Structural aspects

Structural Aspects.—Further complexes of nonacarbonyldi-iron with divinyl-cyclopropanes are reported. Products from semibullvalene (16), homo-semibuljvalene (254), barbaralone, benzosemibullvalene (423), and hydrocarbons (424) and (425) ° are described. The structure of the bullvalene-Fe2(CO)9 complex is established by 2f-ray analysis, and the 

Reactions.—Oxymetallation, the formation of carbon-carbon bonds using intermediate u-allylnickel compounds,the reaction of olefins with derivatives of Group VIII metals, and the vibrational spectra of ti-bonded organo-transition-metal complexes have been reviewed. 

Analysis of the stereochemistry of the products obtained by addition of enrfo-2-norbornylcopper(i) to mesityl oxide shows high stereospecificity. Hence the intermediacy of 2-norbornyl radicals is improbable. 

The reactants as well as the intermediate alkyl species in this reaction have been characterised. Thus, the difficulty associated with interpreting NMR spectra of As and Sb complexes (arising from the unfavourable properties of the nuclei) has been partially circumvented by X-ray structural analysis. 

As shown in Table 1, the As-Rh-As moiety in 2 (4.824(1) A) is significantly longer than that of P-Rh-P in 1 (4.665(1) A). This in turn means there is a larger cavity for entering nucleophiles in the arsine Vaska-type complex compared to the sterically more hindered phosphine analogue. 

The increase in the length of Rh-Cl bonds from complex 1 and 2 suggests more electron density on the Rh-centre in complex 2. The Rh-C(l) and CsO bond distances in the isomorphous complexes 1 and 2 do not differ significantly. 

Of further interest is the fact that the Rh-P and Ir-P bond lengths in the structures listed in Table 2 are virtually identical. However, changing the Group 15 donor atom results in significant increases in the Rh-L bond length (from 2.33 

In Table 2, comparisons between the complexes investigated in this study and a few other relevant iso-structural complexes from the literature suggest that the introduction of ligands with heavier donor atoms, such as arsines and stibines, leads to reduced steric crowding at the metal, creating more space for entering moieties in reactions such as oxidative addition. 

Two types of information are obtained from any molecular mechanics study, the minimum value of the strain energy and the structure associated with that minimum. Agreement between the energy-minimized and experimental (crystallographic) structures has often been used as the primary check on the validity of the force field and to refine the force field further, but often little predictive use has been made of the structures obtained. As force fields become more reliable, the potential value of structure predictions increases. More importantly, when no unequivocal determination of a structure is available by experimental methods then structure prediction may be the only means of obtaining a three-dimensional model of the molecule. This is often the case, for instance, in metal-macromolecule adducts, and structures obtained by molecular mechanics can be a genuine aid in the visualization of these interactions. In this chapter we consider the ways in which structure prediction by molecular mechanics calcluations has been used, and point to future directions. 

In order to get an insight into the diversity of proteins that may contain a Rieske or Rieske-type cluster, a database search was performed in the following way. 

Bacterial Rieske proteins that are not from bci or b f complexes or of unknown origin [9] 

Rieske proteins are constituents of the be complexes that are hydro-quinone-oxidizing multisubunit membrane proteins. All be complexes, that is, bci complexes in mitochondria and bacteria, b f complexes in chloroplasts, and corresponding complexes in menaquinone-oxidizing bacteria, contain three subunits cytochrome b (cytochrome 6e in b f complexes), cytochrome Ci (cytochrome f in b(,f complexes), and the Rieske iron sulfur protein. Cytochrome 6 is a membrane protein, whereas the Rieske protein, cytochrome Ci, and cytochrome f consist of water-soluble catalytic domains that are bound to cytochrome b through a membrane anchor. In Rieske proteins, the membrane anchor can be identified as an N-terminal hydrophobic sequence (13). 

In addition to the four ligands of the Rieske cluster, three residues are fully conserved in all Rieske proteins  

The residues printed in bold are the ligands of the cluster the two underlined cysteine residues form a disulfide bidge stabilizing the cluster (9). 

