The structures of two cofacial metalloporphyrins anchored by rigid pillaring spacer groups of anthracene and biphenylene have been determined by using X-ray crystallographic methods (DP-A and DP-B, respectively). The cobalt complexes of these two porphyrin hosts have demonstrated electrocatalytic activity of mediating the four-electron reduction of dioxygen to water. The structures reported here are as the dinickel (DP-A) and the dicopper (DP-B) complexes. The porphyrins of both molecules slip with respect to each other: 2.40 A in DP-A giving a Ni-Ni distance of 4.566 and 1.60 A in DP-B giving a Cu-Cu distance of 3.807 A. The slip of the former leads to an average porphyrin plane-to-plane separation of 3.88 A while that of the latter corresponds to about 3.45 A. The larger distance of the DP-A is a consequence of the greater lack of planarity of the Ni porphyrin structure. The slip of the porphyrin rings appears to be an optimizational positioning of the rings with respect to van der Waals interactions and its extent is limited by the following: ( I ) repulsive interactions between pyrrole substituent methyl groups and atoms of the pillaring connector group and (2) the ruffling of the porphyrin rings by the metal. The structures of the pyrroles adjacent to the aromatic connector groups are different from the outer pyrroles while the structures of the connectors compare quantitatively with those of the isolated molecules. Interestingly, although both crystal structures are triclinic, the crystal packing of the two is very different. The results of these studies suggest indirectly that an exact metal-metal distance is not absolutely crucial for four-electron dioxygen reduction.In recent years great interest has been shown in the use of metalloporphyrins for catalyzing dioxygen activation and reduction.',2 The ability of certain cofacial dicobalt porphyrins to catalyze the electroreduction of dioxygen a t unusually positive potentials has made such complexes, supported on inexpensive graphite, promising alternatives to platinum as electrode material in air battery and fuel cell application^.^-^ I t has been shown, however, that the performance of these macrocycle catalysts is extremely sensitive to the porphyrin structure. To date, only three diporphyrins are capable of achieving the four-electron electroreduction of dioxygen to water without accumulating a substantial amount of hydrogen peroxide. Among these, the first compound consists of two stacked porphyrin rings doubly linked via short alkyl amide straps3v4 while the other two are based on a design in which two porphyrins are anchored cofacially onto a rigid pillaring spacer group (e.g., anthracene or biphenylene).s The strapped system is, due to the synthetic approach employed, always composed of more than one stereoisomer6 which undoubtedly has contributed to the difficulty of obtaining good quality crystals for X-ray diffraction studies. There are two published crystallographic diporphyrin ~t r u c t u r e s~-~ but neither is for the active (4-e process) ca...
Movements of tropomyosin play an essential role in muscle regulation. This fibrous protein is a two-chain alpha-helical coiled coil that bonds head to tail to form cables wound in the two long grooves of the actin helix. The regulatory switch consists of tropomyosin and a "globular" Ca2+-sensitive protein complex called troponin. The structure of the tropomyosin filaments has now been determined by x-ray crystallography to approximately 15 A resolution. The complete sequence of alpha-tropomyosin is known; by using mercury markers on the cysteine residues the ends of the molecules in the filaments have been identified. Details of the coiled-coil structure have also been visualized by refinement of models against the diffraction data. The average pitch of the coiled coil is approximately 137 A, so that each tropomyosin molecule can make similar contacts with seven actin monomers. The electron density map also indicates that departures from the alpha-helical coiled coil occur in a few localized regions of the molecule, especially at the overlapping ends. Motions of tropomyosin in the crystal lattice are displaced by the character of the Bragg reflections and the strong diffuse scatter. These effects depend markedly on temperature. It appears that the molecular filaments fluctuate freely in a direction perpendicular to their axes. Moreover, the C-terminal half of the molecule "unfolds" to some degree at less than physiological temperatures. Crystallographic results on co-crystals of tropomyosin and a component of troponin (TnT) suggest that this subunit consists of structurally distinct domains, so that the troponin complex is not in fact simply "globular". The interactions of the extended alpha-helical region of TnT may "stiffen" tropomyosin and influence its motions. We picture the tropomyosin/troponin switch in muscle as a restless cable, perpetually making and breaking bonds as it vibrates on the thin filament. These movements of tropomyosin probably depend on two aspects of its design: the regular pattern of coiled-coil linkages with actin; and the aperiodic features that allow flexibility and motion.
The X-ray crystal structure of the psychoactive agent clozapine as its dihydrobromide salt [8-chloro-ll-(4-methyl-1-piper azinyl)-5-dibenzo [b,e] [Petcher & Weber (1976). J. Chem. Soc. Perkin Trans. 2, pp. 1415-1420], the bond lengths and angles associated with the distal N of the piperazine ring become characteristic of a protonated tertiary N, while the bond lengths and angle associated with the imino N of the seven-membered ring increase upon protonation, causing a concomitant increase in the angle formed between the normals to the benzene rings.
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