Structural classification

The Stemona alkaloids are structurally characterized by the presence of either an exposed or hidden pyrrolo[l,2-a]azepine (k=1) moiety (5), also named perhydro-azaazulene (12) or 4-aza-azulene (13), or a pyrido[l,2-a]azepine (n = 2) nucleus (A, Fig. 1) and currently comprises 68 alkaloids. 

Xu and coworkers have previously suggested that the Stemona alkaloids can be separated into eight structural groups according to the sites of connection between the 

The numbering system of the structures was based on that described in the literature 12,13,16,17), although in some cases the numbering of some alkaloids may differ from that originally published in order to provide a consistent numbering system for all representatives in each group. 

Stenine Type Stemona Alkaloids Occurrence and Properties. 

Isostenine (3) also named Neostenine Source S. collinsae Craib. (roots) 20), S. tuberosa Lour, (roots) (2) 

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SCOP Structural Classification of Proteins. Hierarchical protein structure database... 

CATH, FSSP Sequence-structure classification databases... 

Brenner S E, C Chothia and T ] P Hubbard 1997. Population Statistics of Protein Structures Lessons from Structural Classifications. Current Opinion in Structural Biology 7 369-376. 

Murzin A G, S E Brenner, T Hubbard and C Chothia 1995. SCOP A Structural Classification of Proteins Database for the Investigation of Sequences and Structures. Journal of Molecular Biology 247 536-540. 

TJ Hubbard, B Alley, SE Brenner, AG Murzm, C Chothia. SCOP A structural classification of proteins database. Nucleic Acids Res 27 254-256, 1999. 

Structural classifications of oxides recognize discrete molecular species and structures which are polymeric in one or more dimensions leading to chains, layers, and ultimately, to three-dimensional networks. Some typical examples are in Table 14.14 structural details are given elsewhere under each individual element. The type of structure adopted in any particular case depends (obviously) not only on the... 

Ramsdellite is thermodynamically unstable toward a transformation into the stable ft -modification. Hence, it is rarely found in natural deposits. Natural ramsdellite has a stoichiometry close to the composition of Mn02 and can be considered another true modification of manganese dioxide. Attempts to synthesize ramsdellite in the laboratory usually lead to materials of questionable composition and structural classification. It is very likely that synthetic ramsdellite materials are more or less well-crystallized samples of the y-modification that will be described in more detail below. 

For a long time the structural classification of the mineral todorokite was uncertain, until Turner and Buseck [4] could demonstrate by HRTEM investigations that the crystal structure of that mineral consists of triple chains of edge-sharing octahedra, which form [3 x 3] tunnels by further corner-sharing. These tunnels are partially filled by Mg2+, Ca2+, Na+, K+, and water (according to the chemical analysis of natural todorokites). In 1988 Post and Bish could perform a Rietveld structure determination from XRD data taken for a sample of natural todorokite [25], This diffraction study confirmed the results of Turner and Buseck. The cations... 

In order to make as much data on the structure and its determination available in the databases, approaches for automated data harvesting are being developed. Structure classification schemes, as implemented for example in the SCOP, CATH, andFSSP databases, elucidate the relationship between protein folds and function and shed light on the evolution of protein domains. 

Other ion channels are closed at rest, but may be opened by a change in membrane potential, by intracellular messengers such as Ca + ions, or by neurotransmitters. These are responsible for the active signalling properties of nerve cells and are discussed below (see Hille 1992, for a comprehensive account). A large number of ion channels have now been cloned. This chapter concerns function, rather than structure, and hence does not systematically follow the structural classification. 

Different ways of the structural classification of deposits exist. In one system, the following structures are distinguished arbitrarily (1) fine-crystalline deposits lacking orientation, (2) coarse-crystalline deposits poorly oriented, (3) compact textured deposits oriented in field direction (prismatic deposits), and (4) isolated crystals with a predominant orientation in the field direction (friable deposits, dendrites). The structure of metal deposits depends on a large number of factors solution composition, the impurities present in the solntion, the current density, surface pretreatment, and so on. 

