Rapid, hydrolytically stable modification of aldehyde-terminated proteins and phage libraries

We describe the rapid reaction of 2-amino benzamidoxime (ABAO) derivatives with aldehydes in water. The ABAO combines an aniline moiety for iminium-based activation of the aldehyde and a nucleophilic group (Nu:) ortho to the amine for intramolecular ring closure. The reaction between ABAO and aldehydes is kinetically similar to oxime formations performed under stoichiometric aniline catalysis. We characterized the reaction by both NMR and UV spectroscopy and determined that the rate-determining step of the process is formation of a Schiff base, which is followed by rapid intramolecular ring closure. The relationship between apparent rate constant and pH suggests that a protonated benzamidoxime acts as an internal general acid in Schiff-base formation. The reaction is accelerated by substituents in the aromatic ring that increase the basicity of the aromatic amine. The rate of up to 40 M(-1) s(-1) between an electron-rich aldehyde and 5-methoxy-ABAO (PMA), which was observed at pH 4.5, places this reaction among the fastest known bio-orthogonal reactions. Reaction between M13 phage-displayed library of peptides terminated with an aldehyde moiety and 1 mM biotin-ABAO derivative reaches completion in 1 h at pH 4.5. Finally, the product of reaction, dihydroquinazoline derivative, shows fluorescence at 490 nm suggesting a possibility of developing fluorogenic aldehyde-reactive probes based on ABAO framework.