Crystal structure determination pigments

The X-ray crystal-structure determination revealed a lack of any direct, helix-helix interaction between the a- and P-polypeptides within the hydrophobic core, as had been expected on the basis of earlier models [see Fig. 4 (B) and (C) above]. The association of the fMet-1 residue at the N-terminal of each a-polypeptide to a B800 BChl-a molecule prevents direct interaction between radially paired a-and P-helices. Interactions between the membrane-core helices are mediated only via pigment molecules or buried water molecules. The only polypeptide interaction occurs at the N- and C-terminal tails. In the C-terminal tail the large aromatic residues (a-Trp 40, a-Tyr 44, a-Trp 45 and P-Trp 39) contribute to binding between polypeptides via hydrogen bonds and hydrophobic interactions [see Fig. 5 (C)]. 

Schmidt, M.U. Energy Minimisation as a Tool for Crystal Structure Determination of Industrial Pigments. In Crystal... 

The crystal structure of Pigment Blue 80 (4d) was determined analogously, but since the powder diagram could be indexed, the lattice parameters and the space group could be used as input... 

Substitution patterns, especially that of the diazotized aromatic amine, determine the color of a pigment to some extent but empirical data do not lead to unambiguous conclusions as to the exact influence of a particular substituent on the shade. The problem is intricate, since the substitution pattern also has a bearing on the size and orientation of a pigment molecule and therefore on its crystal structure, including all the interactions associated with it. 

The crystallinity of organic pigment powders makes X-ray diffraction analysis the single most important technique to determine crystal modifications. The reflexions that are recorded at various angles from the direction of the incident beam are a function of the unit cell dimensions and are expected to reflect the symmetry and the geometry of the crystal lattice. The intensity of the reflected beam, on the other hand, is largely controlled by the content of the unit cell in other words, since it is indicative of the structural amplitudes and parameters and the electron density distribution, it provides the basis for true structural determination [32],... 

The crystal structure of the pigments is determined by X-ray analysis which is sensitive enough to determine 0.3-0.5% anatase in the presence of 99.7-99.5% rutile. For standards, see Table 1 (Titanium dioxide pigments Methods of analysis and Specification ). 

The value of pigments results from their physical—optical properties. These are primarily determined by the pigments physical characteristics (crystal structure, particle size and distribution, particle shape, agglomeration, etc) and chemical properties (chemical composition, purity, stability, etc). The two most important physical—optical assets of pigments are the ability to color the environment in which they are dispersed and to make it opaque. 

MacLean et al. (2000) have recently smdied the dimorphic behaviour of the pigment precursor ( latent pigment) derivative of 8-VI (R = COOr-but, R = H) (abbreviated DPP-Boc). The latency is due to the thermal decomposition reaction of both polymorphs resulting in the commercially important pigment DPP. The a form of DPP-Boc contains three half molecules in the asymmetric unit (see also Ellern et al. 1994) while the form contains one half molecule per asymmetric unit. Hence, they are easily distinguishable by solid state NMR as well as by X-ray powder diffraction. The crystal structure solution from powder data and Rietveld refinement of both polymorphs is an exemplary smdy demonstrating the potential of these methods in determining the detailed crystal structure of these compounds which are often difficult to crystallize. 

The refractive index of a pigment at each wavelength is determined by its crystal structure. Titanium dioxide pigments (rutile and anatase) differ from each other and from other white pigments or crystalline substances, like silicon dioxide, in the proportion of radiant energy that is transmitted, absorbed, or re-... 

With sufficiently large crystals of the bacterial reaction centers available, the determination of crystal structure by X-ray diffraction became possible and in 1983, Deisenhofer, Epp, Miki, Huber and Michel determined tbe crystal structure of the Rp. viridis reaction centers at 3 A resolution [later refined to 2.3 A ]. From the electron-density map, the spatial arrangement of the polypeptide subunits, the pigment molecules and the electron carriers in the reaction center was determined. For this work, Deisenhofer, Michel and Huber were awarded the Nobel Prize in 1988. 

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