Azobenzenes are of particular interest as a photochromic scaffold for biological applications because of their high fatigue resistance, their large geometrical change between extended (trans) and bent (cis) isomer, and their diverse synthetic accessibility. Despite their wide-spread use, there is no reported photochromic inhibitor of the well-investigated enzyme -galactosidase, which plays an important role for biochemistry and single molecule studies. Herein, we report the synthesis of photochromic competitive -galactosidase inhibitors based on the molecular structure of 2-phenylethyl -d-thiogalactoside (PETG) and 1-amino-1-deoxy--d-galactose (-d-galactosylamine). The thermally highly stable PETG-based azobenzenes show excellent photochromic properties in polar solvents and moderate to high photostationary states (PSS). The optimized compound 37 is a strong competitive inhibitior of -galactosidase from Escherichia coli and its inhibition constant (K-i) changes between 60 nM and 290 nM upon irradiation with light. Additional docking experiments supported the observed structure-activity relationship
