Structural origin

M.o. theory and the transition state treatment In 1942 Wheland proposed a simple model for the transition state of electrophilic substitution in which a pair of electrons is localised at the site of substitution, and the carbon atom at that site has changed from the sp to the sp state of hybridisation. Such a structure, originally proposed as a model for the transition state is now known to describe the (T-complexes which are intermediates in electrophilic substitutions... 

The structure originally proposed for cordycepic acid, [a]u = +40.3°, has been shown to be incorrect. Suggest a reason to be skeptical about the original structure, which is given below. 

Compound A possesses the structure originally proposed for the methyl ester of Aao (see K. Nakanishi, M. Endo, U. Naf, and L. F. Johnson, J. Am. Chem. Soc, 1971, 93, 5579). However, the 270-MHz NMR spectrum of synthetic A was different from that reported for Aah methyl ester with respect to the protons at C(3) and C(5). Therefore, the 3a-alcohol, compound B, was synthesized as shown below. 

Therefore the relationship between these interconvertible structures originates from a cubic anion lattice of 32 0 ions in the cell. With 32 Fe ions in the octahedral holes stoichiometric FeO is formed. Replacement of a number of Fe ions with two-thirds of their number of Fe ions maintains electrical neutrality but provides non-stoichiometric Fei 0. Continual replacement in this way to leave 24 Fe atoms in the cubic cell produces Fej04, and... 

Ohmiya, Y., Hirano, T., and Ohashi, M. (1996). The structural origin of the color differences in the bioluminescence of firefly luciferase. FEBS Lett. 384 83-86. 

A zeolite membrane, where the pores originate from the structure, presents only one type of (ultramicro)pore and therefore seems to be a good candidate for CMRs application. Moreover the structural origin of the pores should induce a much better thermal stability of the... 

The GC-MS data (Figure 16.11) of the violet zone of B. carterii revealed that the unchanged diterpenes (verticillatriene, cembrene A, and cembrene C) and the nortriterpenes with carbohydrate structure originated from the pyrolyzed triterpenes (Figure 16.12) of the a- and (3-boswellic acids, named 24-norursa-3,12-diene (compound 7), 24-norursa-3,9(ll),12-triene (compound 8), 24-noroleana-3,12-diene (compound 9), and 24-noroleana-3,9(ll),12-triene (compound 10). 

For an approximate determination of the sample composition it is often sufficient to measure the peak height of the core level. In general, however, core level structures are asymmetric peaks above a finite background and sometimes accompanied by satellite structures. These structures originate from the many-body character of the emission process. Therefore a peak integration including satellites and asymmetric tails is much more reliable. Due to the above difficulties quantitative analysis of XPS data should be taken as accurate to only within about 5-10%. 

This result is important to fully understanding the biochemical and ultra-structural origin of peaks and the physiological basis for variation. It not only helps in designing the analytical strategy (e.g., in selection of cleanup columns) but, more important, in making a decision on whether the marker should be used for strain or species identification or for biodetection. For example, there are a number of low-molecular weight peptides (1500-8000 kDa) present in... 

B. anthracis and related species.41,44 6 Some of these peaks have been identified (e.g., as small acid soluble spore proteins and cyclic lipopeptides), but others remain uncharacterized. There is no agreement among different laboratories as to which markers are suitable for chemotaxonomic differentiata-tion of species (i.e., are consistently found in one species versus another) or for strain identification (i.e., are reproducibly found in one strain but not another). Further, although it might be anticipated that surface proteins can be preferentially ionized or extracted, the ultra-structural origin of some peptides within the cell is not always clear. 

Weber, P. C. Ohlendorf, D. H. Wendoloski, J. J. Salemme, F. R., Structural origins of high-affinity biotin binding to Streptavidin, Science 1989, 243, 85-88. 

How then, can one recover some quantity that scales with the local charge on the metal atoms if their valence electrons are inherently delocalized Beyond the asymmetric lineshape of the metal 2p3/2 peak, there is also a distinct satellite structure seen in the spectra for CoP and elemental Co. From reflection electron energy loss spectroscopy (REELS), we have determined that this satellite structure originates from plasmon loss events (instead of a two-core-hole final state effect as previously thought [67,68]) in which exiting photoelectrons lose some of their energy to valence electrons of atoms near the surface of the solid [58]. The intensity of these satellite peaks (relative to the main peak) is weaker in CoP than in elemental Co. This implies that the Co atoms have fewer valence electrons in CoP than in elemental Co, that is, they are definitely cationic, notwithstanding the lack of a BE shift. For the other compounds in the MP (M = Cr, Mn, Fe) series, the satellite structure is probably too weak to be observed, but solid solutions Coi -xMxl> and CoAs i yPv do show this feature (vide infra) [60,61]. 

Fig. 31. Molecular structure of 40h in the crystal. This figure was generated using data downloaded from The Cambridge Crystallographic Data Centre (CCDC) and corresponds to a structure originally reported in Ref. (212).

The conformational mobility of a chromophoric main-chain polymer is often connected to its electronic structure. Therefore, changes in the UV-visible absorption spectra and/or chiroptical properties are spectroscopically observable as thermo-, solvato-, piezo-, or electrochromisms. It is widely reported that o-conjugating polysilanes exhibit these phenomena remarkably clearly.34 However, their structural origins were controversial until recently, since limited information was available on the correlation between the conformational properties of the main chain, electronic state, and (chir)optical characteristics. In 1996, we reported that in various polysilanes in tetrahydrofuran (THF) at 30°C, the main-chain peak intensity per silicon repeat unit, e (Si repeat unit)-1 dm3 cm-1, increases exponentially as the viscosity index, a, increases.41 Although conventional viscometric measurements often requires a wide range of low-dispersity molecular-weight polymer samples, a size exclusion chromatography (SEC) machine equipped with a viscometric detector can afford... 

R. Ciriminna, L. M. Ilharco, A. Fidalgo, S. Campestrini and M. Pagliaro, The structural origins of superior performance in sol-gel catalysts, Soft Matter, 2005, 1, 231. 

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