Page 78 - Macromolecular Crystallography

FIRST ANALYSIS OF MACROMOLECULAR CRYS TALS 67

Cold gaseous
(a) nitrogen stream (b)
(t = 180˚C)
Shock-cooled
crystal
(in propane)
(e)
Goniometer head
assembled on X-ray
camera

(c) (d)

Figure 4.5 Procedure for transfer of shock-cooled macromolecular crystals to the X-ray camera. (a) The goniometer head is assembled on the
X-ray camera and the cold gaseous nitrogen stream is centred on the eucentric point of the camera. (b) For samples prepared with propane or
ethane, a cryovial containing a crystal shock-cooled in propane is selected and allowed to partially thaw (5–10 s). Once the external layer of
propane has thawed, a pair of forceps is used to lift the loop-mounted crystal, still encased in solid propane, out of the cryovial and placed on the
goniometer head. The solid propane is allowed to thaw completely to liquid, which falls away from the crystal. Excess liquid should be collected
and disposed of properly. (c) For samples prepared with liquid nitrogen, a cryovial is selected and placed in a bath of liquid nitrogen (held in a
shallow Dewar flask). The pin containing the loop-mounted crystal is removed from the cryovial and captured with the magnetic crystal wand as
shown above. The mounting pin (held by the magnetic crystal wand) is manoeuvred into the head of the cryotong. (d) With the cryotongs in its
locked position, the crystal is lifted out of the liquid nitrogen bath and the mounting pin positioned on the magnetic mount held on the goniometer
head. (e) The cryotongs are quickly unlocked to expose the crystal to the nitrogen stream.

Figure 4.6 A crystal lattice. Circles represent lattice points; heavy lines represent a unit cell.