Cellulose is reputedly the most abundant organic material on Earth, being the main constituent in plant cell walls. It is composed of glucopyranose units linked pi 4 in a linear chain. Alternate residues are rotated in the structure, allowing hydrogen bonding between adjacent molecules, and construction of the strong fibres characteristic of cellulose, as for example in cotton. 

Unusual Structures. Former reviews treating clusters according to their basic structures (241, 316) have revealed that the most common metal atom arrangements are triangle, tetrahedron, and octahedron. Relatively few other oligonuclear species were observed. This situation has changed during the last five years, and unusual structures with four to six metal atoms and a number of clusters with more than six metal atoms have been described. 

Among the tetranuclear clusters there exists the full range of possible structures now between the tetrahedron and the rectangle. The open tetrahedron with the 

A wider open butterfly angle is found in the tetrameric arylcopper [J] 194, 263), whereas a similar alkylcopper [-7] 225, 269) has a square arrangement of copper atoms. 

Yet another arrangement of four metal atoms, a triangle with one external metal, 

In the hexanuclear clusters the dominating shape is that of the octahedron, which is even able to dimerize as in Rhi2(CO)3Q (90). The few other structures are that of a trigonal prism as in Ptg(CO)i2 (60)] (22) and 

Since the primary and three-dimensional structures of cytochrome c peroxidase are yet to be established, any discussion on its molecular 

Rossi-Fanelli, E. Antonini, and A. Caputo, Advan. Protein Chem, 19, 73 

A proposed orieatatioa of the protoheme prosthetic group in cytochrome c peroxidase. 

The sixth coordination position of the heme iron in cytochrome c peroxidase, which is normally occupied by H2O, is available for reactions with extraneous ligands such as fluoride, cyanide, and azide, as well as substrates and substrate analogs. The acidic-alkaline transition of a hemo-protein, which is caused by the ionization of the bound water ligand, is usually accompanied by significant spectral changes. However, the visible absorption spectrum of cytochrome c peroxidase is not appreciably 

The reaction of cytochrome c peroxidase follows a general form of compulsory order mechanism for two-substrate enzymes as indicated by Eqs. 

PS II is made up of a cluster of polypeptides, several of which span the membrane. The location of the various components dealt with in previous sections is in many cases not clearly demonstrated. At present, then, models of the structure of the PS II reaction centre are rather speculative. The model shown in Fig. 5 is no exception. 

When the crystal structure of the Rps. viridis reaction centre was published [133], along with the primary structure of the L and M polypeptides, a basis was provided for a model of the PS II reaction centre based on sequence homologies with the 32 kDa polypeptides [203-204], Not only were remarkable homologies of secondary structure obtained but also all of the specific amino acid changes associated with herbicide resistance were found to be clustered around the predicted Qb binding site [204]. 

The model in Fig. 5 is based on the X-ray structure of the purple bacterial reaction centre. Since no analogies to Z and D are present in purple bacteria it is reasonable to suggest that these components originate in polypeptides other than the 32 kDa (Dj and D2) polypeptides. An obvious candidate is the 47 kDa polypeptide which forms part of the PS II core. 

Cyt 6-559 is closely associated with the PS II reaction centre. A structural model has recently appeared in which the haem, which is oriented perpendicular to the membrane, is liganded to two histidines each on different membrane-spanning polypeptides (9 kDa). Changes of the relative orientation of the imidazole rings from parallel to perpendicular have been proposed to be responsible for the high-potential to low-potential redox form transition [212]. It is still not clear whether 1 or 2 cytochromes are present per reaction centre. The conflicting reports may be 

The orientation of some of the chromophores has been determined and is included in Fig. 5 [210,213,214]. It is of interest that the ultra-rapid electron transfer reaction that takes place between P-680 and Pheo occurs between chromophores that are perpendicular to each other. This is also the case in purple bacteria [211]. 

The surface of a crystalline solid is strongly correlated to its bulk structure. The atoms of a crystal are arranged in a periodical sequence forming the crystal lattice. Most frequently metals and metal alloys tend to form close-packed sphere (cps) arrangements reflecting the isotropy of the forces of atomic interaction. 

