Behavior of copper complexes in very restricted fields provided by reversed micelles

Abstract

Copper ion, in its various forms, plays an important role in many biological reactions. This versatility of copper stems from its feasible redox property and possibility of coordination to a number of common ligands to form complexes of different structure, e.g., octahedral, tetrahedral or square-planar, and mono-nuclear or polynuclear structures. To date, numerous studies have been performed on the preparation and characterization of model copper complexes in conjunction with the structure and function of the copper metallo-enzymes. However, metal ions in reversed micelles, one of enzyme pocket models, have not attracted much attention. As part of our investigations on the metal ion behavior in reversed micelles [1] correlating with the activity of water and the restricted field effect provided in the micelles, we recently explored redox properties of copper ion and chelation of common ligands of biochemical interest such as imidazole to Cu(I) or Cu(II) ion. When aqueous Cu(II) chloride is solubilized in chloroform containing 0.20 M hexadecyltrimethylammonium chloride (CTACl) and 0.20 M or less of water, the predominant species formed is the chlorine-bridged polymeric complex. This complex shows absorption bands at 294 and 408 nm. Upon the addition of imidazole to this system, the intensity of both bands decreases and a new band appears at about 280 nm. From an analogous experiment in aqueous media, this was assigned to a charge-transfer band of the copper–imidazole complex. The change of the band intensity as a function of imidazole concentration showed a clear break at the point where [Im]/[Cu(II)] ≑ 2. In contrast, the presence of 1.0 M water abolished the break, just like in the bulk aqueous solution. In chloroform containing CTACl, Cu(I) chloride is also readily solubilized and the Cu(I) ion is subject to a very slow oxidation by oxygen present in the medium. The oxidation is drastically facilitated by the addition of aqueous hydrogen peroxide. Interestingly, the resulting absorption spectrum was exactly the same as that of Cu(II) chloride itself dissolved in the reversed micelles except the apparent extinction coefficients at λ max′s. On the other hand, Cu(II) chloride complex is reduced instantaneously by the addition of 2-mercaptoethanol as evidenced by a disappearance of 294 and 408 nm bands. Chelation of mercapto and imidazole groups to Cu(II) ion in reversed micelles is also studied by ESR and NMR. Results obtained will be discussed with particular attention to the properties of copper metalloenzymes.