Structural changes within unit cell subunits in crosslinked alpha-chymotrypsin crystals.

The structural effects of bifunctional and polyfunctional protein crosslinking agents on protein molecules are, in principle, studied most exactly by modifying and examining a repeating ordered array of protein molecules, i.e. as in single crystals. While X-ray crystallography was possible with crosslinked carboxypeptidase A (Quiocho &; Richards, 1964), with other protein crystals disorder frequently obscured any reasonably resolvable electron density map. We have examined α-chymotrypsin crystals using electron spin resonance spin labels as a probe of the active site region only, i.e. observing one local region of a protein (crystal) structure in the midst of a (potentially) overall disordered structure. The methods and techniques of handling spin-labeled protein crystals were outlined by Bauer &; Berliner (1979).