There are two possible cps arrangements, the hexagonal close-packed (hep) and the face centered cubic (fee) crystal lattice. In both cases the planes of highest atomic density, (0001) and (111), respectively, have the same two-dimensional (2D) hexagonal close-packed (2D hep) structure, Fig. 2.1. The sequence of consecutive planes forming the bulk cps structure, however, are different  

In the first case the atoms of every second layer (i.e., every even layer) occupy one of the three (every second out of six) concavities, e.g., B in Fig. 2.1, around each 

In the second case every third consecutive layer occupies a concavity lying above a concavity of the first layer. Therefore, an ABCABC. . . sequence of planes results, forming a face-centered cubic (fee) 3D Me bulk lattice, Fig. 2.3. The unit cells of both crystal lattice structures are also given in Figs. 2.2 and 2.3. 

Generally the following imperfections of a 3D crystal lattice can be distinguished  

The famous double helix, at the first glance, seems to be a comparably simple and rather uniform structure of a biopolymer. In fact however, not only may the double helix adopt several different conformations, the best known being the A-, B- and Z-types, but also single-stranded oligonucleotides, depending on their sequence and derivatization and conditions such as humidity, ion strength, counter ion charge, pH, buffer salts, solvent and last but not least temperature, display numerous types of polymorphism, the study of which has become an important field of biophysical research. 

With respect to pharmacokinetics it is obvious that biotransformation of natural or only slightly modified natural oligonucleotides - mainly by action of nucleases - is a major obstacle towards application of such compounds. It is not surprising that first of all a variety of modifications of the labile phosphodiester bond has been developed, with phosphorothioate and methylphosphonate groups being hitherto the most successful approaches (see section 6). This report will also include structures such as peptide nucleic acids (PNAs), which are promising bio-isosteres of natural lead structures (see section 10.1). 

A variety of phosphorus containing ligands have been utilized in the preparation of binuclear complexes. Among the most versatile are the diphosphines of the type Ph2P(CH2)nPPh2. These are readily available (at the present time from commercial sources) and are easily handled, air-stable solids. The length of the methylene chain can be varied to give different 

Phosphide anions, i 2P are also known to function as bridging ligands/ In this regard, they resemble the more familiar bridging halide and chalconide ligands. We have, somewhat arbitrarily, excluded these from this review. 

Similar structures with large gaps between metal ions are seen for the 

Other variations of the face-to-face structure can be identified. An additional ligand can be present on one or both metals to give five-coordinate species. The two metals may actually be directly bonded. Such is the case for (/Lt-dpm)2Mo2Cl4 where a Mo-Mo quadruple bond is present, and the Mo-Mo separation is only 2.138 

Recently the problem of constructing bifunctional phosphine ligands with two different binding sites, each capable of bonding a different type of metal ion, has received attention. Two such ligands are 2(diphenylphos- 

Type Layered Framework Corpuscular/ polymeric structures classy/ skeleton 

SiOj-based (silicates) Clays LDHs Zeolites MCM-41, MCM-48, FSM, PMOs Silica gel, precipitated silica, fumed silica Porous glasses (VPG, CPG), sintered glasses 

Carbon-based Graphite, graphene Carbide- derived carbons (CDCs) Bucky balls, carbon nanotubes Soot, carbon black Activated carbons, charcoals 

Al/AljOj- based Pseudo- boehmite AlPOs (SAPOs, MeAPSOs) Anodic oxidized Aluminum oxide gels Foams 

The group of crystalline materials can be divided into the layered and framework types, while the group of amorphous materials is subdivided as possessing corpuscular or glassy character. 

Silicon carbide exists in several modifications, being polymorphic and polytypical and crystallizing in a diamond lattice [7]. 

As silicon carbide exists predominantly in its beta form at temperatures below 2000 °C, this is referred to as the low-temperature modification. Cubic P-SiC is metastable and, in accordance with Ostwald s rule, is formed initially in SiC production from silicon dioxide and carbon. P-SiC can also be prepared at about 1450 °C from simple mixtures of silicon and carbon, or by the hydrogen reduction of organosilanes at temperatures below 2000 ° C. Above 2000 ° C, only the hexagonal and 

The layer sequences can repeat themselves in the cycles ABC, ABC... (zincblende, type 3C) or AB, AB. .. (wurtzite, type 2H), according to cubic or hexagonal close packing. In addition, numerous other stack sequences are formed in the case of silicon carbide, resulting in many similar polytypes. 

