Extended structures

2-Dithiolium ions, l,2-dithiole-3-thiones, and l,6,6a(S -trithia-pentalenes show structural relationships which can be further extended. Compounds of this series are of peculiar interest because of the structural problems involved concerning the nature of the chemical bond and the aromaticity of the ring systems. 

When the compounds contain an odd number of sulfur atoms, they are neutral molecules, and monomeric thioketones may be considered as the first member of the series. Compounds containing an even number of sulfur atoms are positive ions. In the following formulas, only one resonance structure is given for the sake of simplicity. 

2-Dithiol-3-ylidenemethyl)-l,2-dithiolium ions may be prepared by reacting a 3-methylthio-l,2-dithiolium ion with a 3-methyl-1,2-dithiolium ion, either in ethanol or in acetic acid with catalytic amounts of pyridine (Eq. 38).  

It has been noted earlier (Section IV, A) that carbon disulfide can be condensed with trithiapentalenes having a 2- or 5-methylene group, provided that the corresponding carbanion does not rearrange too easily. 

For example, trithiapentalene (71) reacts with carbon disulfide in the presence of a strong base ( -AmONa NaH) giving the dianion 72 which can be methylated to 73 or oxidized to a dithiolylidenealkyl-dithiolethione (74).  

Here there are many possibilities, especially if systems with differently hybridized sites are considered. Only the purely hybridized systems are considered in the present section, but many examples with mixed hybridization may be generated from the pure hybridized systems via the poly-yne transformation of Section 9.6.5. 

---

R. Hoffmann, Solid and Surfaces A Chemist s View of Bonding in Extended Structures, VCH Publishers, New York, 1988. 

Since the main chain in this representation is in an entirely staggered conformation, whereas in the Fischer projection formulas the conformation represented is completefy eclipsed, an anti relationsh between two adjacent substituents in an extended structure corresponds to being on the same side in a Fischer projection formula (erythro) whereas a syn relationship corresponds to being on opposite sides in die Fischer projection (three). 

We showed the possible existence of various forms of helically coiled and toroidal structures based on energetic and thermodynamic stability considerations. Though the formation process of these structures is not the subject of this work, the variety of patterns in the outer and inner surface of the structures indicates that there exist many different forms of stable cage carbon structures[10-19]. The molecules in a onedimensional chain, or a two-dimensional plane, or a three-dimensional supermolecule are possible extended structures of tori with rich applications. 

Ta) contains a discrete [MeSn] anion, the stoichiometrically analogous M4Nb60n0.3H20 has an extended structure. 

The consideration made above allows us to predict good chromatographic properties of the bonded phases composed of the adsorbed macromolecules. On the one hand, steric repulsion of the macromolecular solute by the loops and tails of the modifying polymer ensures the suppressed nonspecific adsorptivity of a carrier. On the other hand, the extended structure of the bonded phase may improve the adaptivity of the grafted functions and facilitate thereby the complex formation between the adsorbent and solute. The examples listed below illustrate the applicability of the composite sorbents to the different modes of liquid chromatography of biopolymers. 

Annealing drawn PE hydrostatically at high pressure, generates a wide spectrum of crystal thicknesses varying from the common oriented chain folded to the chain-extended structures — where folds and ties tend to disappear63 —. This range of crystal thicknesses coupled with the chain axis orientation, offers a suitable model in... 

P406 and P4Ow are the anhydrides of phosphorous acid and phosphoric acid, respectively. Polyphosphates are extended structures used (as ATP) by living cells to store and transfer energy. 

The simplest indicator of conformation comes not from but the sedimentation concentration dependence coefficient, ks. Wales and Van Holde [106] were the first to show that the ratio of fcs to the intrinsic viscosity, [/ ] was a measure of particle conformation. It was shown empirically by Creeth and Knight [107] that this has a value of 1.6 for compact spheres and non-draining coils, and adopted lower values for more extended structures. Rowe [36,37] subsequently provided a derivation for rigid particles, a derivation later supported by Lavrenko and coworkers [10]. The Rowe theory assumed there were no free-draining effects and also that the solvent had suf-... 

By making use of the spatial information, the velocity field of an extended, structured object can be obtained unambiguously without errors caused by uncertainty in the position of a feature within the slit. 

Define a crude initial structure by either distance geometry algorithms or by model building. The latter starts by defining elements of secundary structure (helices, 3 Sheets) fhom the NMR data. Even starting from an extended structure is feasible (53) ... 

Extended structure contributes to their high viscoelasticity. 

As a result, the branches in the G TPPH2 family are more flexible, thus causing a decrease in the hydrodynamic volume compared to the theoretical fully extended structure in the gas phase. 

Activated PAMAM dendrimers interact with DNA to form a DNA-dendrimer complex with a toroid-like structure (Fig. 2). Such DNA-dendrimer complexes have diameters of 50-100 nm [10],which means that the DNA molecules are highly condensed in these complexes. A 6-kb plasmid alone, for example, has an extended structure several hundred nanometers in diameter. In transfection experiments, typically an 8- to 12-fold excess of positive amino groups over negatively... 

Gelation involves an extended structure and some type of linking between chains. The concept of salt-like crosslinks has already been described (Section 5.5). Other possibilities may be considered. Hill, Wilson Warrens (1989) examined the possibility that chain entanglements might account for the strength of polyelectrolyte cements. They used in particular... 

Liu, Y. H., and Zumbrunnen, D. A., Toughness enhancement in polymer blends due to the in-situ formation by chaotic mixing of fine-scale extended structures, J. Mater. Sci. 34, 1921-1931 (1999). 

Fig. 1. Schematic diagram of nuclease A131A in the folded conformation. The alpha helices and beta strands are labeled. NMR analysis suggests the two turns and one helix in black are modestly populated in the denatured state, whereas the shaded helix is slightly populated. Strands / l-/ 2-/ 3 form an extended structure about which littie is known. Reproduced from Barron, L. D., Hecht, L., Blanch, E. W., and Bell, A. F. (2000). Prog. Biophys. Mol Chem. 73, 1-49. 2000, with permission from Elsevier Science.

Figures 9 and 10 represent a selected comparison of amide V and I+II FTIR and VCD for four proteins in D2O solution. Of these, myoglobin (MYO) has a very high fraction of a-helix, immunoglobulin (IMU) has substantial /1-sheet component, lactoferrin (LAF) has both a and j3 contributions, and a-casein (CAS) supposedly has no extended structure. The FTIR spectra of these proteins change little, the primary difference...

The LBP-20 proteins were predicted to be rich in a-helix, and with very little, if any, (i/extended structure, and this has been confirmed by circular... 

---