X-ray Diffraction Analyses of Trypsin Crystals Grown in the Presence of Additives

Four porcine crystals, in three different unit cells, and bovine trypsin crystals, studied at 173°K and 295°K, were analyzed by X-ray diffraction. Crystals were grown in experiments designed to discover small molecules or additives that enhance crystallization. Mother liquors of crystals each contained a set of additives as well as known salts, buffers, and precipitants. Noteworthy were mellitic acid, pyromellitic acid, and other small organic acids and anions, benzamidine, a buffer molecule, HEPES, sucrose, and PEG fragments ranging from three to six units in length. Except for PEG molecules, most of these bound to trypsin directly and generally at protein–protein interfaces. Preponderance of carboxyl containing molecules provides further evidence of their usefulness in protein crystallization. Because of symmetry, some additive molecules were seen lying on special positions. The appearance of small molecules in protein crystals is a further testament to the value of exploring additives in crystallization screens. In porcine trypsin crystals, PEG in the solvent regions has received particular attention. Ubiquitous, extended electron density peaks plausibly interpreted as PEG molecules suggest that the solvent regions in crystals derived from PEG should be treated in a more granular manner than those using only bulk solvent corrections in modeling and refinement.