Imaging bacterial peptidoglycan with near-infrared fluorogenic azide probes

Significance Bioorthogonal chemistry has created new opportunities for interrogating biomolecules in cells and organisms. Near-infrared (NIR) fluorogenic probes activated by bioorthogonal chemistry are ideal for imaging studies, as tissue penetrance is highest and background fluorescence from excess probe and endogenous biomolecules is minimized at these wavelengths. Here, we present the development and optimization of NIR fluorogenic azide probes that undergo a fluorescence enhancement of up to 48-fold upon reaction with terminal or strained alkynes. In conjunction with a new class of cyclooctyne d -amino acids, the NIR probes were used to image bacterial peptidoglycan (PG) without the need for cytotoxic copper catalyst or washing away excess probe. This platform should facilitate the in vivo imaging of PG in pathogenic bacteria with bioorthogonal chemistry.