No abstract

We have observed condensation of X-rays emitted from an edge of a thin Ge parallel crystal, using X-rays from synchrotron radiation. When highly parallel X-rays with the energy near an absorption edge of an atom or a nucleus are incident on a thin crystal in the Bragg case, a part of X-rays is confined in the crystal and do not come out either from the top or the bottom surface. The density of confined beam increases as the width of the incident beam is increased. The confined beam can come out from an edge of a thin crystal with higher density than the incident beam.

Crystal and molecular structures of the two diastereomeric title complex ions have been determined by X-ray diffraction. Crystals of the orange isomer, [Co(taa) (GlyO)] Cl[ClO,], are monoclinic, space group P2,/a, with a = 14.1 35(4), b = 9.725(4), c = 11.544(4) A, = 98.1 3(13)", Z = 4, while those of the red isomer, [Co(taa)-(GlyO)] [C104]2, are orthorhombic, space group P212121. with a = 16.990( 4), b = 8.382(4), c = 12.034(4) A, Z = 4. The structure of the orange isomer has been determined by the heavy-atom method, and of the red isomer by direct methods. The structures have been refined to R 0.039 (orange, 2 425 reflections) and 0.035 (red, 1 708 reflections). The glycine is chelated in the octahedral complex through its NH, group trans to the tertiary amine of taa in the orange isomer, and cis in the red isomer. These geometries are adequately reproduced by strain-energy minimization calculations and the latter are correlated with the stability relation. Comparison of the two isomeric complexes shows how the tripod ligand is accommodated in the octahedral structure, and reveals a special steric situation which labilizes a substituent cis to Nhd in taa complexes of C O ' " .The specific reaction path with which [Co(taa) (OH) (0H2)l2+ hydrolyzes the N-terminal residue of peptides to afford the orange isomeric [Co(taa)L]"+ ( L = amino-acid anion) is rationalized by the present studies.

The molecular structures of several tetranactin complexes have been determined with X-ray reflexion data collected on a four-circle diffractometer. The structures of the form I crystals of the Rb + complex [space group P21/n, Z=4, a=15.927 (9), b=20.555 (14), c=15.495 (6) A~, fl=90.38 ° (3)] and K + complex were solved by the heavy-atom method, and refined by the block-diagonal matrix least-squares method: the R indices are 8.4 and 9"9 %, respectively, including H atoms. The structures of the form II crystals of the K + complex [space group C2/c, Z=8, a=27.455 (6), b= 12.420 (1), c= 30.211 (6) A, fl=97.15 ° (2)] and the Na + complex were determined by the symbolic addition procedure; the R indices were 11.1 and 11-9%, respectively, excluding hydrogen atoms. The molecular structures are approximately identical with those of the nonactin complexes in which all ethyl groups are replaced by methyl. Conformational energies of the complexes were calculated and the dependence of the cation selectivity on intra-ligand steric interactions was estimated. Strong attractive ion-dipole interactions were observed in an inverse proportion to ionic radii of the central cations. However, it was concluded that the ion selective behavior of the macrotetrolide must be closely related to the free energy of the hydration of cations in aqueous solutions. i ¸ • .: i

The structure of the Cs+-tetranactin complex [space group P2~/n, Z = 4, a = 16.022 (6)