The evolution with pressure of the CH4 vibrational modes related to C–H stretching has been followed by in situ Raman spectroscopy on a single crystal of cubic melanophlogite from Varano Marchesi (Parma, Italy) containing only CH4 in the cages. The sample was compressed hydrostatically in a diamond anvil cell up to 6.5 GPa, using a non-penetrating P-transmitting medium. Two modes at 2900 and 2910 cm–1 (ν1 and ν2) were followed in response to the applied pressure, corresponding to C–H stretching of CH4 enclathrated in the 51262 and 512 cages, respectively. A change in slope of the frequency vs. P linear trend of the sharper peak at 2900 cm–1, observed between 1 and 1.5 GPa, is interpreted as evidence of the P-induced cubic to tetragonal transition, previously observed by X-ray diffraction at P ≥ 1.2 GPa. At pressures below the transition, the shift with P of the CH4 modes is comparable to that observed in methane ice hydrate, which becomes hexagonal at P ≥ 0.9 GPa however. The ratio of the integrated areas of the two C–H stretching Raman peaks does not change significantly with pressure across the transition. At P ≥ 5.1 GPa, a shoulder appears, close to the n1 peak, along with a slight change in slope of peak shift and peak broadening. The shoulder is maintained in decompression down to P = 4.4 GPa, showing slight hysteresis. At the same pressures however, X-ray diffraction shows no evidence of a phase transition, suggesting that a rearrangement of CH4 configuration occurs, without any effect on the tetrahedral framework.

The unrefined fold of Escherichia coli β-galactosidase based on a monoclinic crystal form with four independent tetramers has been reported previously. Here, we describe a new, orthorhombic form with one tetramer per asymmetric unit that has permitted refinement of the structure at 1.7 Å resolution. This high-resolution analysis has confirmed the original description of the structure and revealed new details. An essential magnesium ion, identified at the active site in the monoclinic crystals, is also seen in the orthorhombic form. Additional putative magnesium binding sites are also seen. Sodium ions are also known to affect catalysis, and five putative binding sites have been identified, one close to the active site. In a crevice on the protein surface, five linked five-membered solvent rings form a partial clathrate-like structure. Some other unusual aspects of the structure include seven apparent cis-peptide bonds, four of which are proline, and several internal salt-bridge networks. Deep solvent-filled channels and tunnels extend across the surface of the molecule and pass through the center of the tetramer. Because of these departures from a compact globular shape, the molecule is not well characterized by prior empirical relationships between the mass and surface area of proteins. The 50 or so residues at the amino terminus have a largely extended conformation and mostly lie across the surface of the protein. At the same time, however, segment 13–21 contributes to a subunit interface, and residues 29–33 pass through a “tunnel” formed by a domain interface. Taken together, the overall arrangement provides a structural basis for the phenomenon of α-complementation.