Polyene sensitizers

The general theory is that these antibiotics cause a change in the permeability of the fungal cell membrane with attendant depletion of essential cellular constituents and that this change in permeability is in some way triggered by a binding of the antibiotic to the cell membrane via a steroid-antibiotic complex. Further support for this theory, v/hich in effect says that the presence of a steroid in the cell membrane is indeed a necessary prerequisite for polyene sensitivity, came as the result of an in vitro examination of the effects of polyene antibiotics on model bilayer membrane systems of egg lecithin, with and without cholesterol. ... 

In fungi, increases in sterol content also increases polyene sensitivity when the yeast Schizosaccharomyces faponicus was grown aerobically it contained ergosterol and was sensitive to polyenes, but when grown anaerobically this yeast lacked sterol and became polyene-insensitive [137]. When the sterol level of Candida albicans was increased, this was accompanied by increased antibiotic sensitivity [138]. In yeasts, lowered sterol levels resulted in greater resistance to polyene antibiotics [138,139]. 

A small group of C albicans mutants, induced by A -methyl-A -nitroso-guanidine, were found to contain more ergosterol than the wild type [311] and it has been suggested that this additional sterol is either re-orientated or masked in some way so that polyene binding is lowered. The observation that sterol incorporation per se does not always increase the polyene sensitivity of liposomes, i.e. it is the membrane state induced by sterol, not the total quantity which confers polyene sensitivity, which may provide a partial explanation of this case. 

Studies with isolated mitochondria from rat liver of Neurospora crassa have shown that, even though polyene antibiotics are bound, they have little effect on oxidative phosphorylation, ion uptake, or electron transport [375]. The mitochondria of Neurospora, which contain ergosterol, remained viable after binding filipin [375]. Mitochondria have high phospholipid sterol ratios in their membranes (approximately 40 1) and this may explain low polyene sensitivity. 

Spectroscopic methods such as uv and fluorescence have rehed on the polyene chromophore of vitamin A as a basis for analysis. Indirectly, the classical Carr-Price colorimetric test also exploits this feature and measures the amount of a transient blue complex at 620 nm which is formed when vitamin A is dehydrated in the presence of Lewis acids. For uv measurements of retinol, retinyl acetate, and retinyl palmitate, analysis is done at 325 nm. More sensitive measurements can be obtained by fluorescence. Excitation is done at 325 nm and emission at 470 nm. Although useful, all of these methods suffer from the fact that the method is not specific and any compound which has spectral characteristics similar to vitamin A will assay like the vitamin... 

LiBF4, wet CH3CN, 96% yield. Unsubstituted 1,3-dioxolanes are hydrolyzed only slowly, but substituted dioxolanes are completely stable. This reagent proved excellent for hydrolysis of the dimethyl ketal in the presence of the acid-sensitive oxazolidine " and polyene. ... 

Carotenoids are known to be sensitive to oxygen, and are said to be unstable in presence of air. Because of their polyene structures, carotenoids are susceptible to reactions with the so-called ROS that may be radicals (02", HO ) or non-radicals (H2O2, O2). 

These compounds have been the subject of several theoretical [7,11,13,20)] and experimental[21] studies. Ward and Elliott [20] measured the dynamic y hyperpolarizability of butadiene and hexatriene in the vapour phase by means of the dc-SHG technique. Waite and Papadopoulos[7,ll] computed static y values, using a Mac Weeny type Coupled Hartree-Fock Perturbation Theory (CHFPT) in the CNDO approximation, and an extended basis set. Kurtz [15] evaluated by means of a finite perturbation technique at the MNDO level [17] and using the AMI [22] and PM3[23] parametrizations, the mean y values of a series of polyenes containing from 2 to 11 unit cells. At the ab initio level, Hurst et al. [13] and Chopra et al. [20] studied basis sets effects on and y. It appeared that diffuse orbitals must be included in the basis set in order to describe correctly the external part of the molecules which is the most sensitive to the electrical perturbation and to ensure the obtention of accurate values of the calculated properties. 

Wang, X. F., Y. Koyama, H. Nagae, Y. Yamano, M. Ito, and Y. Wada. 2006. Photocurrents of solar cells sensitized by aggregate-forming polyenes Enhancement due to suppression of singlet-triplet annihilation by lowering of dye concentration or light intensity. Chem. Phys. Lett. 420 309-315. 

Carotenoids are a class of lipophilic compounds with a polyisoprenoid structure. Most carotenoids contain a series of conjugated double bonds, which are sensitive to oxidative modification and cis-trans isomerization. There are six major carotenoids (ji-carotenc, a-carotene, lycopene, P-cryptoxanthin, lutein, and zeaxanthin) that can be routinely found in human plasma and tissues. Among them, p-carotene has been the most extensively studied. More recently, lycopene has attracted considerable attention due to its association with a decreased risk of certain chronic diseases, including cancers. Considerable efforts have been expended in order to identify its biological and physiochemical properties. Relative to P-carotene, lycopene has the same molecular mass and chemical formula, yet lycopene is an open-polyene chain lacking the P-ionone ring structure. While the metabolism of P-carotene has been extensively studied, the metabolism of lycopene remains poorly understood. 

It is seen that the fluorescence quantum yield and lifetime of G19 gradually decreases with increasing solvent polarity. For example, the insertion of 20% ACN by volume into toluene leads to a decrease of a factor of two. Based on these results we can conclude that G19 is very sensitive to solvent polarity and can be used as an efficient probe to test the polarity of its microenvironment. A reverse trend of the absorption peak at 1 1 mixture of ACN and toluene (50%T in Fig. 22b) corresponds to a change of the sign of due to a transition from a polyene-like structure in nonpolar toluene to a polymethine-like structure in polar ACN. 

The only materials that have been examined for this purpose to date have been dienes and other polyenes.12 The major liability of these substances is their chemical reactivity. The finding that 1,3,5-trans-hexatriene quenches the phosphorescent emission from phenanthrene but does not have a significant effect on the fluorescent emission from 1,3,5,7-tetraphenylisobenzofuran indicates that chemical reactivity may not be too severe a problem, and that triplet quenchers may be effective tools for mechanistic work in ECL. Here too, further study is indicated. Unfortunately, or perhaps significantly, the compounds that produce the brightest emission on redox stimulation—1,3,4,7-tetra-phenylisobenzofuran and rubrene—have no detectable phosphorescence. Studies with sensitizers and quenchers have failed to produce any evidence regarding their triplet levels.12 Thus the mechanism of anni-hilative emission remains uncertain. 

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