Chemical probe method

R267 C. Brunei and P. Romby, Probing RNA Structure and RNA-Ligand Complexes with Chemical Probes , Methods EnzymoL, 2000, 318, 3... 

A chemical probe method originates from experiments with simple model compounds which indicate that a particular reagent may act in... 

NMR methods can be applied to give quantitative determination of initiator-derived and other end groups and provide a wealth of information on the polymerization process. They provide a chemical probe of the detailed initiation mechanism and a greater understanding of polymer properties. The main advantage of NMR methods over alternative techniques for initiator residue detection is that NMR signals (in particular nC NMR) are extremely sensitive to the structural environment of the initiator residue. This means that functionality formed by tail addition, head addition, transfer to initiator or primary radical termination, and various initiator-derived byproducts can be distinguished. 

Probe methods like particle insertion and test particle methods (29-32) are quite useful for computing chemical potentials of constituent particles in systems with low densities. Test particles are randomly inserted the average Boltzmann factor of the insertion energy yields the free energy. For dense systems these methods work poorly because of the poor statistics obtained. 

Scanning Probe Methods The main problem with scanning probe methods is the lack of chemical information. Methods have to be devised to get chemical information simnltaneonsly with the images. 

Unfortunately, we do not have microinterface voltmeters, and we can use only indirect methods like various chemical probes and electrokinetic methods. These methods make it possible to estimate only part of the Galvani potential, denoted here Aj[Pg.35]

Despite the enormous impact that scanning probe methods have had on our understanding of reactions at oxide surfaces, both STM and AFM suffer from the lack of chemical specificity. The application of STM-inelastic electron tunneling spectroscopy is a potential solution as it can be used to measure the vibrational spectrum of individual molecules at the surface [69, 70]. 

Fluorescent probes containing sulfhydryl-reactive groups can be coupled to DNA molecules containing thiol modification sites. The chemical derivatization methods outlined in Section 2.2 (this chapter) may be used to thiolate the oligo for subsequent modification with a fluorophore. Appropriate fluorescent compounds and their reaction conditions may be found in Chapter 9. The protocol discussed in the previous section can be used as a general guide for labeling DNA molecules. 

Access to modified nucleosides. Base-modified nucleosides and nucleotides are very important for their biological properties. They can be found as antiviral agents (HBH, VZV, AIDS),93 95 in the study of DNA degradation,96,97 as fluorescent agents and as chemical probes of DNA structure.98 101 The access to nucleosides can be achieved by different methods ... 

Chemical probes such as titrations using Hammett indicators [188,189] and test reactions [190] have been often employed as well. Given that each method has its own strengths and limitations, a rational combination of specific techniques is often the best approach to the study of a given catalytic system. 

The structure of a reacting molecule can be used as the chemical probe for the reaction mechanism in several ways. Ample experience is available with these methods from the research of noncatalytic homogeneous reactions, and their possibilities and limitations are well known. However, the solid catalyst restricts the scope to some extent on the one hand, but opens new applications on the other. For this reason, the methods of physical organic and inorganic chemistry developed for noncatalytic reactions cannot simply be transferred into the field of heterogeneous catalysis. The following remarks should identify some of the problems. 

Identification by Methods of Chemical Probes 4.3.6.1 Initiation of Polymerization of Vinyl Additives... 

By combining with a time-of-flight atom-probe method, FIM is capable of providing information about the chemical identity and even the isotope number of the surface atoms, when it is evaporated from the surface (Tsong, 1990). In this application, FIM is unique. 

Clearly there must be a price to pay for determining rate constants by a method where all the participants (reagents and products) are invisible to the technique. The reaction of Figure 18.8 provides an excellent example. In principle, 1,7-octadiene can react with tert-butoxyl radicals by all the mechanisms of Scheme 18.5. The probe method cannot distinguish them, the rate constant obtained (2.3 X 10 A/ s ) includes all possible modes and sites of reaction. It is simply the rate constant with which 1,7-octadiene removes tert-butoxyl from the system. In this case the first reaction in Scheme 18.5 accounts for the reactivity of tert-butoxyl, perhaps with a minor contribution from the second reaction. In all cases, the site and form of reaction must be known independently. Chemical intuition, product... 

The GRID program [44] has been used by a number of workers as an alternative to the original CoMFA method for calculating interaction fields. An advantage of the GRID approach, apart from the large number of chemical probes... 

Samples were characterized by X-ray diffraction, magnetic susceptibility and chemical analysis with some results summarized in Table 1. The electrical resistivity measurements were made down to 80 K using a four-probe method. Raman scattering experiments used the excitation line A = 514.5 nm of an Ar+ laser in a quasi-backscattering geometry. The laser power of 5 mW was focused to a 0.1 mm diameter spot on the (010) surface. The averaged laser power density amounts to 6 105 W/m2 which is much less compared to earlier Raman studies in manganites [12-15],... 

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