Figure 5.

Comparing tryptophan fluorescence with tryptophan absorbance. The cover slip holding the IRBIT protein crystal in Fig. 3 ▶(b) is now flipped, removed from the tray and directly exposed to BF (a) and UV (b, c) light microscopy. Here, the crystals are confirmed to be protein (i.e. knowing that there is no other aromatic component in the crystallization condition) by tryptophan fluorescence (bright crystals in b) and by tryptophan absorbance (black-colored crystals in c). If the crystals were salt they would appear dark in (b) and white in (c).