Alkyl diazirines are photo-reactive functional groups frequently used in photo-affinity labeling (PAL). They are compact in size, being nearly isosteric to a methyl group, and they are activated at a wavelength of light (~355 nm) that is not damaging to protein(s). Photo-crosslinked protein targets are then visualized by the reporter group (e.g. fluorophore, biotin, or radioactive label). Covalent bond formation between the probe and targets enable the subsequent purification and identification of the targets using techniques such as SDS-PAGE, immunoprecipitation, biotin-streptavidin affinity purification and mass spectrometry. Alkyl diazirines are photo-stable under typical laboratory lighting conditions so there is no need to perform experiments in the dark.