The stability of pigments

Chlorophyllase is an intrinsic membrane glycoprotein situated in the thylakoid membrane (Terpstra, 1981). It does not normally interact with its chlorophyll substrate, although chlorophyll is situated in the same membrane. In intact plant tissue, chlorophyllase is kept in an inactive and stable conformation by the adjacent lipids of the thylakoid membrane (Lambers and Terpstra, 1985). In order to activate chlorophyllase, a substantial disruption of the thylakoid membrane has to 

Hydroxychlorophylls are oxidized chlorophylls with the hydroxy gronp being loeated at C-10. The hydroxychlorophylls of chlorophylls a and b are ealled 

During autooxidation of the carotenoids, alkylperoxyl radicals are formed. These radicals mainly attack the double bonds resnlting in formation of epoxides. Autooxidation of /3-carotene has been studied in model systems. The products being formed during autooxidation of /3-carotene were identified as a mixture of products with epoxy, hydroxy and carbonyl gronps (Packer et al., 1981 Mordi et al., 1991 Liebler and Kennedy, 1992 Liebler, 1993 Yanishlieva et al., 1998). 

Carotenoid breakdown Lipoxygenase, unsaturated lipids, O2 Carotenoids Autooxidation, O2 Carotenoid breakdown 

As described above, different cA-isomers are formed during oxidation. The carotenoid stractnre makes a c/x-tranx-isomerisation possible around the double bonds (Fig. 8.2). In vegetables, the carotenoids are predominantly the aS -trans isomers, which may be converted to the cix-isomers. Cis-trans isomerisation resnlts in colonr changes in vegetable products as the spectral properties of the cA-carotenoids are different from the corresponding frawx-carotenoids. Insertion of one or more cA-double bonds in an aH-trans conjugated system results in a 

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The photochemical properties of titania surfaces are of interest for several reasons. They determine the stability of pigmented paint systems [76], the rate at which pollutants can be degraded in systems designed to purify air and water [77], and are the root cause of poorly understood phenomena such as water photolysis [78] and super hydrophilicity [79]. Using thin rutile epilayers with five low index orientations, it has been shown that the relative rates of photochemical reactions catalyzed by titania depend on the surface orientation [80]. In this chapter, experiments used to map the complete orientation dependence of the relative photochemical reactivity of TiO are described [81-83]. In this case, the relevant reactions are carried out at room temperature and this gives us the opportunity to fix both the surface morphological structure and stoichiometry. For the reactions described here, all of the surfaces were... 

Thus, the stability of aqueous dispersions of CuPc depends on the efficiency of the stabilizer adsorption on its surface. Treatment in an ultrasonic field allows to increase the efficiency of the stabilizer adsorption and therefore to increase the stability of pigment dispersions. 

Isostearic acid is used in applications which require thermal stability in the case of a lubricant, odor stability for a cosmetic formulation, and oxidation stability for products with long shelf-life requirements. The branched structure of isostearic acid also enhances its dispersing power, and it is used in cosmetic and industrial applications for the stabilization of pigments and mineral particles in oils and solvents. 

This area has recently been extensively investigated by flow adsorption microcalorimetry, mostly in respect of the strength of adsorption of surfactants and resins on the surfaces of the above materials and the effect on the stability of pigment dispersion and adhesion properties as a function of the surface chemistry of solids. A major contribution in this field has been made by the late Prof. F. Fowkes whose work was recently summarised by D. P. Ashton and D. Briggs [30],... 

For all studied compounds the same type efifect of aza-substitution is observed, the most dramatic change of photocurrent (2-5 times) and photopotential (1.3-1.7 times) is the first step—the transition from the TBP to a mono-aza-substituted derivatives. Each subsequent step makes a smaller contribution, but the overall increase in the transition to the structure of phthalocyanine for Up reaches 185 percent, and for —1,000 percent. The stability of pigments changes significantly— the pH value, at which protonation of the molecules begins, decreases by fom to five imits as the proportion of nitrogen atoms in the macrocycle grows, which is associated with the increase of redox potential with... 

Because of the high melt viscosity of polyolefins, normal spinning melt temperatures are 240—310°C, which is 80—150°C above the crystalline melting point. Because of the high melt temperatures used for polyolefin fiber spinning, thermal stabilizers such as substituted hindered phenols are added. In the presence of pigments, the melt temperature must be carefully controlled to prevent color degradation and to obtain uniform color dispersion. 

The Atlas Type FDA-IR Fadeometer is used primarily to check and compare color stability. Besides determining the stability of various pigments needed to provide both standard and custom colors, the Fadeometer is helpful in preliminary studies of various stabilizers, dyes, and pigments compounded in plastics to prolong their useful life. 

Nowadays, ultramarine-type pigments are produced synthetically. Inside the zeolite structure the highly reactive sulfur radical anions are well protected which explains the stability of the blue color over thousands of years in air. However, the species responsible for the blue color should not be confused with the sulfur radical cations responsible for the blue color of sulfur solutions in fuming sulfuric acid (oleum) and similar oxidizing mixtures... 

The thermal degradation of anthocyanins, both in extracts and model systems, was reported to follow first-order reaction kinetics in all studies. The stability of anthocyanins and all pigments found in foods decreased with increases in temperature. 

Escribano-Bailon, T. et al.. Color and stability of pigments derived from the acetaldehyde-mediated condensation between malvidin 3-O-glucoside and (-t-)-catechin, J. Agric. Food Chem., 49, 1213, 2001. 

An anthocyanin occurs in solution as a mixture of different secondary structures, a quinonoidal base, a carbinol pseudobase, and a chalcone pseudobase. ° hi addition, different mechanisms for the stabilization of anthocyanins lead to the formation of tertiary structures such as self-association, inter-, and intra-molecular co-pigmentation. ... 

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