How to Prepare Heavy Atom Derivatives with Dimethyl Mercury and Tetraethyl Lead
Duilio Cascio and Dan Anderson
This technique was first used by Ward Smith at UCLA in the early 80s with RuBisCo. These are some of the first heavy atoms we try at UCLA. The success rate has been very high.
DANGER: DIMETHYL MERCURY HAS NOW CAUSED THE DEATH OF A LABORATORY SCIENTIST AT DARTMOUTH, AND ITS USE SHOULD BE DISCONTINUED AS A PROTEIN HEAVY ATOM DERIVATIVE
DMM or Dimethyl Mercury ( (CH3)2 Hg ) and TEL or Tetraethyl Lead ( (C2H5)4 Pb ) are liquids at room temperature and not miscible with water, but their vapors can diffuse into protein crystals and produce useful derivatives. Both compounds are extremely toxic by inhalation and skin contact. They are fat soluble, and therefore absorption is probably permanent. Please use extreme caution in handling these compounds and in disposing of contaminated objects and crystals after the experiment is done. DMM will permeate in few seconds through disposable latex gloves. A highly resistant laminate glove (Silver Shield or 4H) should be worn under a pair of long cuffed, unsupported neoprene, nitrile, or similar heavy-duty gloves. See report on DMM accident.
- Here at UCLA, the management has already purchased DMM and TEL from ALDRICH (catalog numbers 32,808-1 ($120.00) and 40,269-9 ($47.50)). Please, labmates, don’t duplicate our efforts; this is nasty stuff to store.
- At UCLA, the management does this: In a fume hood break open an ampule of the liquid and transfer the whole contents to several previously labeled 1 ml REACTI-VIAL vials from PIERCE (catalog 13221Z; $69 for 12 vials; the cap liner must have a Teflon face to contain these materials). The transfer is safest with a positive displacement pipet (Gilson Microman M250). DO NOT use an air displacement pipet (Pasteur or Gilson Pipetman). Close each vial with the special cap provided. Wrap the cap with a strip of Parafilm. Each vial is stored inside a 50 ml polypropylene centrifuge tube with a plug seal screw cap. Cover the cap with Parafilm. Store each individual centrifuge tube inside a larger container (glass jar or metal) sealed with parafilm and label it EXTREMELY TOXIC.
These compounds should be stored in a freezer to minimize their vapor pressures. No food should be stored in this freezer. In our lab we keep these vials in a freezer in room 219.
- To prepare the protein heavy atom:
- Keep all your tools used for handling DMM and TEL inside a fume hood. Leave the reagent in its vial in its centrifuge tube in the hood to warm it to room temperature (to minimize condensation of water). Preheat the wax melter in the hood and keep it on.
- Outside the fume hood: Mount the crystal using your usual technique. Seal one end of the capillary with wax. Insert into the open end a small piece of filter paper. Leave the paper filter flush with the end of the capillary. Pay attention to the size of the paper so it doesn’t touch the crystal.
- The rest of the procedure MUST be done with double gloves and inside the fume hood (preferably dissimilar glove materials, such as Silver Shield over long cuffed nitrile, neoprene or similar heavy duty gloves).
- Quickly move the mounted crystal to the fume hood, open the vial with DMM or TEL and with a 10 ul Hamilton syringe add just enough liquid to completely wet the paper. Be careful and do not leave a heavy atom liquid plug inside the capillary.
- Seal the other end of the capillary: If you use wax, be careful not to warm the paper strip because this causes distillation of the compound onto the nearest cooler thing (your crystal). With 5 minute epoxy, there isn’t a heating problem, but the gyrations needed to hold the liquid epoxy on the end of a capillary sometimes causes the paper strip to fall into the epoxy.
- Wait for 10-15 minutes to be sure the vapor diffuses into the crystal (see the references, below). TEL is less volatile than DMM, so maybe on average it takes longer. Your crystal is now read for data collection.
Leave the Hamilton syringe in the fume hood for few days. Label it so it is only used for Hg or Pb Heavy Atom experiments.
The gloves, wax and any other disposable tools that touched the compound should be disposed in a appropriate container (We have a container labeled HEAVY SOFT). After data collection the crystal and capillary should be disposed in the sharp Heavy atom waste container. (labeled HEAVY SHARPS)/.