BINDING OF SMALL ELECTRON-DENSE LIGANDS IN LARGE PROTEIN CAVITIES. Michael L. Quillin, Walter A. Baase, and Brian W. Matthews, Howard Hughes Medical Institute, Institute of Molecular Biology University of Oregon, Eugene, Oregon 97403.

The extent to which disordered water molecules occupy hydrophobic cavities in proteins has been the subject of considerable debate. In some cases, the techniques of NMR spectroscopy and X-ray crystallography have provided seemingly contradictory estimates of the solvent content of apolar cavities (Ernst et al. (1995), Science 267, 1813-1817; Matthews et al. (1995), Science 270, 1847-1848). In an effort to resolve whether it is possible to detect fully occupied yet disordered molecules in a protein cavity using crystallographic methods, we have determined the crystal structures of several complexes of small, electron-dense molecules bound within cavity-containing mutants of T4 lysozyme. Two classes of probes have been studied in this manner: noble gases, such as krypton and xenon; and alkyl halides, including alkyl bromides and iodides. Although these ligands bind within the cavity, they do not do so in a disordered fashion. Rather, it appears that there are preferential binding sites which are highly conserved among the different ligands. Factors which influence the location of these sites will be discussed.