Structure, chemically modified

Until recently, the catalytic role of Asp ° in trypsin and the other serine proteases had been surmised on the basis of its proximity to His in structures obtained from X-ray diffraction studies, but it had never been demonstrated with certainty in physical or chemical studies. As can be seen in Figure 16.17, Asp ° is buried at the active site and is normally inaccessible to chemical modifying reagents. In 1987, however, Charles Craik, William Rutter, and their colleagues used site-directed mutagenesis (see Chapter 13) to prepare a mutant trypsin with an asparagine in place of Asp °. This mutant trypsin possessed a hydrolytic activity with ester substrates only 1/10,000 that of native trypsin, demonstrating that Asp ° is indeed essential for catalysis and that its ability to immobilize and orient His is crucial to the function of the catalytic triad. 

Some porous ceramic structures of oxides on titanium (CT2O3, RuOj, MnOj, VOJ obtained by baking films of metal complexes like acetylacetonates on titanium surfaces can also be regarded as chemically modified electrodes Applications... 

Barhanin et al. (26) chemically modified the side chains of several residues to correlate structure and function in As II. Their results established the importance of charged residues for the function of the toxin. They showed that Arg-13 is essential for binding to the sodium channels as well as for toxicity, while the aspartate, glutamate, and lysine residues in the N-terminal segment of the protein are... 

Coordination of reactive and/or unstable molecules to metal centers is a useful approach for their stabilization,1 and it presents unique opportunities for their characterization by spectroscopic methods and for elucidation of their structure. Moreover, under appropriate conditions the coordinated species can be chemically modified. In addition, displacement of the coordinated compound from the metal and its trapping in solution by reactions with suitable substrates can form the basis for useful synthetic methodology. 

The indicated formal potential E° n of the corresponding monomer (-1.17V) in solution is very near that of the surface film (-1.13V vs. SSCE). That formal potentials of surface films on chemically modified electrodes are near those of their corresponding dissolved monomers (13,18) is actually a common, and quite useful, observation. In the present case, it demonstrates that the electronic structures of the porphyrin rings embedded in the polymer film are not seriously perturbed from that of the monomer. 

The silver(I) complexes with the tetrakis(methylthio)tetrathiafulvalene ligand have been reported, the nitrate salt presents a 3D structure with an unprecedented 4.16-net porous inorganic layer of silver nitrate,1160 the triflate salt presents a two interwoven polymeric chain structure.1161 The latter behaves as a semiconductor when doped with iodine. With a similar ligand, 2,5-bis-(5,5,-bis(methylthio)-l,3,-dithiol-2 -ylidene)-l,3,4,6-tetrathiapentalene, a 3D supramolecular network is constructed via coordination bonds and S"-S contacts. The iodine-doped compound is highly conductive.1162 (Methylthio)methyl-substituted calix[4]arenes have been used as silver-selective chemically modified field effect transistors and as potential extractants for Ag1.1163,1164... 

In Situ Surface Extended X-ray Absorption Fine Structure at Chemically Modified Electrodes... 

Pickles had proposed that rubber was composed of covalently bound chains of isoprene, and that variations in the chains accounted for differences in the properties of rubbers (40). Pickles was the first to assign a chain structure of rubber on the basis of the properties of the chemically modified material. He noted that saturation of the double bond with bromine did not destroy the "colloidal nature" of the material. In a remarkably accurate proposition of structure, he made but one error. He assumed that the chain ends combined to form a ring of eight isoprene units. As we shall see later, he was not alone in this ascertation. 

Staudinger, like Pickles in 1910, chemically modified rubber and noted its failure to lose colloidal properties as evidence of chain structure (49). His experimental proof was impressive for he had catalytically hydrogenated natural rubber and then thoroughly studied the properties of the saturated product. He reasoned that the disappearance of the double bonds of natural rubber should result in a loss of "residual valence", and failure to do so was conclusive. 

Gebhard and coworkers42 reported a synthesis and spectroscopy of chemically modified spheroidenes. The structure and numbering of the system is shown in 66. The syntheses and spectroscopic properties of the all-E isomers of 1112 dihydrospheroidene (67),... 

Figure 6.6. Structures of chemically modified fluorescein derivatives with long-wavelength emission (I) and with novel energy transfer acceptor characteristics (II).

Gas phase approaches have the advantage that the nanocarbons do not need to be filtered or washed after hybridization making them ideal for nanocarbons produced on substrates such as CVD grown graphene films or CNT forests which tend to lose their structure upon immersion and/or drying. Consequently they are not ideal for chemically modified GO or CNTs. 

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