Polycrystalline silicon carbide obtained by the Acheson process exhibits a large number of different polytypes, some of which dominate. More than 200 different polytypes are currently known these can be classified into the cubic, hexagonal, and rhombohedral crystal system, and all have the same density of 3.21 gcm . Written polytype nomenclature [11] indicates the number of layers in the repeating layer pack by a numeral, while the crystal system is denoted by the letters C, H, or R. 

The most frequently encountered structures 3C, 4H, 6H, and 15R, are called short-period polytypes long-period polytypes are muchlesscommonandconsistofblocks of short-period polytypes which are broken by regularly occurring stacking faults. 

The same approach can obviously be applied to substituted dienes, with however a rapidly increasing complexity 6 isomers for isoprene, still more isomers, with some having asymmetric carbons in the chain for 1,3 pentadiene, and so on. 

Marcel Dekker, Inc. 270 Madison Avenue, New York, New York 10016 

In addition to the epithelial cells, the dendritic cells, which reside at the base of the epithelium and reach up to the tight junctions, with numerous intercellular (dendritic) cytoplasmic processes, constitute a principal cell population in the airways. The dendritic cells are extensively described in Chapter 11. 

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The existence of osmiophilic material in various forms in the aqueous lining layer has been previously reported (34,35,41,45,47,48). The appearance of osmiophilic material at the air-liquid interface in the form of a film covering the aqueous lining layer is of special interest for the retention of particles and will be discussed later in further detail. 

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R), i.e. there is no effect due to caging of the encounter complex in the common solvation shell. There exist numerous modifications and extensions of this basic theory that not only involve different initial and boundary conditions, but also the inclusion of microscopic structural aspects [31]. Among these are hydrodynamic repulsion at short distances that may be modelled, for example, by a distance-dependent diffiision coefficient... 

Takemura K, Minomura S, Shimomura O, Fu]ii Y and Axe J D 1982 Structural aspects of solid iodine associated with metallization and molecular dissociation under high pressure Phys. Rev. B 26 998... 

Yarkony D R 1995 Electronic structure aspects of nonadiabatic processes in polyatomic systems Modern Electronic Structure Theory vo 2, ed D R Yarkony (Singapore World Scientific) pp 642-721... 

S. Maim and C. C. Perry, Structural Aspects of Biogenic Silica, Ciba Poundation Symposium 121, John Wiley Sons, Inc., New York, 1986, pp. 40—58. 

Batsanov, S.S., Structural Aspect of Shock-Wave Propagatio in Crystals, in Proceedings, First All Union Symposium on Shock Pressures, Vol. 2 (edited by Batsanov, S.S.), Moscow, 1974, pp. 1-10. Translation, Sandia National Laboratories Report No. SAND80-6009, April 1980. 

Bohm, A., Gaudet, R., Sigler, P.B. Structural aspects of het-erotrimeric G-protein signaling. Cun. Opin. Struct. Biol. 8 480-487, 1997. 

Schulze, A. J., Huber, R., Bode, W., and Engh, R. A., 1994. Structural aspects of serpin inhibition. FEES Letters 344 117-124. 

Cramer, W. A., et al., 1996. Some new structural aspects and old controversies concerning die cytochrome b /complex of oxygenic photosyndie-... 

Some structural aspects of the organometallic compounds of the alkali metals have already been briefly mentioned in Section 4.3.6. The diagonal relation of Li with Mg (p. 76), coupled with the known synthetic utility of Grignard reagents (pp. 132-5), suggests that Li, and perhaps the other alkali metals, might afford synthetically... 

Figure 2. Rotation of the NASICON structure represented in Fig. 1 by 90°. For clearer representation of the structural aspects, the sodium ions are indicated by yellow.

Traditionally, the chemical stability of the electrode/electrolyte interface and its electronic properties have not been given as much consideration as structural aspects of solid electrolytes, in spite of the fact that the proper operation of a battery often depends more on the interface than on the solid electrolyte. Because of the high ionic conductivity in the electrolyte and the high electronic conductivity in the electrode, the voltage falls completely within a very narrow region at the electrolyte/electrode interface. 

