Color centers structure determination

The dendrites of a melanocyte contact about 36 keratinocytes and are able to transfer melanosomes to these adjacent cells. The numbers and sizes of the melanosomes as well as melanin structure determine differences in skin color.131 Similar cells in amphibians, the melanophores, also contain light receptors p Their melanosomes are not transferred to other cells but may be either clustered near the center of the cell or dispersed. The location can be changed quickly by transport of the melanosomes along a network of microtubules allowing the animals to change in response to changes in light color.0)... 

Band structure details of insulators can be determined from their UV/VIS spectra. Defects in the crystal produce electronic levels within the gap between the conduction and the valence bands. Spectroscopic measurements at low temperature allow the investigation of the phonon structure of a crystal. Absorptions due to lattice or point defects can be used to describe the optical and electronic properties of the insulator. For example, Cr in AI2O3 crystals leads to an intense color change of the crystal. Many so-caUed color centers are based on lattice defects caused by intercalation of atoms in the crystal lattice. 

Chapter 4 is concerned with a technically important group of leuco compounds which like the spiropyrans are not formed by reduction of the parent dye, but by formation of a spiro structure from the dye in such a way that the newly created sp3 center destroys the conjugation, and hence, the color of the chromophore. These are the phthalides (spirolactones) and the position of equilibrium is determined by pH rather than a redox process. Such materials are used mainly as color formers in pressure-sensitive... 

Various spectroscopic methods have been used to probe the nature of the copper centers in the members of the blue copper oxidase family of proteins (e.g. see ref. 13). Prior to the X-ray determination of the structure of ascorbate oxidase in 1989, similarities in the EPR and UV-vis absorption spectra for the blue multi-copper oxidases including laccase and ceruloplasmin had been observed [14] and a number of general conclusions made for the copper centers in ceruloplasmin as shown in Table 1 [13,15]. It was known that six copper atoms were nondialyzable and not available to chelation directly by dithiocarbamate and these coppers were assumed to be tightly bound and/or buried in the protein. Two of the coppers have absorbance maxima around 610 nm and these were interpreted as blue type I coppers with cysteine and histidine ligands, and responsible for the pronounced color of the protein. However, they are not equivalent and one of them, thought to be involved in enzymatic activity, is reduced and reoxidized at a faster rate than the second (e.g. see ref. 16). There was general concurrence that there are two type HI... 

Fig. 15 Ribbon diagrams of tubulin in complex with MT-stabilizing drugs as determined by EC or NMR. Left PTX/tubulin determined by EC (PDB entry 1JFF [83]). Center EpoA/tubulin determined by EC (PDB entry 1TVK [26]). Right EpoA/tubulin determined by solution NMR [76]. Drug molecules are represented as stick models (C red, O blue, N green, S yellow). Some secondary structure elements of tubulin are colored helix H7 (yellow), M loop (green), S9-S10 loop (blue). The side chain of His227 is depicted as cyan sticks...

The type I copper sites function as electron transfer centers in the blue copper proteins and in multicopper enzymes, particularly oxidases (33). They are characterized by their intense blue color, their unusually small A values, and their very positive redox potentials (Table II). X-ray crystal structures of several blue copper proteins have been determined, notably plastocyanin (34), azurin (35), cucumber basic blue protein (36), and pseudoazurin (37). The active site structures show marked similarities but also distinct differences (Fig. 8). 

Typical metastatic liver tumors have a thick hypoechoic rim with a relatively hyperechoic center, known as the huU s-eye sign (Fig. 16.10a) however, this finding cannot he applied to all metastatic liver tumors. Penetration of the normal vascular structure through the tumor proven hy color/power Doppler, ring-shaped enhancement in the arterial phase, and loss of enhancement in the postvascular phase as determined hy contrast ultrasound, are helpful findings for the diagnosis of metastatic liver tumors. 

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