Structures of furanosides : geometrical analysis of low-temperature X-ray and neutron crystal structures of five crystalline methyl pentofuranosides

Crystal structures of all five crystalline methyl D-pentofuranosides, methyl alpha -D-arabinofuranoside (1), methyl beta -D-arabinofuranoside (2), methyl alpha -D-lyxofuranoside (3), methyl beta -D-ribofuranoside (4) and methyl alpha -D-xylofuranoside (5) have been determined by means of cryogenic X-ray and neutron crystallography. The neutron diffraction experiments provide accurate. unbiased H-atom positions which are especially important because of the critical role of hydrogen bonding in these systems. This paper summarizes the geometrical and conformational parameters of the structures of all five crystalline methyl pentofuranosides, several of them reported here for the first time. The methyl pentofuranoside structures are compared with the structures of the five crystalline methyl hexopyranosides for which accurate X-ray and neutron structures have been determined. Unlike the methyl hexopyranosides, which crystallize exclusively in the C-1 chair conformation, the five crystalline methyl pentofuranosides represent a very wide range of ring conformations

Symmetry lowering in crystalline solid solutions: A study of cinnamamide-thienylacrylamide by x-ray and neutron diffraction and solid-state photochemistry

Principles are outlined for symmetry lowering of a mixed crystal. A survey is given of methods used to detect reduced symmetry: changes in crystal morphology, detection of enantiomeric segregation of chiral additives in centrosymmetric'' crystals, generation of second harmonic optical signals, optical birefringence, asymmetric photoreactions in the crystalline state and X-ray and neutron diffraction. The last two methods are applied to mixed crystals of cinnamamide host and thienylacrylamide. Diffraction demonstrated that the mixed crystals are composed of six sectors of reduced symmetry, from monoclinic centrosymmetric P2[sub 1]/c to triclinic P1 in four sectors and possibly Pc in the remaining two. The X-ray diffraction data were not sufficiently accurate for assigning the absolute structures of the PI sectors of anomalous X-ray scattering. Thus, by this method one could not ascertain the absolute orientation of the guest molecules on the surface sites through which they were selectively occluded. This ambiguity was resolved by assignment of the absolute configuration of the chiral heterophotodimers, between host and guest, in enantiomeric excess in the PI sectors, after irradiation with UV light. This leads to the conclusion that the selective occlusion of thienylacrylamide arises from replacement of attractive C-H[pi] (electron) interactions between host molecules by a repulsive sulfur (lone pair electron)[pi](electron) interactions between guest and host at the crystal surfaces

Characterization of single crystals of the large ribosomal particles from a mutant of Bacillus stearothermophilus

Single, three-dimensional crystals of 50S ribosomal subunits, from a mutant of Bacillus stearothermophilus that lacks the protein L11, have been characterized using a synchrotron X-ray source. The crystals of the mutated particles grow under the same conditions and are isomorphous to those of the wild type of the same bacteria. They are orthorhombic, contain at least one 2-fold screw axis, and have unit cell dimensions of a = 350(±10) A ̊, b = 670(± 10) A ̊, and c = 910(±10) A ̊. They diffract to 15 to 18 A ̊ resolution at 4 °C and are stable in the synchrotron beam for several hours

TPA labelled oligonucleotides for long range distance measurements by EPR

The folding of the RNA and their three-dimensional structure are areas of the great interest of biological and medical research. Besides X-ray, NMR and FRET, Electron Paramagnetic Resonance (EPR) can be applied to elucidate RNA's three-dimensional structure and also the dynamics of the system. Indeed, EPR spectroscopy has already shown to be a powerful technique to characterize the local surrounding of the paramagnetic center in proteins or oligonucleotides(1,2). Pulsed ELectron Double Resonance (PELDOR) enable us to measure long range distances between two nitroxides TPA(1) in DNA(1) and RNA(2) and TEMPA (2 and 2*)(3). The spinlabel was introduced during the solid-phase oligonucleotide synthesis on different nucleobases(4).</p