Enkelmann, V. Structural Aspects of the Topochemical Polymerization of Diacetylenes. Vol. 63, pp. 91-136. 

Stereochemical and electronic structural aspects of five coordination. J. S. Wood, Prog. Inorg. Chem., 1972,16, 227-486 (566). 

Structural aspects of metal complexes with some tetradentate Schiff bases. M. Calligaris, G. Nardin and L. Randaccio, Coord. Chem. Rev., 1972, 7, 385-403 (56). 

Structural aspects and biochemical functions of erythrocuprein. U. Weser, Struct. Bonding (Berlin), 1973,17, 1-65 (226). 

Structural aspects and coordination chemistry of metal porphyrin complexes with emphasis on axial ligand binding to carbon donors and mono- and di-atomic nitrogen and oxygen donors. P. D. Smith, B. R. James and D. H. Dolphin, Coord. Chem. Rev., 1981,39, 31-75 (170). 

Structural aspects of zirconium chemistry. A. Clearfield, Rev. Pure Appl. Chem., 1964, 14, 91-108 (94). 

There is a wealth of information concerning preparative and structural aspects of the fluoro- and oxyfluorocomplexes of... 

The techniques used in the work have generally been spectroscopic visible-uv for quantitative determinations of species concentrations and infrared-Raman for structural aspects of the polymer. Although the former has often been used in the study of plutonium systems, there has been considerably less usage made of the latter in the actinide hydrolysis mechanisms. 

Even when they have a partial crystallinity, conducting polymers swell and shrink, changing their volume in a reverse way during redox processes a relaxation of the polymeric structure has to occur, decreasing the crystallinity to zero percent after a new cycle. In the literature, different relaxation theories (Table 7) have been developed that include structural aspects at the molecular level magnetic or mechanical properties of the constituent materials at the macroscopic level or the depolarization currents of the materials. 

Esterases have a catalytic function and mechanism similar to those of lipases, but some structural aspects and the nature of substrates differ [4]. One can expect that the lessons learned from the directed evolution of lipases also apply to esterases. However, few efforts have been made in the directed evolution of enantioselective esterases, although previous work by Arnold had shown that the activity of esterases as catalysts in the hydrolysis of achiral esters can be enhanced [49]. An example regarding enantioselectivity involves the hydrolytic kinetic resolution of racemic esters catalyzed by Pseudomonasfluorescens esterase (PFE) [50]. Using a mutator strain and by screening very small libraries, low improvement in enantioselectivity was... 

F. Tuinstra, Structural Aspects of the Allotropy of Sulfur and the Other Divalent Elements, Waltman, Delft, 1967... 

Weser U (1985) Redox Reactions of Sulphur-Containing Amino-Acid Residues in Proteins and Metalloproteins, an XPS Study. 61 145-160 Weser U (1973) Structural Aspects and Biochemical Function of Erythrocuprein. 17 1-65 Weser U, see Abolmaali B (1998) 91 91-190... 

Minkin VI, Clukhoytsev MN, Simkin BY (1994) Aromaticity and Antiaromaticity, Electronic and Structural Aspects. Wiley, New York... 

Modern representations of the virtual heart, therefore, describe structural aspects like fibre orientation in cardiac muscle, together with the distribution of various cell types, active and passive electrical and mechanical properties, as well as the coupling between cells. This then allows accurate reproduction of the spread of the electrical wave, subsequent contraction of the heart, and effects on blood pressure, coronary perfusion, etc. It is important to point out, here, that all these parameters are closely interrelated, and changes in any one of them influence the behaviour of all others. This makes for an exceedingly complex system. 

This volume covers the synthetic and structural aspects of all types of terpenoids ar>d staroids based on the irtcreasirtg importance of btogenetic theory in this work It covers the period September 1969 to August 1970 557pp tIl OO... 

This report covers the synthetic and structural aspects of the subiect with discussion of the biological aspects included where relevant It covers the 1970 literature 2S4pp 16 00 (Still eveilsbie Vo 1 14 50 Vof 2 6 00)... 

Chen, L.J. and Hrazdina, G., Structural aspects of anthocyanin-flavonoid complex formation and its role in plant color. Phytochemistry, 20, 297, 1981. 

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