Local chemical information

Count descriptors give local chemical information, are insensitive to isomers and to conformational changes and show a high level of degeneracy. However, due to their immediate availability, they are among the most used descriptors. 

For many years neuropharmacologists, neurochemists, and behavioral psychologists have been obtaining qualitative and quantitative localized chemical information from the brains of test animals in a rather unsatisfying manner. Following a controlled behavioral or chemical manipulation the animal is sacrificed, usually via cervical dislocation, decapitated, and its brain... 

Because there are more ex situ characterization techniques than this chapter could cover, here we only describe briefly the principles of some selected ex situ techniques. These characterization tools could provide average or collective chemical information, local chemical information with spatial resolution at micrometer and submicrometer ranges, and local structural information at nanometer and atomic scales for heterogeneous catalysts. Examples are selected that demonstrate the performances of these techniques, with a special focus on their applications in characterizing nanocatalysts for energy production and energy conversion. 

The hybrid technique of SECM-SICM, first reported concurrently by two separate research groups, combines the capability of SICM to attain topographical images of samples within biologically relevant media with the ability of SECM to measure localized electrochemistry. Like SICM, the ion current flow between bulk solution and a nanopipette is utilized in this system as a feedback control for the probe position so that a topographical image of the sample can be obtained. Simultaneously, an independent electrode on the probe measures the faradaic current to provide localized chemical information of redox-active species. This hybrid SPM technique has been utilized to measure the transport of redox probes through polymer nanoporous membranes, immobilized enzymes, and live cells. ... 

Turning from chemical exchange to nuclear relaxation time measurements, the field of NMR offers many good examples of chemical information from T, measurements. Recall from Fig. 4-7 that Ti is reciprocally related to Tc, the correlation time, for high-frequency relaxation modes. For small- to medium-size molecules in the liquid phase, T, lies to the left side of the minimum in Fig. 4-7. A larger value of T, is, therefore, associated with a smaller Tc, hence, with a more rapid rate of molecular motion. It is possible to measure Ti for individual carbon atoms in a molecule, and such results provide detailed information on the local motion of atoms or groups of atoms. Levy and Nelson " have reviewed these observations. A few examples are shown here. T, values (in seconds) are noted for individual carbon atoms. 

Microscopy methods based on nonlinear optical phenomena that provide chemical information are a recent development. Infrared snm-frequency microscopy has been demonstrated for LB films of arachidic acid, allowing for surface-specific imaging of the lateral distribution of a selected vibrational mode, the asymmetric methyl stretch [60]. The method is sensitive to the snrface distribntion of the functional gronp as well as to lateral variations in the gronp environmental and conformation. Second-harmonic generation (SHG) microscopy has also been demonstrated for both spread monolayers and LB films of dye molecules [61,62]. The method images the molecular density and orientation field with optical resolution, and local qnantitative information can be extracted. 

A major deficiency in scanning tunnelling microscopy is the absence of direct chemical information, but the dependence of the tunnelling current on the local... 

Chemical Information, Irvine CA Tripos, Inc. St. Louis MO), similarity searching can be carried out around a well-defined compound class using local descriptors such as atom pairs [46, 47] or topomeric shape [48, 49]. Also, ligand-based pharmacophore searches are able to identify follow-up compounds that are less obvious and more diverse than similarity searches [30, 50-54]. The problem with the latter methods is defining the molecular shape or pharmacophore specifically enough to be useful when there are few hits within a compound class and they cannot be reliably aligned (as is often the case for NMR hits in the absence of detailed structural information). 

A third way to gain some knowledge about the concentrations of chemicals in the environment involves some type of modeling. Scientists have had, for example, fair success in estimating the concentrations of chemicals in the air in the vicinity of facilities that emit those chemicals. Information on the amount of chemical emitted per unit time can be inserted into various mathematical models that have been designed to represent the physical phenomena governing dispersion of the chemical from its source. Certain properties of the chemical and of the atmosphere it enters, together with data on local weather conditions, are combined in these models to yield desired estimates of chemical concentrations at various distances from the source. These models can be calibrated with actual measurement data for a few chemicals, and then used for others where measurement data are not available. 

Bergstrom, C.A.S., Wassvik, C.M., Norinder, U., Luthman, K. and Artursson, P. (2004) Global and local computational models for aqueous solubility Prediction of drug-like molecules. Journal of Chemical Information and Computer Sciences, 44, 1477-1488. 

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