Hydrophobic heavy atom derivatives
Most derivatives are limited to binding sites on the protein surface due to
their charged nature or their large size. However, several compounds have
been reported to be able to penetrate the hydrophobic protein interior and
could therefore be interesting compounds to test. The only truely hydrophobic
compound is the noble gas Xenon which can occupy hydrophobic pockets.
Xenon has to be added under pressure (2 Atm or more) which presents some
technical problems. A special device has been described by Schiltz et al.
(J. Appl. Cryst. 27, 950-960, 1994). An alternative procedure has been to
produce liquid Xenon by immersing the X-ray capillary in liquid N2. After
sealing the capillary the evaporating liquid Xenon reaches a sufficient
pressure to derivatize the crystal (or explode the capillary :-). For
references see Schiltz et al. A commercial device is now being sold
by MSC to make nobel gas derivatives for cryocrystallography.
Other compounds with a hydrophobic nature are:
- Ethyl mercury chloride or ethyl mercury phosphate
- the latter is the favourite of Prof. Petsko but cannot be used in
- This compound is formed in approx. 24 hours from (PtCl4)2- in AS
precipitants at above neutral pH.
- This compound is formed from K2HgI4. Its formation can be stimulated by
addition of excess KI.
- This anion has sufficient hydrophobic character to enter proteins.