Bioconjugatable azo-based dark-quencher dyes: synthesis and application to protease-activatable far-red fluorescent probes

We describe the efficient synthesis and one-step derivatization of novel, nonfluorescent azo dyes based on the Black Hole Quencher-3 (BHQ-3) scaffold. These dyes were equipped with various reactive and/or bioconjugatable groups (azido, α-iodoacetyl, ketone, terminal alkyne, vicinal diol). The azido derivative was found to be highly reactive in the context of copper-catalyzed azide-alkyne cycloaddition (CuAAC) reactions and allowed easy synthetic access to the first water-soluble (sulfonated derivative) and aldehyde-modified BHQ-3 dyes, the direct preparation of which failed by means of conventional azo-coupling reactions. The aldehyde- and α-iodoacetyl-containing fluorescence quenchers were readily conjugated to aminooxy- and cysteine-containing peptides by the formation of a stable oxime or thioether linkage, respectively. Further fluorescent labeling of the resultant peptide conjugates with red- or far-red-emitting rhodamine or cyanine dyes through sequential and/or one-pot bioconjugations, led to novel Förster resonance energy transfer (FRET) based probes suitable for the in vivo detection and imaging of urokinase plasminogen activator, a key protease in Cancer invasion and metastasis.