The interaction of transition metals with the coenzyme α-lipoic acid: synthesis, structure and characterization of copper and zinc complexes

Abstract

The reaction of DL-α-lipoic acid or DL-α-lipoamide with copper(I) chloride in acidic aequeous solution leads to the copper(I) lipoic acid complexes catena-poly[Cu 1(lip 0)Cl[ form I ( A) and form II ( B) and poly[Cu I 3(lip 0) 2Cl 3] ( C). From an aqueous suspension of lipoic acid, crystals of the copper(II) lipoic acid complex [{;Cu 11(lip −) 2> 2] ( D) and of the zinc(II) complex [Zn 11(lip − 2(H 2O) 2] ( E) can be grown on the surface of pressure pills of the corresponding insoluble metal hydroxide salts. The crystal structures of A, B, C and E have been established by X-ray diffraction. Crystal data are as follows A: monoclinic, In the three copper(I) complexes A, B and C, α-lipoic acid is coordinating to the metal atom via its cyclic disulfide group and bridges two adjacent copper atoms. Five-membered rings of the type CuClCuSS- are building blocks in each of these three structures. A four-fold bridging μ 4-chlorine atoms is observed in C. The structure of E is built up by neutral molecular [Zn 11(lip −) 2(H 2O) 2] unitsl the anionic α-lipoic acid ligand is coordinating via its bidentate, chelating carboxylate group. A trend is evident, whereby the SS bond distance of the cyclic disulfide unit increases and the CSSC torsion angle decreases upon coordination to metal ions. This geometrical influence upon coordination may be paralleled by an increase of the corresponding ring strain and, hence, by an enhanced redox reactivity of the disulfide moiety of α-lipoic acid. Spectroscopic and variable temperature magnetic studies indicate the presence of dimeric [Cu II 2(lip −) 4] unit with antiferromagnetic coupling (−2 J = 315 cm −1) between the copper(II) centers in D.