The structure of the title compound has been refined from three-dimensional X-ray diffractorneter data. Crystals are monoclinic, space group C2/c, with Z = 4 in a unit-cell of dimensions: a = 13. 1 68(2). b = 8.564(2), c = 13.858(2) 8, and = 11 7.02(1)". Full-matrix least-squares refinement, using 1280 independent reflections, has reached R 0.047. The refinement has, as expected, confirmed the original two-dimensional structure determination and much more accurate bond lengths and bond angles have been obtained : mean Cu-O(acetate) 1-969 8, Cu-O(water) 2.1 56, and Cu * * Cu is 2.61 6 8. The geometry of the acetate bridges is planar and bond lengths and angles are as expected.

The Kunitz-type soybean trypsin inhibitor (STI) has played a key role in the early study of proteinases, having been used as the main substrate in the biochemical and kinetic work that led to the de®nition of the standard mechanism of action of proteinase inhibitors. A partial structure of STI complexed with porcine trypsin has previously been reported, in which the ®rst 93 residues of the inhibitor, including the region of contact with trypsin, were relatively well de®ned, whereas for the remaining part of the peptide chain only some C atoms were located. The structure of the inhibitor in its free form has now been determined by molecular replacement to 2.5 A Ê , using the coordinates of the homologous Erythrina trypsin inhibitor as a search model. When the re®ned atomic coordinates of STI are compared with the partial model previously available, the conformation of the reactive-site loop and its position with respect to the main body of the molecule does not change when the inhibitor interacts with trypsin. There are instead, despite the high similarity in the overall tertiary structure, signi®cant differences between STI and Erythrina trypsin inhibitor (ETI) in the region which is in contact with the enzyme in the STI:trypsin crystal structure. Some of these differences can explain the unique speci®city of ETI and its ability to inhibit the ®brinolytic enzyme tissue-type plasminogen activator.

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