Exploring hydrophobic sites in proteins with xenon or krypton

X‐ray diffraction is used to study the binding of xenon and krypton to a variety of crystallised proteins: porcine pancreatic elastase; subtilisin Carlsberg from Bacillus licheniformis; cutinase from Fusarium solani; collagenase from Hypoderma lineatum; hen egg lysozyme, the lipoamide dehydrogenase domain from the outer membrane protein P64k from Neisseria meningitidis; urate‐oxidase from Aspergillus flavus, mosquitocidal δ‐endotoxin CytB from Bacillus thuringiensis and the ligand‐binding domain of the human nuclear retinoid‐X receptor RXR‐α. Under gas pressures ranging from 8 to 20 bar, xenon is able to bind to discrete sites in hydrophobic cavities, ligand and substrate binding pockets, and into the pore of channel‐like structures. These xenon complexes can be used to map hydrophobic sites in proteins, or as heavy‐atom derivatives in the isomorphous replacement method of structure determination. Proteins 30:61–73, 1998. © 1998 Wiley‐Liss, Inc.