Structure Modulations

Figure 4.12 Schematic diagram illustrating the role of the conserved leucine residues (green) in the leucine-rich motif in stabilizing the P-loop-(x structural module. In the ribonuclease inhibitor, leucine residues 2, 5, and 7 from the P strand pack against leucine residues 17, 20, and 24 from the a helix as well as leucine residue 12 from the loop to form a hydrophobic core between the P strand and the a helix.

A constellation of genes code for PBPs of varying amino acid sequences and functionalities. PBPs occur as free-standing polypeptides and as protein fusions. This combinatorial system of structural modules results in a large increase in diversity. 

The recommended Mg-Au phase diagram is that of ref. 3 amended by the addition of several new compounds close to MgAu3 they are a family of close-packed structures modulated both by antiphase boundaries of the superstructure and by a... 

Notes on the crystallography of quasi-periodic structures. A general way to face the problems related to the interpretation of quasi-periodic structures (modulated structures, quasicrystals) is based on the introduction and application of higher-dimensional crystallography (de Wolff 1974, 1977, Janner and Janssen 1980, Yamamoto 1982, 1996, Steurer 1995). 

Extensive structure modulations were performed to obtain more potent and less toxic anti-cancer agents, such as etoposide used in the therapy of numerous cancers (Fig. 41) [113], In contrast to podophyllotoxin, etoposide derivatives act as DNA topoisomerase II inhibitors. Tafluposide (F 11782) is an etoposide where both hydroxyl functions of the glycoside moiety are acylated with the pen-tafluorophenoxyacetic acid (Fig. 41). It has been demonstrated that tafiuposide does not act as a pro-drug of etoposide, but through a specific mechanism of interaction with both topoisomerases I and lla [114]. 

Fluorine substitution can take places at 9a, 6a or at both of these positions (Fig. 49). Avery large number of compounds have been designed from corticoid core with these common structural elements, that is, 1,4-diene, 17-hydroxymethylacetyl, 11 S-hydroxyl and the main following structural modulation ... 

To all these structural modulations, the esterification of 17- and 21-hydroxyl has been added with the aim to prepare oral, topical, spray and delayed-delivery formulations. A huge number of medicines have been marketed from more than 20 basic molecules (Figs. 48-55). Despite the fact that fluorocorticosteroids... 

K. Fujimoto, R. Doi, R. Hosotani, M. Wada, J.-U. Lee, T. Koshiba, T. Ibuka, H. Habashita, K. Nakai, N. Fujii, M. Imamura, Effects of structural modulation on biological activity of bombesin analogues with ( )-alkene bond. Life Sci. 60 (1997) 29-34. 

Astruc has developed the concept of transition metal sandwiches acting as electron reservoir complexes. The characteristic of an electron reservoir is that the reduced form is easily generated and does not decompose to increase stability, the radical center can be sterically protected in the heart of a bulky molecular framework. The [FeCp(arene)] series of complexes, e.g. 7, are prime examples, for which variation of the arene structure modulates the redox potential. 

Table 1. Crystallographic data are presented for the new phases which resulted from a 3d-metal substitution at the copper site in the n=l, 2 and 3 Bi-based cuprates. The periodicity of the structural modulation (p) is given by bold numbers when obtained from single crystal x-ray studies, otherwise the values were obtained from TEM studies. 

A number of models have been proposed to explain the crystal chemistry associated with the structural modulations. The following paragraphs survey the current understanding of the origin of the modulations ... 

Figure 10 (a) Schematic representation of structural modulations showing repeated shortening and widening of Bi-Bi distances, (b) Modulations in Bi-Bi separation in adjacent layers, indicating a sinusoidal variation. 

Interpretations of Structural Modulations The following paragraphs summarize the reported models concerning the origin of the modulations and report some of the recent experiments. The origin of the modulations, the role of anion stoichiometry and of the cation substitution affecting the modulations and Tc are not yet fully resolved and need further experimental clarification. 

Figure 19 (a) HREM image of (Tl,Pb)Sr2Ca2Cu309showing a 4-layer intergrowth (arrowed), (b) SADP in [010]. (c) [001] SADP with no superstructure spots, indicating that structural modulations are not present, (d) EDX spectrum showing the presence of H and Pb. Quantification is shown in Table 3. (e) Flux exclusion behavior for n=2 and n=3.. ... 

Therefore the ideal structure of Y7O6F9 can be depicted as a stacking of. .. AjBiAjBj. .. along [100], as shown in Fig. 2.36. In the real crystal, shown in Fig. 2.35, mutual correlations among these atomic planes exist. These result in structural modulation, i.e. the movement of not only anions but also cations from the ideal positions, although the dimensions of the subcells of each plane (Aj., Bj, Aj, B2) are assumed to be constant in the ideal structure, and, moreover, the site occupancy of anions is differentiated. 

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