Koshland reagent

One example of a reactive ortho-hydroxybenzyl derivatives is the Koshland reagent I (Fig. 11.1).11 13 Its hydrolysis half-life in water (pH 3.5 and 25 °C) is... 

Tryptophan cleavage with NBS can be blocked by reaction of the tryptophan peptide or protein with 2-hydroxy-5-nitrobenzyl bromide the Koshland reagent (see Section III.4.) or by ozonolysis of tryptophan residues to formylkynurenine (313, 419). These modification reactions of the indole nucleus restrict NBS cleavage to tyrosyl and histidyl peptide bonds. 

Several studies of proteins have been made on the assumption that specific reactions occur between the reagents used and the tryptophan residues in proteins. This ideal condition is rarely fulfilled. It is worth recalling that, although extensively used for tryptophan modification studies, NBS is by no means selective for the indole nucleus of tryptophan and also the final modification products cannot be defined unambiguously (438). A similar situation is also presented by the Koshland reagent, which was shown to give, upon reaction with model tryptophanyl compounds, up to ten different products (168, 236, 253, 378). 

Horton, H. R. Koshland, D. E. Jr. A highly reactive coloured reagent with selectivity for the tryptophan residue in proteins 2-hydroxy-5-nitrobenzyl bromide. J. Amer. Chem. Soc. 1965, 87, 1126-1132. 

Carraway, K.L., and Koshland Jr, D.E (1968) Reaction of tyrosine residues in proteins with carbodiimide reagents. Biochim. Biophys. Acta, 160, 272-274. 

Koshland devised the following reagent for the indole rings of tryptophan residues (Eq. 3-46). 

H.R. Horton, H. Kelly and D.E. Koshland, Environmentally sensitive protein reagents 2-methoxy-5-nitrobenzyl bromide, J. Biol. Chem. 240 (1965) 722-724. 

Figure 7. Modifications of indole groups (a) HNBB or Koshland s reagent (b) photooxidation (c) 4-nitrophenylsulfenyl chloride.

One common way to modify tryptophan residues occurs through the addition of 2-hydroxy-5-nitrobenzyl bromide (HNB). Also known as Koshland s reagent (68), this compound first alkylates the 3-position of the ring and eventually leads to the formation of a product mixture through subsequent intramolecular cyclization pathways. Fig. 6c. This reagent exhibits good tryptophan selectivity, although some levels of... 

Koshland DE, Karkhanis YD, Latham HG. Environmentally- 86. sensitive reagent with selectivity for tryptophan residue in proteins. J. Am. Chem. Soc. 1964 86 1448-1450. 

These derivatives of methionine (isomers) may be obtained by mild oxidation of proteins or peptides with reagents like H2O2 under controlled conditions (see also 3.5). They are also found in peptides and proteins (particularly if denatured) which have been exposed to air for prolonged periods. Since the usual conditions of acid hydrolysis convert much of the sulfoxide to methionine (see Ray and Koshland 1960), the extent of methionine oxidation may not be recognized, unless other analytical procedures are utilized (see below). In our own laboratory we have studied peptides in which the methionine sulfoxides are recovered from acid hydrolysates in 5-30% yield, and the combined recoveries of methionine and methionine sulfoxides are usually in the range of 85-95 %... 

Hydroxy-5-nitrobenzyl bromide (Koshland s reagent) 8.0. Crystallise the bromide from benzene or benzene/ligroin. C6H6 and AcOH, and very soluble in ligroin. [Beilstein 6 H 367.]... 

Let us start our analysis with consideration of the first factor, i.e., with the entropic explanation of enzyme activity. This approach has been used by Koshland to describe those kinds of processes that, when drawing together substrate molecules, ions and substrates, cofactors and substrates, etc., are the obligatory intermediary steps determining the overall reaction rate [7,8]. In the frame of the transition state theory, the configuration of the activated complex, that corresponds to the lowest potential barrier, is reached with the precise position and mutual orientation of the reactants. This means that such ordering of the reagents, in the course of an activated complex formation, would be accompanied by the initial entropy decrease, i.e., by the positive contribution to the activation entropy, S, of a model system. 

Bromo-H-(2-hydroxy-5-nitrophenyl)acetamide, 9CI. 2-Bromo-2 -hydroxy-5 -nitroacetanilide. Koshland III reagent. Koshland s reagent III [3947-58-8]... 

Me ether [3913-23-3]. 2- Bromomethy[)- J-methoxy-4-nitrobenzene, 9CI. 2-Methoxy-5-nitrobenzyl bromide. 2-Bromomethyl-4-nitroanisole. MNBB. Koshland s reagent II... 

Koshland and coworkers (25, 193, 79 ) introduced 2-hydroxy-5-nitro-benzyl bromide (HNB-Br) (116) for the modification of tryptophan residues in proteins. The reagent has been found to possess striking selectivity in modifying tryptophan residues at acid pH, although the... 

Casnati and coworkers (69, 70) found that y, y-dimethylallyl bromide reacts with N-acetyl-tryptamine similarly to the Koshland s reagent. Two compounds were isolated and their structure (125) and (126) were determined (70). 

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