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Figure 2.
Figure 2 He high-pressure cryocooling and S SAD phasing of
thaumatin. (a) Thaumatin crystal in a polycarbonate capillary at
110 K. The entire sample, the crystal and mother liquor in the
capillary, was He high-pressure cryocooled without adding
penetrative cryoprotectants. This clear sample could not be
obtained by conventional (room-pressure) flash-cryocooling when
cryoprotectants were not added. (b) Diffraction image of the
thaumatin crystal grown in a polycarbonate capillary that was He
high-pressure cryocooled at 170 MPa. The diffuse background
scatter from the capillary ranges from 4.5 to 5.5 Å. The
lack of ice rings on the image confirms that water vitrification
was successfully achieved under high pressure. The resolution
limit [I/  (I)
 equal
to] 5.0] is approximately 1.9 Å and the crystal mosaicity
is 0.34°. The diffraction spots in the enlarged region look
compact. (c) Anomalous difference map (5 level)
generated with the refined phases. All 17 sulfurs that naturally
present in thaumatin are clearly visible. The shape of the
electron density at disulfide bonds is dumbbell-shaped, so
two-sulfur positions could be easily distinguished. The peak
height was over the 10 level
(red) for most of the sulfur sites and the peak at Met112 was
even visible at the 15 level.
(d) F[o] electron-density map (1 level)
after S SAD phasing and density modification at 1.9 Å
resolution. The final refined model solved by molecular
replacement was superimposed for visual map evaluation. The
figure of merit is 0.824 and the map correlation coefficient
calculated with the final refined 2F[o] - F[c] density map is
0.85 for the main chain and 0.76 for side chains.
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