Reaction Relative specificity

In recent years, biochemists have developed an arsenal of reactions that are relatively specific to the side chains of particular amino acids. These reactions can be used to identify functional amino acids at the active sites of enzymes or to label proteins with appropriate reagents for further study. Cysteine residues in proteins, for example, react with one another to form disulfide species and also react with a number of reagents, including maleimides (typically A ethylmaleimide), as shown in Figure 4.11. Cysteines also react effectively... 

Acetic anhydride is the only monocarboxylic acid anhydride that is important in modification reactions. The acetylation of the amino groups of proteins can be made relatively specific if the reaction is done in saturated sodium acetate, since the o-acetyltyrosine derivative is unstable to an excess of acetate ions (Fraenkel-Conrat, 1959). The tyrosine derivative rapidly hydrolyzes in alkaline reaction conditions, even in the absence of added acetate buffer (Uraki et al., 1957 Smyth, 1967). Treatment with hydroxylamine also cleaves any o-acetyltyrosine modifications, forming acetylhydroxamate, which can be followed by its purple complex with Fe3+ at 540 nm... 

The principal advantages of RIA are its sensitivity, specificity and simplicity of operation. There are three main disadvantages. Firstly, the lengthy development periods for new methods often result from the difficulties of producing the specific binding agent. Secondly, cross-reactions with other molecules similar to the analyte can sometimes interfere. Thirdly, poor precision can result unless careful control of experimental conditions is employed to ensure reproducible binding reactions. Relative precisions of 1-3% are typical. 

Isopropylbenzene (A) is alkylated with propylene (P) using HF catalyst. The mono (B), di (C), tri (D) and tetra (E) derivatives are formed. Relative specific rates are given by Rodiguin Rodiguina (Consecutive Chemical Reactions, 1964) for the case of a large excess of propylene which makes the reactions pseudo first order. The relative specific rates used here are kx = 1.0, k2 = 0.5, k3 = 0.3 and k4 0.2. The system of linear differential... 

This is a spectrophotometric assay based on the reaction of diphenylamine with the deoxyribose moiety of DNA to produce a complex that absorbs at 600 nm. The reaction is specific for deoxyribose and RNA does not interfere. It can be used on relatively crude extracts where direct spectrophotometric determinations of DNA concentration are not possible. 

The simplest protonated amine to be studied in detail was metastable CII3NII31, which loses H2 with a large and relatively specific KE release (T = 78 kJmol-1)123. This reaction has a high regioselectivity ( and it was originally classified, together... 

The proteolytic enzymes are classified into endopeptidases and exopeptidases, according to their site of attack in the substrate molecule. The endopeptidases or proteinases cleave peptide bonds inside peptide chains. They recognize and bind to short sections of the substrate s sequence, and then hydrolyze bonds between particular amino acid residues in a relatively specific way (see p. 94). The proteinases are classified according to their reaction mechanism. In serine proteinases, for example (see C), a serine residue in the enzyme is important for catalysis, while in cysteine proteinases, it is a cysteine residue, and so on. 

The use of enzymes in PET chemistry for the introduction of the radioactive label is an attractive strategy because enzymatic reactions are specific, often fast and without side products. Until recently this approach was restricted to carbon-11 chemistry and relatively few examples exist largely because of the lack of available suitable enzymes. 

Chemical Stability. Chemical stability is just as important as the physical stability just discussed. In general, chemical deterioration of the polymers is no problem, and they can be stored at room temperature for years. However, the polymeric surfaces are subjected to an extreme variety of chemicals during the accumulation process. Some of these may react with the polymer. For example, reactions of styrene-divinylbenzene polymers and Tenax with the components of air and stack gases have been documented (336, 344, 540). The uptake of residual chlorine from water solutions has also been observed in my laboratory and elsewhere (110, 271, 287). Although the homogeneous nature of synthetic polymers should tend to reduce the number of these reactions relative to those that occur on heterogeneous surfaces of activated carbons, the chemical reaction possibility is real. In the development of methods for specific chemicals, the polymer stability should always be checked. On occasion, these checks may lead to... 

The influence of these various effects may be manifested in measurable parameters of the reaction like the overall quantum yields (On) and the photoproduct ratios for fragmentation to cyclization (E/C) and for trans to cis cyclobutanol formation (t/c) as shown in Scheme 41. The values of these quantities and their variations as the media are changed can provide comparative information concerning the relative importance of solvent anisotropy on Norrish II reactions, also. Specifically, they reveal characteristics of the activity of the walls and the size, shape, and rigidity of the reaction cavities occupied by electronically excited ketones and their BR intermediates. 

MKTAI.LOPROTEINS. Proteins, especially in solution, readily participate in a greater variety of chemical reactions than any other class of compounds of biological interest. This reactivity is a function primarily of the many polar side chains containing Oil. -COOII. -NHy. -SH. and other groups, all of which can. to vary ing extents, interact with metal ions. Proteins can hind metals, some of them very tightly. However, relatively specific and nonspecific binding should Ik- differentiated. 

Enzyme-catalyzed reactions involve specific, rapid combination of substrate and enzyme to form a complex that is rapidly converted to products through transition states that are controlled by the enzyme s environment. Since enzymes are homogeneous chemical catalysts, we expect them to operate by routes that parallel some of the same processes in reactions that do not involve enzymes. The relative magnitude of enzymic and nonenzymic catalytic parameters has been called catalytic proficiency by Wolfenden6,17 24 and this has been a subject of intense current interest.7,25 32 Wolfenden noted that while nonenzymic reactions have diverse rates, enzyme-catalyzed processes are highly evolved to be comparable in rate, no matter how slow their nonenzymic counterparts. 

When switching from water to an organic solvent, or switching between organic solvents, the substrate specificity can change. In the example of the standard reaction, transesterification of N-acetyl-i-phenylalanine ethyl ester with n-propanol by Subtilisin Carlsberg, which has been mentioned several times in this chapter already, the relative specificity between the rather hydrophobic phenylalanine compound and its more hydrophilic analog N-acetyl-L-serine ethyl ester varies with the solvent (Table 12.8) (Wescott, 1993). 

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