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The structural phase transitions occurring in the chain compounds (CH3)4NMnCl3 (TMMC) and (CH3)4NCdCl3 (TMCC) are studied through X-ray diffraction measurements. In both compounds, the existence of a new high-temperature orientationally disordered phase is established (phase I') with space group P63/mmc and Z=2 (parent phase). The room-temperature phase I of TMMC and TMCC (space group P63/m and Z=2) derives from I' by a rotation of the octahedra chains about the c axis; orientational disorder of the (CH3)4N+ group (TMA) in phase I is described by a complex Frenkel model involving at least five different orientations of the TMA. Phase II of TMMC (space group P21/b with Z=4) is derived from phase I by antiphase translational displacements of the octahedra chains along the c axis; an important amount of residual disorder of the TMA is observed just below the I to or from II transition temperature. In TMCC, the space group P21/m (Z=2) of phase III is confirmed, whereas phase IV exhibits a complex structure (space group P21/b with Z=12) corresponding to a trebling of the lattice constant along c and a doubling along b. X-ray diffuse scattering experiments on TMMC and TMCC are also reported. In phase I, the diffuse scattering patterns are interpreted in terms of linear translational disorder of the octahedra chains along the c axis. In phase II of TMMC, a residual translational disorder of the chains is evidenced, just below the I to or from II transition temperature. These results show that orientational disorder of the TMA group is coupled to translational disorder of the octahedra chain sublattice.

Monoclinic uncomplexed double-stranded, antiparallel, left-handed f35-6-helix (TIJ5-6) structure ABSTRACTA comparison of the monoclinic and orthorhombic crystal structures of the uncomplexed doublestranded, antiparallel, left-handed 8-helix (5.6 amino acid residues per turn) (1135.6) conformers of gramicidin A reveals marked differences in the tryptophan side-chain orientations and the degree of helical uniformity of the dimer and in the manner in which these helical dimers associate with one another in the crystal. The helix of the orthorhombic dimer exhibits a regular pattern of bulges and constrictions that appears to be induced by crystal packing forces affecting tryptophan side chains that are aligned parallel to the helix axis. The monoclinic dimer is more uniform than the orthorhombic dimer as a consequence of vi stacking interactions between dimers in which orientation of tryptophan side chains

Anodic oxidation of two series of alpha-(2)- and alpha-(4-methoxyphenoxy)alkanoic acids were studied both at the analytical and preparative scales in order to delineate mechanistic aspects of electrochemically induced spirolactonization and to develop synthetically useful orthoquinone bis- and monoketals. Although alpha-monomethylated carboxylic acids and acetic acid derivatives do not undergo any spiroannulation, alpha-dimethylated carboxylic acids furnished spirolactones in high yields. A gem-dimethyl effect is invoked to explain these differences in cyclization capacity. Electrooxidation conditions can be selected to furnish either quinone spirolactone bis- or monoketals. Chemoselective monohydrolysis of bisketals can also be accomplished in a stepwise fashion to furnish the corresponding spirolactone monoketals, but the ortho compound unfortunately dimerized in situ via a Diels-Alder process. An ECEC pathway is proposed to rationalize the observed spirolactonizations on the basis of cyclic voltammetry analyses.

Ferritin, the iron-storage protein, binds porphyrins, metalloporphyrins and the fluorescent dyes ANS (8-anilino-1-naphthalenesulfonic acid) and TNS (2-p-toluidinyl-6-naphthalenesulfonic acid), similarly to apo-myoglobin. Octahedral crystals of horse-spleen apo-ferritin (HSF; 174 amino acids) complexes prepared by the addition of haem, hematoporphyrin or Sn-protoporphyrin IX to a solution of apo-ferritin crystallize in space group F432 with cell parameter a = 184.0/~,. X-ray crystallographic analysis of single crystals prepared from a mixture containing haem or Sn-protoporphyrin IX shows that the haem-binding sites in these crystals are occupied by protoporphyrin IX, which is free of metal, rather than by the original metalloporphyrin. The present paper describes the structure of horse-spleen apo-ferfitin cocrystallized with Sn-protoporphyrin IX. The 6797 reflections up to 2.6 A resolution used in the refinement were obtained from a data set recorded on a Nicolet/Xentronics area detector with Cu Ks radiation from a Rigaku RU 200 rotating anode. The final structure comprises 1613 non-H atoms, two Cd atoms and 170 solvent molecules. Four residues are described as disordered. The root-meansquare deviations from ideal bond lengths and angles are 0.013 A and 2.88 °, respectively. Protoporphyrins are observed in special positions on the twofold axes of the ferritin molecule with a stoichiometry of 0.4 per subunit.