A soybean trypsin inhibitor. Crystallization and x-ray crystallographic study.

Five trypsin and alpha-chymotrypsin inhibitors which have low molecular weights (ranging from 6800 to 8600) and are present in soybean seeds of the Tracy variety have been isolated and purified, and single crystals which give x-ray diffraction data beyond 3-A spacings have been obtained from one of them. The trypsin inhibitor crystallizes in a monoclinic unit cell of symmetry P2(1) and dimensions a = 25.919(7) A, b = 43.23(1) A, c = 19.905(5) A, and beta = 103.63(2) degrees. The assymmetric unit contains 1 molecule of molecular weight 6800. The crystal, which has been found to be unusually stable to x-radiation, has solvent content of approximately 26% by volume.