Cluster structure elements

Figure 3 Schematic sketch of a two-dimensional percolation cluster structural elements are single connecting bonds, dead (or dangling) ends, and loops (blobs) on all scales (this is only an illustration and not a real generated cluster)...

This short description of the various elementary modifications of gallium shows its high structural variety, which also includes structural elements that are atypical for metals. This variety is also reflected in the inner composition of the metalloid gallium clusters, as discussed below. 

For niobium and cobalt clusters structures have been proposed based upon the elements behavi or (71). Niobium s specific inertness has been associated with structures that are analogous to close-packed surface of W(110) which also has an activation barrier for hydrogen chemisorption. Since the IPs are also expected to be higher for closed packed structures these two sets of observations are in agreement. This model at its current stage of development requires different structures for each system and as yet has not been useful in making predictions. 

This may be the reason why silicon is essential, namely that it keeps aluminium in a non-toxic form as aluminium silicate. While silicon is required as a trace element in most animals, in plants, particularly grasses, and in many unicellular organisms, such as diatoms4, it is a major structural element. The importance of phosphorus and sulfur is obvious, the latter often associated with iron in an important family of proteins that contains iron-sulfur clusters. 

Bare group 13 metal vertices (e.g., Ga, In, Tl) provide, as noted above, only one skeletal electron each to polyhedral cluster structures. Thus it is not surprising that the bare metal cluster ions Enz (E = group 13 element) found in homonuclear alkali-metal/group 13 intermetallic phases [86-89] (mainly for In and Tl) have charges less negative than the — (n + 2) (i.e., z [Pg.21]

Fig. 4. Stereoviews of TmCsp. (A) Stereoview of a superposition of the backbone atoms of the 21 accepted structures of TmCsp. (B) Stereoview of a representative ribbon diagram of TmCsp. The secondary-structure elements are indicated. (C) Stereoview of the solution NMR structure of TmCsp (rotated ribbon diagram). Location of a possible peripheral ion cluster. Reprinted from Eur. J. Biochem., Vol. 268, W. Kremer, B. Schuler, S. Harrieder, M. Geyer, W. Gronwald, C. Welker, R. Jaenicke and H. R. Kalbitzer, Solution NMR structure of the cold-shock protein from the hyperthermo-philic bacterium Thermotoga maritima , pp. 2527-2539, Copyright 2001, with permission from Blackwell Science.

It is interesting that a unique secondary structural element, designated the half-turn, was indentified in preliminary NMR studies of rabbit metallothionein-2 (Wagner etal., 1986). The half-turn element is defined as a type II turn with (f>3 rotated from 90° to -90° its occurrence in the metallothionein-2 structure arises from the constraints placed on the relatively short polypeptide chain by the metal clusters. Although these constraints are not well understood and are certainly difficult to predict, the continued biophysical study of metallothionein-2 will certainly improve our understanding of protein-metal cluster interactions. 

Growth of Nuclei to Metal Nanoparticles. If the elemental cluster of 13 atoms is the nucleus, the growth of nuclei to metal nanoparticles could proceed by deposition of atoms or microclusters on the surface of nuclei. This process is understandable based on the consideration of the formation of monodispersed nanoparticles. However, structural analysis has often proposed the aggregation of elemental clusters to form fundamental clusters (64). A similar idea is discussed for the structural analysis of bimetallic nanoparticles with cluster-in-cluster structure (40,61). 

Mercury(II) forms a series of compounds in which it is bonded to a transition metal to form heteronuclear Hg—M bonds.540,541 The most widely used synthetic routes have been reviewed elsewhere.542-549 Besides heteronuclear bonds M—Hg there are structural elements M—Hg— M, 550 e.g. (31), or cyclic arrangements as in Os3(CO)uHg 3415 or (1/5-MeC5H4)Mn(CO)2Hg 4.416 A trigonal prismatic coordination of mercury has been reported in the green zerovalent mixed metal cluster [Hg Pt(2,6-Me2C6H3NC)fi ]. a... 

Numerous transition metals ions form cluster complexes with chalcogenide anions [42-52], Iron and sulphur are unique elements in the sense that no two other elements can generate such a large diversity of cluster structures. This is the consequence of two stable oxidation states of iron ions and strong Fe-S bonds of significantly covalent character [53], Moreover, numerous structures are stable in several oxidation states, so these clusters serve as electron reservoirs in biological systems [51], This is why iron-sulphur proteins usually catalyze redox reactions. 

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