N-ethyl-5-phenylisoxazolium-3′-sulfonate (Woodward's reagent K) as a reagent for nucleophilic side chains of proteins

Llamas K., Owens M., Blakeley R.L. and Zerner B. (1986) N-ethyl-5-phenylisoxazolium-3′-sulfonate (Woodward's reagent K) as a reagent for nucleophilic side chains of proteins. Journal of the American Chemical Society, 108 18: 5543-5548.

Author Llamas K.
Owens M.
Blakeley R.L.
Zerner B.
Title N-ethyl-5-phenylisoxazolium-3′-sulfonate (Woodward's reagent K) as a reagent for nucleophilic side chains of proteins
Journal name Journal of the American Chemical Society   Check publisher's open access policy
ISSN 0002-7863
Publication date 1986
Sub-type Article (original research)
Volume 108
Issue 18
Start page 5543
End page 5548
Total pages 6
Subject 1600 Chemistry
Abstract N-Ethyl-5-phenylisoxazolium-3′-sulfonate (1) is converted to the corresponding keto ketenimine [ArC(O)CH=C=NEt, 2] in a hydroxide-promoted reaction with k- OH = 6.64 × 10 5 M -1 s -1 at 25°C. The rate law for spontaneous hydration of 2 to form N-ethyl-3-oxo-3-(3-sulfonatophenyl)propanamide is k sp obsd = k H+[H +] + k w + k -OH[ -OH], where k H+ = 442 M -1 s -1 k w = 0.0116 s -1, and k- OH = 254 M -1 s -1 at 25°C (μ = 0.1). In acetate buffers, k obsd = k sp obsd + k AcO[AcO -] + k AcOH[AcOH], where k AcO- = 0.245 M -1 s -1 and k AcOH = 2.33 M -1 s -1. Both catalytic terms lead to the enol ester [3-acetyloxy-N-ethyl-3-(3-sulfonatophenyl)propenamide], and it is likely that the general acid catalysis term corresponds to specific acid catalysis of the attack of acetate ion on 2. Imidazole, N-methylamine, the phenoxide ion of N-acetyl-L-tyrosinamide, and 2-hydroxyethanethiolate react nucleophilically with 2 with rate constants of 4.70, 125, 82, and 57 600 M -1 s -1, respectively, at 25°C to form relatively stable enamine adducts with absorption maxima at 348.5 nm (ε = 23 200 M -1 cm -1), 319 nm (ε = 16990 M -1 cm -1) 329 nm (ε = 19200 M -1 cm -1), and 349 nm (ε = 24 200 M -1 cm -1), respectively. The results establish that 1 may be used as a spectrophotometric probe for imidazole, lysine, cysteine, and tyrosine residues under appropriate conditions. The selectivity of 1 for aspartate and glutamate residues may be maximized by adding it as an acetonitrile solution of the keto ketenimine 2, to an enzyme solution maintained at low pH for the short time required for reaction.
Q-Index Code C1
Q-Index Status Provisional Code
Institutional Status Unknown

Document type: Journal Article
Sub-type: Article (original research)
Collection: Scopus Import
 
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