Extrinsic reagent

The catalytic effect of enzymes is based in part on their capacity to endow the substrates with highly increased chemical reactivity. This superreactivity of enzyme-activated substrates manifests itself in the increased rate of the chemical transformation of the specific substrate into the specific product it may, however, also comprise enhanced reactivity toward extrinsic reagents that are not constituents of the normal enzyme-substrate system. Enzyme-substrate intermediates may thus react with extrinsic reagents and branch off the normal catalytic pathway. The term paracatalytic enzyme reaction is suggested to describe these reactions that, except in the first stages of their pathway, do not correspond with the normal catalytic specificity of the enzyme. 

Paracatalytic enzyme modification is a new type of catalysis-linked and, hence, substrate-dependent enzyme modification. In all instances in which the substrate promotes inactivation of an enzyme by a chemical reagent, particularly by an oxidant, paracatalsrtic modification should he considered to be the underlying mechanism. In contrast to ligand-induced and syncatalytic modifications, paracatalytic modification involves a direct chemical interaction between enzyme-activated substrate and extrinsic reagent. In this respect, it is similar to the chemical trapping of covalent enzyme-substrate intermediates, e.g., the reduction of enzyme-substrate Schiff bases by sodium borohydride in class I fructose-l,6-bis-phosphate aldolases - or in acetoacetate decarboxylase. ... 

Therefore, other factors that have not yet been studied and are not easily quantifiable, such as the absorption properties of the C.-T. adduct at the surface of the metal powder and the solubility of the formed species should be important in determining the oxidation properties of C.-T. adducts towards metal powders. Furthermore, some extrinsic factors inherent to the experimental conditions, such as reaction temperature, reagent concentration, and nature of the solvent have been reported to affect the overall yield or the course of the reaction, and led to separation of different products in some cases.55 59 In any case, it appears that the simultaneous presence of the donor molecule and the di-/inter-halogen lowers the oxidation potentials of the metals, allowing their oxidation, dissolution, and complexation. 

Extrinsic fluors are produced via a chemical reaction where the added reagent either enhances emission of a weak emitter through association or the analyte is derivatized with a fluor tag. 8-Hydroxyquinoline (HQS) is an example of an extrinsic complexing reagent (Reaction 11.1) where the native ligand is a marginal fluorophore but forms intense emitting metal chelates. This approach affords sensitive detection of... 

Extrinsic fluorescence is used whenever the natural fluorescence of a macromolecule is inadequate for accurate fluorescence measurement. In this case, one can attach a fluorescent reporter group by using the reactive isocyanate or isothiocyanate derivatives of fluorescein or rhodamine, two intensely fluorescent molecules. One can covalently also label a protein s a- and e-amino groups with dansyl chloride (/.e., A,A-dimethylaminonaphtha-lenesulfonyl chloride). Another useful reagent is 8-ani-lino-l-naphthalenesulfonic acid (abbreviated ANS). This compound is bound noncovalently by hydrophobic interactions in aqueous solutions, ANS is only very fluorescent, but upon binding within an apolar environment, the quantum yield of ANS becomes about 100 times greater. 

Fig. 9 Four waveguide-based sensors. Left Extrinsic sensors a in direct analyte measurements or c in indirect measurements by means of indicators or other reagents immobilized in membranes. Right Intrinsic sensors for measurements of changes in the light guided though the waveguide by b the adsorption of the analyte into its surface or by d the interaction of the analyte with a recognition phase, which is in direct contact with the waveguide...

In addition, the reagent and reaction conditions are sometimes critical and may have an extrinsic and secondary effect on the course of ring closure. 

Extrinsic. Many of the traditional bioassays are based on optical measurements, but these are solution assays without immobilised reagents. Optical biosensors are concerned with interrogation of reactions at surfaces, but often the reagents are the same the optical fibres acting, in the extrinsic mode as light guides to transmit information to and from a remote target reaction at its terminus. 

The authors further found that y-subunit rotation was blocked when the Fq complex was modified by dicyclohexylcarbodiimide (DCCD), the lipid-soluble carboxyl reagent that is known to inhibit proton transport by reacting with a carboxylate residue on one of the c-subunits in Fq (Glu in MFo Asp in EcFq). These results demonstrate that the reconstituted EcFi can rebind to Fq to form a functional, membrane-bound EcFo F complex and that y-subunit rotation in F, is functionally coupled to Fq. This is also consistent with the long-held notion of a long-range conformational interaction between Fq within the membrane and the catalytic nucleotide-binding site on the extrinsic F complex. 

Direct physical contact between the sensor reagent or transducer and the fiber is not a requirement. The indicator can also be in a sample that is viewed through a window the optical fiber, such as in a flow-injection-analysis sensing scheme used for process control ( ). Although such applications will be an Important part of optical sensor technology, the more demanding approach is the preparation of extrinsic sensors with the reagent phase attached directly to the fiber tip, a requirement for in vivo and in situ applications. The inventive work of sensor chemists now focuses... 

Safety, health concerns (represented in this context as toxicity), and environmental impact are a third class of criteria in assessing the suitability of a reagent selected on the basis of the above chemical criteria. These extrinsic criteria are inextricably linked with the laboratory equipment and the infrastructure of the place of work. This may mean that a specific reagent can be used without any problem in labo-... 

Specifically designed sustainable phosgene substitutes for specific applications will be increasingly used (like the already used BOP, BDDC, CDC, and Burgess reagent see Chapter 4) to perform selective reactions that comply with the intrinsic yield, reactivity, handling, work-up) and extrinsic (safety, toxicity, environmental... 

Extrinsic fiber-optic chemical sensors are constructed by immobilizing indicator chemistries on the fiber tip or on the annulus of the fiber. In this section, the mechanism of the chemical and biological reagents employed for sensing are described as well as methods for immobilizing sensing materials. 

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