A new crystal phase of barium nitroprusside trihydrate studied by neutron diffraction at 20 K.

The crystal of barium pentacyanonitrosylferrate trihydrate [barium nitroprusside trihydrate, Ba[Fe(CN)5(NO)] x 3H2O] has been studied by neutron diffraction at 20 K. The study was performed to characterize the structural phase generated by the phase transition undergone by the crystals at 80 K, at which temperature the unit-cell volume doubles. This crystal phase still exists at 20 K. The crystal structure, in space group P1, is completely ordered. The positional changes of the water molecules in the present structure with respect to those of the compound at 105 K are presented.

An ordered low-temperature phase of barium nitroprusside trihydrate studied by neutron diffraction.

Crystals of barium pentacyanonitrosylferrate trihydrate (barium nitroprusside trihydrate), Ba[Fe(CN)(5)(NO)].3H(2)O, have been studied by neutron diffraction in order to examine the structural behaviour of the compound in the 20-120 K temperature range and to determine the structure at 105 K. The results show the existence of a new crystal phase of the compound at 80 K (with a duplicated a parameter), which still exists at 20 K. The crystal structure at 105 K shows a rearrangement of the water molecules, which results in an ordered structure with P1 symmetry. Two of the four independent nitroprusside cations are rotated by 4.5 degrees around the [100] direction.

Redox signalling in the chloroplast: structure of oxidized pea fructose-1,6-bisphosphate phosphatase.

Sunlight provides the energy source for the assimilation of carbon dioxide by photosynthesis, but it also provides regulatory signals that switch on specific sets of enzymes involved in the alternation of light and dark metabolisms in chloroplasts. Capture of photons by chlorophyll pigments triggers redox cascades that ultimately activate target enzymes via the reduction of regulatory disulfide bridges by thioredoxins. Here we report the structure of the oxidized, low-activity form of chloroplastic fructose-1, 6-bisphosphate phosphatase (FBPase), one of the four enzymes of the Calvin cycle whose activity is redox-regulated by light. The regulation is of allosteric nature, with a disulfide bridge promoting the disruption of the catalytic site across a distance of 20 A. Unexpectedly, regulation of plant FBPases by thiol-disulfide interchange differs in every respect from the regulation of mammalian gluconeogenic FBPases by AMP. We also report a second crystal form of oxidized FBPase whose tetrameric structure departs markedly from D(2) symmetry, a rare event in oligomeric structures, and the structure of a constitutively active mutant that is unable to form the regulatory disulfide bridge. Altogether, these structures provide a structural basis for redox regulation in the chloroplast.