Retrieval chemical

The world wide facilities for retrieving chemical information provided by the Chemical Abstract Service (CAS on-line) have made it a relatively easy matter to obtain CAS registry numbers of substances, and most of the numbers in this monograph were obtained via CAS on-line. We should point out that two other available useful files are CSCHEM and CSCORP which provide, respectively, information on chemicals (and chemical products) and addresses and telephone numbers of the main branch offices of chemical suppliers. 

O Leary TJ, Fowler CB, Evers DL, et al. Protein fixation and antigen retrieval chemical studies. Biotech. Histochem. 2009 84 217-221. 

Figure 7.5. Search enzyme information at Brenda. Brenda is the comprehensive enzyme database for retrieving chemical, kinetic, and structural properties of enzymes via EC number, enzyme name, and organism (biological source). The search page by EC number is shown.

The classical idea of molecular structure gained its entry into quantum theory on the basis of the Born Oppenheimer approximation, albeit not as a non-classical concept. The B-0 assumption makes a clear distinction between the mechanical behaviour of atomic nuclei and electrons, which obeys quantum laws only for the latter. Any attempt to retrieve chemical structure quantum-mechanically must therefore be based on the analysis of electron charge density. This procedure is supported by crystallographic theory and the assumption that X-rays are scattered on electrons. Extended to the scattering of neutrons it can finally be shown that the atomic distribution in crystalline solids is identical with molecular structures defined by X-ray diffraction. 

A number of two-dimensional NMR experiments have been introduced in high-resolution solid-state NMR studies, designed, for example, to investigate chemical exchange processes [64], to retrieve chemical shift anisotropies [65] and dipolar couplings [66], and to probe spin-diffusion proces.ses 67j. Opella has proposed an intemuclear distance-determined, spin-diffusion mechanism in molecular crystals [68], and Benn and coworkers have demonstrated C/ C connectivities using the INADEQUATE sequence for the plastic crystal camphor and have used the COSY sequence for Si/ Si connectivities in the reference molecule Q Ms [691. 

A number of 2D NMR experiments have been introduced in high-resolu-tion solid-state NMR studies, for example, to investigate chemical exchange processes (20), retrieve chemical shift anisotropies (21) and dipolar couplings (22) and to probe spin-diffusion processes (23). 

The fuzzy name match is a browse function in which the system locates and retrieves chemical substance names that directly match or are similar to an input name. The match is accomplished by an initial search using generic name search keys to obtain a set of candidate names. A similarity metric based on trigram character strings is then calculated between input name and each candidate name and if a pre-set metric threshold is exceeded the candidate names/substances are retrieved in rank order relative to the input name. Two generic keys are used. One key rectifies differences in name format (inverted vs. uninverted), locants, stereo, alphabetical order of substituents, and some spelling differences. The other key has the same capabilities and, in addition, applies vocabulary control via special dictionaries. For example, tosyT is mapped to the more systematic methylphenylsulfonyl . 

In the final part the application of concepts from information theory is reviewed. After covering the necessary theoretical background a particular form of the Kullback-Liebler information measure is adopted and employed to define a functional for the investigation of density functions throughout Mendeleev s Table. The evaluation of the constructed functional reveals clear periodic patterns, which are even further improved when the shape function is employed instead of the density functions. These results clearly demonstrate that it is possible to retrieve chemically interesting information from the density function. Moreover the results indicate that the shape function further simplifies the density function without loosing essential information. The latter point of view is extensively treated in [64], where the authors elaborately discuss information carriers such as the wave function, the reduced density matrix, the electron density function and the shape function. 

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