The Chemistry of a Reporter Group: 2-Hydroxy-5-nitrobenzyl Bromide

Abstract

The chemistry of the reaction of 2-hydroxy-5-nitrobenzyl bromide (HNB-Br) with tryptophan ethyl ester to form monosubstitution adducts has been elucidated. The initial compounds are formed by addition at position 3 of the indole ring to form two diastereomeric indolenines. These compounds can then cyclize and rearrange in a number of ways. The absolute stereochemistries of the products have been determined. The 2-hydroxy-5-nitrobenzyl moiety has been shown to form complexes with amino acid esters and the spectral shifts of the complexes can be correlated with the hydrophobicity of the amino acid side chains. The specificity of HNB-Br for tryptophan in proteins can be explained by a combination of a hydrophobic attraction and a specific charge-transfer complex of the quinone methide form of the reagent. The hydrophobic affinity of the HNB group, the specificity of HNB-Br for tryptophan, and the number of potential reaction products of HNB-Br lead to a number of possible modes of reaction of HNB-Br with tryptophan in proteins which enhance the value of the reagent as an environmental probe.