Well, the good news is that they don't seem to be "salt" crystals. But have you really ruled that out? With small molecule crystals the diffraction pattern is so sparse, and at such high resolution, that you are likely to miss it altogether in a one-degree oscillation and looking for spots around the beamstop.
Next time you have one in the beam, put the detector up close and take a 180-degree oscillation (one picture, exposure say 10 sec or however fast the phi-axis drive can spin). Now if there are still no spots you can begin work on extending the diffraction out from behind the beamstop. (Don't forget to set delta-phi back to a reasonable value, or the next shot on a protein crystal is going to be very disappointing!) Ed tat cheung cheng wrote:
Hi all, I have got some crystals, the purified protein was in Tris buffer with 300mM NaCl for crystallization. they grew in light weight PEG, PEG400 or monomethyl ethyl PEG500, they were needle shaped, could be long (~0.2mm) but very thin all the time and sometimes grew into sea-urchin like needle cluster. What interesting is, when i gridded crystallization conditions against pH or PEG amount, the crystals sizes and shapes varied, and the crystals were fragile so i believed they were protein crystals in nature. But upon X-ray diffraction, they gave no reflection at all, not even a faint spot. I wonder, beside silly mistakes like misalignment of the crystal to the beam, not enough exposure time, what could be the reason for this mysterious crystals? Are they protein or PEG or what? Thanks very much. Tc
