Fluorescence from protein crystals may also ®nd applications
in the ®eld of instrumentation. For example, ¯uorescence
can greatly facilitate the localization of a crystal mounted in a
large cryo-loop, and therefore its centring on an X-ray goniometer.
We noticed that a low level of UV light at 355 nm was
generally suf®cient to excite residual ¯uorescence from
aromatic amino acids, resulting in an enhanced contrast
between the crystal and the mother liquor.