Circular dichroism spectrophotometric study of [Mo 2(μ-O) 2(O) 2( R-cysteinato) 2] 2− in aqueous micellar solutions

Abstract

[Mo 2(μ-O) 2(O) 2( R-cys) 2] 2− ( R-cys( R)-cysteinate(2−)) undergoes significant change in circular dichroism (CD) spectra on addition of cationic surfactants such as cetyltrimethylammonium bromide (CTAB), decyltrimethylammonium bromide (DTAB) and cetylpyridinium chloride (CPC). The CD dependence on [CTAB] occurs in two steps. At [CTAB]<cmc (critical micelle concentration), the change is interpreted by considering that R-cys partly liberates from MO 2(μ-O) 2(O) 2 (2+). The change observed at [DTAB]>cmc is accounted for by the partial dissocation of a −COO − arm, while the complete liberation of the ligand is suppressed. The addition of a anionic surfactant and simple salts do not cause any change in the CD spectra. A similar CD change is observed for [Mo 2(μ-O) 2(O) 2( S-pen) 2] 2− ( S-pen=( S)-penicillaminate(2−)), which is interpreted similarly. The sulfide bridged R-cys complexes, [Mo 2(μ-O)(μ-S)(O) 2( R-cys) 2] 2− and [Mo 2(μ-S) 2(O) 2( R-cys),] 2−, also undergo even more drastic CD changes on addition of CTAB. By contrast, [Mo 2(μ-O) 2(O) 2( R-pdta)] 2− ( R-pdta=( R)-1,2-propylenedi- aminetetraacetate(4−)) which has a firmly coordinated hexadentate ligand does not show such CD changes on addition of CTAB.