Spheroidal pigments

Fig. 3.5 SEM pictures ofa-FeOOH pigments. A) needle-like pigment (Bayferrox 420) with standard silking, B) spheroidal pigment (Bayferrox 915) with low silking effect.

Cadmium yellow consists of pure cadmium sulfide (golden yellow color) or mixed crystals of zinc and cadmium sulfide [8048-07-5], (Cd, Zn)S, in which up to one-third of the cadmium can be replaced by zinc. The density of this pigment is 4.5-4.8 g/cm3 and its refractive index is 2.4-2.5. The prevalent parcticle size is approx. 0.2 pm with cubic to spheroidal habits. Cadmium yellow is practically insoluble in water and alkali, and of low solubility in dilate mineral acid. It dissolves in concentrated mineral acid with generation of hydrogen sulfide. 

Cadmium red consists of cadmium sulfoselenide [12656-57-4], [58339-34-7], Cd(S,Se), and is formed when sulfur is replaced by selenium in the cadmium sulfide lattice. With increasing selenium content, the color changes to orange, red, and finally dark red. The density of these pigments increases correspondingly from 4.6 to 5.6 g/cm3 and the refractive index from 2.5 to 2.8. The crystals have cubic or spheroidal habits, the prevalent particle size is 0.3-0.4 pm. 

Three main tendencies have been underlined in recent studies of structure and action mechanism ofbacterial photosynthetic reaction centers. The crystallographic structure of the reaction centers from Rps. viridis and Rb. spheroids was initially determined to be 2.8 and 3 A resolutions (Michel and Deisenhofer et al., 1985 Allen et al., 1986). Resolution and refinement of these structures have been subsequently extended to 2.2, 2.3 and 2.6 A. (Rees et al., 1989 Stowell et al., 1997, Fyfe and Johns, 2000 Rutherford and Faller, 2001). Investigations of the electronic structure of donor and acceptor centers in the ground and exited states by modern physical methods with a combination ofpico-and femtosecond kinetic techniques have become more precise and elaborate. Extensive experimental and theoretical investigations on the role of orbital overlap and protein dynamics in the processes of electron and proton transfer have been done. All the above-mentioned research directions are accompanied by extensive use of methods of sit-directed mutagenesis and substitution of native pigments for artificial compounds of different redox potential. 

Shneour EA (1962a) Carotenoid pigment conversion in Rhodopseudomonas spheroides. Biochim Biophys Acta 62 534-540... 

Shneour EA (1962b) The source of oxygen in Rhodopseudomonas spheroides carotenoid pigment conversion. Biochim Biophys Acta 65 510-511... 

The effect of pigment shape on the properties of an industrial latex coating was studied in detail. Two red iron oxide pigments with similar properties were studied - one rhombohedral (produced by precipitation) and the other spheroidal (produced by calcination). They were added in the same volume concentration to the coating formulations and several liquid, dry-film, wet film and free-film properties were determined. 17 refs. 

Specialized cells which make large amounts of heme or chlorophyll have been examined to discover the controls involved in the synthesis of these pigments. In most of the cell types studied, the generalization can be made that all the enzymes of heme synthesis, except the first one, are normally present in excess. Only the first enzyme, -aminolevulinic acid (ALA) synthetase, is present at a limiting activity (Table VI). The activity of this enzyme not only controls the rate of heme synthesis but in plants it also controls the rate of chlorophyll and bacteriochloro-phyll synthesis. In Rhodopseudomonas spheroides, a photosynthetic bacterium, one and the same enzyme controls both heme and bacterio-chlorophyll synthesis. In this organism the enzymes of the Mg branch may be present in only limited sufficiency, but in higher plants the activities of the enzymes of the Mg branch do not limit chlorophyll synthesis. 

FuUer (1973) ejqtlains diat diis term refers to synthetic analogues of calcined yellow ochres or goethite (qq.v.) by thermal treatment of synthetic yellow iron oxides a series of red pigments can be produced. These pigments apparently differ from so-caUed copperas reds (thermally decomposed iron(II) sulfate hydrate) by having acicular (needle-like) particle morphology rather than a spheroidal shape. 

In the presence of nicotine (7.5 mmol" ), Rps. spheroides accumulates neurosporene in place of the two main pigments. - On re-suspending cells which had been washed free from nicotine, neurosporene disappeared and the normal pigments were synthesized. Experiments with Rps. gelatinosa showed that the same pathway existed in this organism, and emphasized the precursor-product relationship between spheroidene (151) and spheroidenone (153). These observations, combined with the characterization of the previously undetected demethylspheroidene (154) from cultures of both species, provide strong direct support for the biosynthetic pathway just indicated. No 3,4-didehydro-derivative of neurosporene (155) was detected in nicotine-inhibited cells, which suggests that the insertion of the double bond at C-3 does not normally occur before the hydration of the double bond at C-1. 

In reaction centers (RC) of Rhodobacter spheroides (Rb.), the tetra-pyrrole pigments at the sites B ("monomeric" bacteriochlorophylls,... 

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