Direct observation by X-ray analysis of the tetrahedral "intermediate" of aspartic proteinases.
B. Veerapandian, J. B. Cooper, A. Sali, T. L. Blundell, R. L. Rosati, B. W. Dominy, D. B. Damon, and D. J. Hoover
Protein Sci. 1992 March; 1(3): 322–328. PMCID: PMC2142209