Sterols dimers

Formation of sterol dimers. These are apparently formed during bleaching and can be used to detect this process (Schulte and Weber, 1991). 

VM, percentage matter volatilized in pyrolysis CHYDR, carbohydrates with pentose and hexose subunits PHLM, phenols and lignin monomers LDIM, lignin dimers LIPID, lipids, alkanes, alkenes, bound fatty acids, and alkylmonoesters ALKY, alkylaromatics NCOMP, mainly heterocyclic N-containing compounds STEROL, sterols PEPTI, peptides SUBER, suberin FATTY, free fatty acids in % of total ion intensity. 

If sterol content and conformation are so important for membrane stability, we should study the biosynthesis of sterols (Figure 3). The first enzyme in terpenoid biosynthesis is the 3-Hydroxy-3-Methyl-Glutary1-Coenzyme A-reductase (HMG-CoA-reductase) that catalyzes the synthesis of mevalonate. Two phosphorylations and decarboxylation of mevalonate lead to isopentenylpyrophosphate, the basic C -unit in sterol synthesis. Isopentenylpyrophosphate reacts with its isomer, the dimethylally1-pyrophosphate, in a head/tail-reaction to geranyl-pyrophosphate reaction with another C -unit leads to farnesyl-pyro-phosphate, that dimerizes in a tail/tail-reaction to squalene. After expoxidation of its A -double bond, squalene cyclizes to lano-... 

We also investigated chelate binding by dimers of a synthetic hydrophobic macrocycle, in place ofthecyclodextrins [187]. In the systems examined the chelate effect was weaker than that seen with the cyclodextrin dimers. We also studied the strong binding of cholesterol by some cyclodextrin dimers and a cyclodextrin polymer, and saw that the large sterol could occupy parts of two binding cavities [188]. 

As shown in Figure 2-23, oil breakdown during frying can be caused by oxidation and thermal alteration. Oxidation can result in the formation of oxidized monomeric, dimeric, and oligomeric triglycerides as well as volatile compounds including aldehydes, ketones, alcohols, and hydrocarbons. In addition, oxidized sterols may be formed. Thermal degra-... 

Different studies revealed that there is a conserved sequence in the ERG19 allele encoding MVD. The yeast mutated recessive allele ergl9 led to a decrease of sterol production. The mutated erg 19 allele bears a single point mutation shown to impair the dimerization of the mutated enzyme leading to a less active form of MVD [303]. In addition, a single change in the conserved amino acid sequence (Leu for Pro) accounts for the temperature sensitive phenotype of the mutant [304]. 

Cholesterol, on reaction with dibenzoyl peroxide followed by trichloroacetic acid, gave a blue colour attributed to cations with conjugated unsaturated structures. Cholesta-2,4,6-triene and 3-(cholest-5-en-3/8-yl)cholesta-2,4,6-triene were isolated from the product mixture. Some other sterols gave diverse colours.The coloured solutions formed from cholesterol or oestrone with antimony trichloride give e.s.r. spectra suggesting the presence of radical-cations. Cholesta-3,5-diene and 3,3 -bicholesta-2,4-diene were isolated from the reaction of cholesterol, and the hexaene (131) from oestrone. Dimeric and trimeric products have been... 

The pioneering paper on metabolites from sponges of Theonella genus appeared in literature in 1981 dealing with the isolation of two new and unusual sterols with the unprecedented 4-methylene nucleus, conicasterol (2) and theonellasterol (3) [3]. Subsequently, the 3-keto derivatives, conicasterone (4) and theonellasterone (5), together with a Diels Alder type dimeric steroid, named bistheonellasterone (6), were reported from the Okinawan marine sponge Theonella swinhoei [4], It is noteworthy that... 

The fatty acid spectrum of rice bran oil is 22-25% palmitic acid, 37-41% oleic acid and 37-41% linoleic acid. More recently, interest in rice oil escalated with its identification as a healthy oil that reduces serum cholesterol. Rice bran is a good source of antioxidants including vitamin E and oryzanol (ferulic esters of sterols and triterpene alcohols), cholesterol-lowering waxes and antitumor compounds like rice bran saccharide.Besides applications in nutrition and in phyto-chemicals, rice bran oil has traditionally been used for industrial applications, such as dimer acid manufacturing, depending on pricing for alternative vegetable oils. 

Morinaka, B.I., Pawbk, J.R., and Molinski, T.E. (2009) Amaroxocanes A and B Sulfated dimeric sterols defend the Caribbean coral reef sponge Phorbas amaranthus from fish predators. /. Nat. Prod., Tl, 259-264. 

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