It should be noted that the above classification system of technetium cluster compounds is not the only possible one. In section 4 another classification is described, which is based on thermal stability and the mechanism of thermal decomposition. Section 2.2 is concerned with the classification based on methods of synthesizing cluster compounds. The classifications based on specific properties of clusters do not at all belittle the advantages of the basic structural classification they broaden the field of application of the latter, because for a better understanding and explanation of any chemical, physico-chemical and physical properties it is necessary to deal directly or indirectly with the molecular and/or electronic structures of the clusters. 

A structural classification of 8 is difficult due to the fact that an arrangement of metal atoms as in 8 is uncommon in the whole field of molecular metal clusters. For this reason, detailed understanding of the bonding properties in 8 requires quantum chemical calculations. Theoretical analysis seems to be especially applicable to learning more about the bond between the two tetrahedra, which appears at first to be an isolated metal-metal bond between two metal atoms in the formal oxidation state zero. 

Room temperature Donor conductivity, ort, Scm Metal-insulator 1 transition, K Structural classification (after Morikami et al.127)... 

In this section the salts based on metallocenium cations and metal bisdichalcogenate anions will be reviewed according to the previously referred structural classification. After referring to the general characteristics of the crystal structures the supramolec-ular features will be correlated with the magnetic properties. 

L. F. Wu, M. A. Mandrand (1993) Microbial hydrogenases primary structure, classification, signatures and phylogeny. FEMS Microbiol. Rev., 104 243-270... 

It must be emphasized that the duodectet rule (4.6) initially has no structural connotation, but is based on composition only. Indeed, the compositional regularity expressed by (4.6) encompasses both molecular species (such as the metal alkyls) and extended lattices (such as the oxides and halides) and therefore appears to transcend important structural classifications. Nevertheless, we expect (following Lewis) that such a rule of 12 may be associated with specific electronic configurations, bond connectivities, and geometrical propensities (perhaps quite different from those of octet-rule-conforming main-group atoms) that provide a useful qualitative model of the chemical and structural properties of transition metals. 

The process of classification is typically based on systematically arranging entities on the basis of their similarities and differences. A bowl of fruit can be systematically arranged to have apples on one side and bananas on the other. Chemical elements can be systematically arranged into distinct families on the basis of their atomic structures. Classification of this sort is relatively easy and can be grounded on any number of relatively distinctive parameters or combinations of parameters, including color, size, shape, structure, taste, and so forth. We have already noted that psychiatric disorders are best characterized as open concepts. In open psychiatric concepts, overt, objective, and distinctive parameters are often less apparent, making their classification considerably more difficult. 

In order to study in more detail the clay minerals, it is first helpful to review briefly the basic structural classification of the silicates in general. Although ultimately complicated, the general progression is logical, and is based on the degree of polymerization of the basic structural unit which is the Si04 tetrahedron (see below). The sequence runs as follows ... 

Organic compounds such as the chlorinated solvents also include a wide range of compounds and do not easily fit into the structural classification as described for petroleum hydrocarbons. Chlorinated hydrocarbons are commonly discussed in terms of their relative density (i.e., LNAPL or DNAPL) or degree of halogenation and degree of volatility (i.e., volatile, semi-volatile). 

Murzin, A. G. (1996). Structural classification of proteins new superfamilies. Curr. Opin. Struct. Biol. 6, 386-394. 

Murzin, A. G., and Bateman, A. (1997). Distant homology recognition using structural classification of proteins. Proteins Suppl. 1, 105-112. 

From this information, general principles for the design of spherical molecular hosts have been developed. [11] These principles rely on the use of convex uniform polyhedra as models for spheroid design. To demonstrate the usefulness of this approach, structural classification of organic, inorganic, and biological hosts - frameworks which can be rationally compared on the basis of symmetry - has revealed an interplay between symmetry, structure, and function. [53]... 

Naumann, T. and Matter, H. Structural classification of protein kinases using... 

For the processing industries, most process structures fit into the special purpose industrial, high hazard industrial, or open structures classification. See NFPA 101 for more information. 

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