Dianion-controlled supramolecular assembly of copper(II)-arginine complex ion

In order to clarify the self-organization and structure-determining factors in the [Cu(l- or d-Arg)2]2+-containing systems (Arg=arginine), we studied the solution and crystal structures of [Cu(l-Arg)2]2+X2− systems, where X refers to dinegative tetrahedral anions, sulfate or 1,3-benzenedisulfonate (m-bs2−), by electronic absorption, circular dichroism (CD), and ESR spectroscopic methods and X-ray structural analysis. The molecular structure of [Cu(l-Arg)2](SO4) exhibited a planar Cu(II) geometry with the two coordinated amino groups in the cis configuration due to the hydrogen bonds with SO4 2−, and the complex with X2−=m-bs2− assumed the trans configuration without hydrogen bonds with m-bs2−. The crystal structure of [Cu(l-Arg)2](SO4) showed that the complex cations were bound with the anions to form a right-handed double-strand single-helical structure, whereas that of [Cu(l-Arg)2](m-bs2−) revealed that the complex ions were bound with each other through the guanidinium–coordinated carboxylate hydrogen bonds to form a 3-dimensional layer structure with the m-bs2− ions connecting the layers by hydrogen bonds. The d-Arg analog of the complex with isophthalate (m-pa2−) used in place of m-bs2−, [Cu(d-Arg)2](m-pa2−), was shown to assume the cis configuration and form a left-handed double-helical structure. The configuration of the Cu(II) plane was predictable on the basis of the structural requirements for the hydrogen bonding with X2−. Comparison of the absorption spectra of [Cu(l-Arg)2](X) in the solid state and in aqueous solution indicated that the complexes in solution are more in the cis configuration than in the trans. The CD spectra of the sulfate system exhibited a unique spectral behavior, which suggested formation of stronger hydrogen bonds between coordinated amino groups and the sulfate ion. Factors determining the observed self-organized structures were comparatively discussed in connection with unique infinite right- or left-handed double-helical structures of the [Cu(l- or d-Arg)2]2+–X2− systems reported previously (N. Ohata, H. Masuda, and O. Yamauchi, Angew. Chem., Int. Ed. Engl. 35 (1996) 531).