The method of three-dimensional image reconstruction from electron micrographs is described with reference to the use of symmetries of specimens. Among the symmetries (two-dimensional translational symmetry, helical symmetry, icosahedral symmetry) which have been dealt with so far, the helical symmetry is fully described concerning its nature and its use. The practical aspects of the method, including manipulation of the phase data, are also presented.
The crystal structures of porphinato-iron (III) complexes with an anomalous inter-mediate-spin state (S=3/2) were compared with those of high-and low-spin iron (III) porphyrins. The intermediate-spin state is arisen in an asymmetric coordination field around the central iron (III) ion with respect to the axial and equatorial ligand strengths, while both the low-and high-spin states have approximately octahedral fields. The axial iron ligand distance in the intermediate-spin state is significantly different from the equatorial distances. The geometric parameters consistent with the electron configuration of the intermediate-spin state. The crystal structures of porphinato-ruthenium (IV) and osmium (IV) complexes as a model of peroxidase compound II were discussed also. The complexes consist of the μ-oxo dimer with the weak ligand (OH-, Cl- or OCH3-) . The quadrivalent metallopor-phyrin was stabilized by a strong ligand of O2- with σ- and π-bonding natures. The diamagnetism in these complexes was explained by the dπ-pπ-dπ interaction in the MIV-O-MIVbond.
Many different ring conformations are possible for cyclen: tetraazacyclododecane. Conformation energies were calculated using molecular mechanical method for cyclo-dodecane. Ten different ring conformations were realized for 1, 4, 7, 10-tetrabenzyl-2, 5, 8, 11-tetraethyl-1, 4, 7, 10-tetraazacyclododecanes and their metal complexes.