Dimeric pigments

Due to the structural asymmetry in the x-ray data of rps.viridis the lowest triplet state is localized on the dimer pigment Pj. The calculated Triplet—Triplet absorption therefore shows a low energy contribution which is mainly due to the Triplet—Triplet absorption of Pj and a stronger line at higher energy which represents predominantly the groundstate absorption of the other dimer half Pj. ... 

Many cyanobacteria live in tropical zones in the vicinity of the water surface and are thus subjected to solar radiation, in particular the ultraviolet B (UV B) wavelength (280-320 nm). Certain species thus developed antiradical systems to protect themselves from this radiation. One of these substances is scytonemin, a brown-yellow dimeric pigment of heteroaromatic nature, of which the first observation was made by Nageli in 1849, who obtained it from a species of the genus Scytonema. This compormd, the structure of which was established in 1993, stiU seems to be specific to cyanobacteria, and in a more general way to prokaryotes, although it is present in very many species (Proteau et al, 1993). For an overview on the anti-UV substances of cyanobacteria, of phytoplankton and macro-algae, see Sinha et ol. (1998). Scytonemin absorbs in the three fields of the ultraviolet rays UV-A (325—425 nm) UV-B (280-320 nm) and UV-C (250 nm). 

Hemozoin, also known as malaria pigment, is, in teims of its chemical composition, identical to (3-hematin. Hemozoin is formed as a crystallization product of heme under the acidic conditions present in the food vacuole of malarial parasites. In the crystal, the heme molecules are linked into dimers through reciprocal iron-carboxylate bonds to one of the propionate side chains of each porphyrin. The dimers form chains linked by hydrogen bonds. 

Despite the importance of the oxidative polymerization of 5,6-dihydroxyin-dole, in the biosynthesis of pigments, little experimental data are known on the oxidation chemistry of the oligomers of 1. For such reasons, three major dimers of 1, such as 2-4 (Scheme 2.9), have been computationally investigated at PBEO/ 6-31+G(d,p) level of theory both in gas and in aqueous solution (by PCM solvation model) to clarify the quinone methide/o-quinone tautomeric distribution. 

The crystal structure of the OCP from Arthrospira maxima has been solved to 2.1 A resolution (Kerfeld et al. 2003). It is composed of two domains and the carotenoid, 3 -hydroxyechinenone, spans both. The carotenoid is almost completely buried within the protein only 3.4% of the pigment surface is accessible to solvent (see Figure 1.3a). The OCP is a dimer in solution the intermolecular interactions are largely mediated by hydrogen bonding among the N-terminal 30 amino acids, as shown in Figure 1.3b... 

Fishkin, N., Sparrow, J.R., Allikmets, R., Nakanishi, K., 2005. Isolation and characterization of a retinal pigment epithelial cell fluorophore An all-fraws-retinal dimer conjugate. Proc Natl Acad Sci USA. 102, 7091-7096. 

Kim, S.R., Jang, Y.P., Jockusch, S., Fishkin, N.E., Turro, N.J., Sparrow, J.R., 2007b. The all-trans-retinal dimer series of lipofuscin pigments in retinal pigment epithelial cells in a recessive Stargardt disease model. Pmc Natl Acad Sci USA. 104, 19273-19278. 

Rhodopsin is a seven ot-helix trans-membrane protein and visual pigment of the vertebrate rod photoreceptor cells that mediate dim light vision. In this photoreceptor, retinal is the chromophore bound by opsin protein, covalently linked to Lys296 by a Schiff base linkage. Kpega et al.64 have studied NMR spectra of Schiff bases being derivatives of all-frans retinal and amino-p-cyclodextrins as a model of rhodopsin, where p-cyclodextrin plays a role of a binding pocket. On the basis of analysis of the chemical shift differences for the model compound in the presence and in the absence of adamantane carboxylate, it has been shown that the derivative of 3-amino-p-cyclodextrin forms dimer in water and retinoid is inserted into p-cyclodextrin cavity [31]. 

Other polymeric binders, natural and synthetic, may be found as paints or varnishes in modern artworks and installations. Artists very easily adopt resins developed as industrial coatings or for specialized applications, and use them according to their creative needs. Natural rubber latex is a water dispersion of 1,4-ds-polyisoprene particles where pigments can be added to give coloured paints. By means of Py-GC/MS the presence of these paints can be easily assessed. As shown in Figure 12.13, the principal marker peaks in the pyrogram are those of isoprene, limonene and other cyclic dimers. 

Molecular dimerization, as in the transition from Hansa Yellow to disazo yellow pigments, increases the color strength considerably. A comparison of a Naph-thol AS/disazo condensation pigment pair gives similar results ... 

The formal structure is exemplified by the red disazo condensation pigments. The structure may be visualized as resulting from the dimerization of two monoazo pigments of the Naphthol AS type ... 

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