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The soybean trypsin inhibitor E-I, known to be relatively rich in methionine and
cysteine, has been crystallized at room temperature in the presence of
polyethylene glycol 4000, sodium chloride, and ammonium sulfate. A pronounced
polymorphism in the crystals has been observed and two distinctly different
cubic forms have been identified. Of the two, one form crystallizes in a unit
cell of symmetry F432 with parameters a = b = c = 128.0 A, and diffracts at
least to 2.6-A resolution. Each of the 96 asymmetric units contains one protein
molecule, and has a solvent content of 60% by volume. The other form is of space
group P4132 or P4332 with the unit-cell edge 87.1 A, and diffracts barely to 3-A
spacings. The unit cell is composed of 24 asymmetric units, each of which
probably also contains one molecule and has a solvent content of 69% by volume.
The former cubic form appears to be more suitable for x-ray study than the
latter in terms of stability and the extent of diffraction.
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