I am performing an in vitro in-gel labeling experiment by photo-activating a fluorescent alkyl diazirine probe with UV light in the presence of BSA to determine the required activation parameters for labeling. However, even when I incubate the probe with BSA and do not expose it to light, I am seeing some residual labeling when visualizing the results in the gel even after thoroughly washing the protein to remove unbound probe. I have the same probe but with an aryl azide photoreactive group for which I do not see this background artifact. Does anyone know of evidence that alkyl diazirines label BSA without light activation?