholesterol hydroperoxides generate singlet molecular oxygen [O 2(1Δg)]: Near-IR emission, 18O-labeled hydroperoxides, and mass spectrometry
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In mammalian membranes, cholesterol is concentrated in lipid rafts. The generation of cholesterol hydroperoxides (ChOOHs) and their decomposition products induces various types of cell damage. The decomposition of some organic hydroperoxides into peroxyl
- Uemi, Miriam,Ronsein, Graziella E.,Prado, Fernanda M.,Motta, Flavia D.,Miyamoto, Sayuri,Medeiros, Marisa H. G.,Di Mascio, Paolo
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experimental part
p. 887 - 895
(2012/04/11)
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- Kinetic model for studying the effect of quercetin on cholesterol oxidation during heating
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Inhibition of the heat-induced cholesterol oxidation at 150°C by incorporation of quercetin was kinetically studied. Results showed that without quercetin, the cholesterol oxidation products (COPs) concentration increased with increasing heating time. A low amount (0.002%, w/w) of quercetin was effective in inhibiting the formation of COPs during the initial heating period (≤30 min) at 150°C. However, after prolonged heating (30-120 min), a low antioxidant activity was observed because of the degradation of quercetin. When using nonlinear regression models for kinetic study of cholesterol oxidation in the absence of quercetin, the epoxidation showed the highest rate constant (h-1 = 683.1), followed by free radical chain reaction (h -1 = 453.5), reduction (h-1 = 290.3), dehydration (h -1 = 155.5), triol dehydrogenation (h-1 = 5.35), dehydrogenation (h-1 = 0.68), thermal degradation (h-1 = 0.66), and triol formation (h-1 = 0.38). However, in the presence of quercetin, the reaction rate constants (h-1) for epoxidation (551.4), free radical chain reaction (111.7), and thermal degradation (0.28) were reduced greatly. The kinetic model developed in this study can be used to predict the inhibition of COPs by quercetin during the heating of cholesterol.
- Chien, John-Tung,Hsu, Da-Jung,Chen, Bing-Huei
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p. 1486 - 1492
(2007/10/03)
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- Kinetic studies of cholesterol oxidation as inhibited by stearylamine during heating
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The formation of cholesterol oxidation products (COPs) during heating in the presence of stearylamine at 140°C was analyzed by high-performance liquid chromatography (HPLC) and kinetically studied by use of nonlinear regression models. Results indicated that the COPs concentration increased with increasing heating time, and stearylamine was shown to reduce both oxidation and degradation rates of cholesterol. Without stearylamine, the highest rate constant (per hour) was observed for epoxidation (545.4), followed by free radical chain reaction (251.0), reduction (147.3), dehydration (95.8), triol dehydrogenation (4.7), degradation (0.34), triol formation (0.31), and dehydrogenation (0.13). With stearylamine, the epoxidation and free radical chain reaction rates could be reduced by about 800- and 3.4-fold, respectively, and triol formation during oxidation could be completely inhibited. In addition, the reactions for reduction, dehydration, degradation, and dehydrogenation could proceed slower in the presence of stearylamine. The kinetic model developed in this study can be used to predict the inhibition of COPs formation by stearylamine during heating of cholesterol.
- Chien, John-Tung,Huang, Dong-Yong,Chen, Bing-Huei
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p. 7132 - 7138
(2007/10/03)
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- Reactivities of Some Allylic Hydroperoxides toward Allylic Rearrangement and Related Reactions
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The allylic rearrangement has been studied of the hydroperoxides that are formed when singlet oxygen reacts with epicholesterol, Δ9,10-octahydronaphthalene, 2,3-dimethylbut-2-ene, cyclopentylidenecyclopentane, and cyclohexylidenecyclohexane.The reactivity in this sense decreases in the above sequence. 1-(Cyclopent-1-enyl)cyclopentyl hydroperoxide rearranges only slowly, but in the presence of triplet oxygen it reacts to give 1-(5-hydroperoxycyclopent-1-enyl)cyclopentyl hydroperoxide, and 1-(cyclohex-1-enyl)cyclohexyl hydroperoxide does not rearrange and shows only the reaction with oxygen to give 1-(6-hydroperoxycyclohex-1-enyl)cyclohexyl hydroperoxide.The various factors that affect the rates of these reactions are discussed.It is suggested that the reactivity and regioselectivity in the autoxidation which leads to the formation of dihydroperoxides implies that the reaction involves not the usual two-step propagation sequence, but a three-step sequence in which the chain carriers are a cycloalkenyl radical, a cycloalkenylperoxyl radical, and a cycloalkylperoxyl radical.
- Dang, Hai-Shan,Davies, Alwyn G.,Davison, Ian G. E.,Schiesser, Carl H.
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p. 1432 - 1438
(2007/10/02)
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- Iron-catalyzed Autoxidation of Cholesterol in the Presence of Unsaturated Long-Chain Fatty Acid
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Oxy-functionalization of cholesteryl acetate (1a) occured, giving 3β-acetoxy-5,6-epoxycholestane (2a), 3β-acetoxycholest-5-en-7-one (3a), 3β-acetoxycholest-5-en-7-ol (4a), and an unidentified product (5), when 1a was oxidized by a system consisting of Fe(acac)3, and the hydroperoxide of an unsaturated long-chain fatty aicd (LH) such as oleic, linolic or linolenic acid (Table I).The epoxidation of stilbene by the same system was found to be non-stereospecific.These results and the fact that the reaction in this system was inhibited by a radical scavanger (BHT) were fairly compatible with those obtained with the Fe(acac)3-tBuOOH system, which is assumed to generate oxy and peroxy radicals.Autoxidation of 1a and cholesterol (1b) in the presence of Fe(acac)3 and LH proceeded after a time-lag of several hours and was also inhibited be BHT.The marked stereoselectivity of β-epoxidation (β/α+β=0.72) and the extent (about 30-40percent) of allylic oxidation in the autoxidation were in fair agreement with those found for 1a in the Fe(acac)3-LOOH system (Table II).Autoxidation of stilbene in the presence of Fe(acac)3 and LH also led to non-stereospecific epoxidation.Thus, the autoxidation of cholesterols (1a and 1b) in the Fe(acac)3-LH system was assumed to be a radical reaction in which LOO. and LO. are the attacking species.Keywords - autoxidation; cholesterol; co-oxidation; epoxidation; hydroperoxide; lipid peroxidation; radical pathway; stilbene; tris(acetylaceto)iron(III); unsaturated long-chain fatty acid
- Muto, Toshiki,Tanaka, Jun,Miura, Toshiaki,Kimura, Michiya
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p. 3172 - 3177
(2007/10/02)
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- A sterospecific synthesis of 7alpha-hydroxycholesterol.
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The five step synthesis of 7alpha-hydroxycholesterol utilizes the solvolysis of 7alpha-bromocholesterol benzoate with potassium acetate in acetic acid as the key step in controlling the stereospecificity of the reaction sequence. This reaction yields 7alpha-acetoxycholesterol benzoate with retention of configuration at position seven. The diester is readily reduced with lithium aluminum to 7alpha-hydroxycholesterol.
- Johnson,Lack
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