Miscellany

Hot structures and materials

The earth's core?

The discovery that Xenon appears to resist forming an alloy with iron even under the most extreme pressures casts doubt on the possibility that the Earth has a store of hidden xenon inside its core. Earth's low abundance of xenon relative to the sun and some meteorites has been referred to among geochemists as the "missing xenon" problem. Using a diamond anvil cell, R. Jeanloz, S. Louis and W.A. Caldwell of U. of California, Berekley subjected Xenon and iron to peak pressures of 75 gigapascals (Gpa) while heating the sample to more than 2,000 K by means of a focused laser beam. X-ray diffraction patterns indicate that, as the pressure increases, the xenon undergoes a transformation from a face-centered cubic structure to a hexagonal close-packed structure, but does not alloy with iron (Science, 277, 930, 1997).

Cubane

A U. of Chicago-NIST collaboration has worked out the structure of the plastic phase of cubane, using x-ray powder data. Surprisingly, the plastic phase of cubane is not face-centered cubic but rhombohedral and undergoes a large volume expansion of 5.4% at the first-order phase transition temperature of 394.3 K (T. Yelderun et al, Phys. Rev. Lett.78, 9438, 1997).

New materials

Exciting new materials recently characterized by X-ray analysis include reversible polymers formed from self-complementing monomers stabilized by quadruple hydrogen bonding (Sijbesma, et.al., Science 278, 1601, 1997) an iron-gallium triple bond of 2.2248Å length, (Robinson etal, Organo-metallius, 16, 4511, 1997) and a second family of bismuth oxide superconductors based upon SrBiO3 (Kazokov et al, Nature, 390, 148, 1997).

Distibene-O2 reserves crystal state

The structure of the first stable distibene, a compound with an antimony-antimony double bond, that undergoes an unprecedented reaction with molecular oxygen in the crystalline state was determined by Y. Ohashi's group at Tokyo Inst. of Technology. Changes in the dimensions and volume of the unit cell were monitored as the distibene was transformed into the dioxadistibetane over the course of 10 hours.

Hormone synthesis

Isoprenoids are found as visual pigments, steroid hormones, and membrane constituents. Abnormalities in their biosynthesis can lead to heart disease and cancer. The diverse isoprenoids are generated from compounds with 5, 10, 15 and 20 carbon atoms that form substrates for many enzymes, primarily cyclases (Lesburg et al.). Starks et al, and Wendt describe the X-ray crystallographic structures of three of these cyclases and the mechanism of protonation are reported by three teams of crystallographers in a single issue of Science, 227, 1811, 1815 and 1820 (1997).

Membrane proteins

If one picture is worth a thousand words, recent advances in X-ray crystallography methods are providing the equivalent of the Encyclopedia Britannica. Crystallographers are now churning out three-dimensional (3D) structures of proteins at the rate of four per day. E. Pebay-Peyroula of the Inst of Bio Structure in Grenoble, France and E. Landau of the U. of Basel, Switzerland. offer a newly detailed 3D structure of bacteriorhodopsin, a key protein enabling the salt-loving bacterium Halobacterium salinarium to convert energy from sunlight to chemical energy that the bacterial cells can use. The structure, with a resolution of 2.5 Å, offers the most detailed look yet inside this solar power plant (Science, 277, 1676, 1997). The determination of the same structure at 3.0 Å resolution using electron cryomicroscopy was published almost simaltaneously (Y. Kimura, et al, Nature, 389, 206, 1997) and a 7.5 Å (projected) electron diffraction structure of frog rhodopsin appeared in the same issue (Unger, et al, Nature, 389, 203, 1997).

Protein folding

The chaperone GroEL with it's cofactor GroES and adenosine diphosphate help proteins self-assembly into correctly folded structures and prevents aggregation into misfolded or nonfunctional forms. The hollow, barrel-shaped GroEL protein has interior hydrophobic surfaces to which unfolded polypeptides can bind. The binding of adenosine triphosphate (ATP) leads to the addition of GroES, which caps one end of the complex and stabilizes conformational rearrangement in which the central cavity roughly doubles in size (P Sigler, A. Horwich, and coworkers, Nature, 388, 741 and 792, 1997). At the same time, the interior surface turns hydrophilic, causing release of the polypeptide into the cavity, where it can begin to fold into its native state. How the molecular chaperone facilitates the folding of other proteins is a wonderfully complex and highly dynamic process, the details of which are only beginning to emerge. Two papers by Xu et al and Rye et al from Yale U. demonstrate the power of crystallography coupled to clever mutagenesis.