Low-resolution detergent tracing in protein crystals using xenon or krypton to enhance X-ray contrast

Xenon and krypton show different solubilities in polar versus apolar solvents. Therefore, these noble gases should accumulate in apolar regions of protein crystals. Specifically, they should accumulate in lipid and detergent solvent regions within crystals of membrane proteins, which can be used as a basis for contrast-variation experiments to distinguish such apolar solvent regions from the aqueous phase by a low-resolution X-ray diffraction experiment. This possibility was explored with the OmpF porin, one of the general diffusion pores of the Escerichia coli outer membrane. Trigonal crystals were exposed to elevated pressures of the two noble gases (up to 10⁷ Pa) for several minutes and subsequently flash-cooled to liquid-nitrogen temperatures. Both rare gases bind to a number of `specific' sites, which can be classified as `typical' noble-gas binding sites. Compared with a representative water-soluble protein, they are however much more abundant in OmpF. In addition, a very large number of weakly populated sites are observed which accumulate in the region of the `detergent belt' for crystals exposed to xenon. After application of a Fourier-filtering protocol, low-resolution images of the detergent belt can be obtained. The resulting maps are similar to maps obtained from low-resolution neutron diffraction experiments on contrast-matched crystals.