Cyclic voltammetric studies with gold wire electrodes covered with lipoyl γ-cyclodextrins: positional isomerism effect of the modified cyclodextrins on monolayer quality and electrochemical responses on soluble probes

Four positional isomers of γ-cyclodextrins (γ-CyD) having two lipoyl residues were prepared for investigating the effect of the positional isomerism of the lipoyl residues on the quality of the resulting monolayers deposited on gold wire electrodes. Charges associated with reductive desorption of lipoyl residues from the gold surface revealed that the 6A,6E-isomer (4) formed a nearly defect-free monolayer from its 1 mM solution, whereas the 6A,6C-, 6A,6D-isomers (2 and 3, respectively) covered 86% and 89%, respectively, of the gold surface, and 6A,6B-isomer (1) covered only 60% of the gold surface. The cyclic voltammetric responses of Fe(CN)63− on the 1–4-modified electrodes was partly blocked by the 1–4 monolayers, and the largest inhibition was achieved by 1 monolayer. The cyclic voltammetric responses of ferrocenecarboxylic acid (FCA) was slightly affected by the presence of 1–4 monolayers. Ursodeoxycholic acid, which is an excellent guest for γ-CyD, reduced the anodic current of FCA at the 1–4-modified electrodes, and the degree in the reduction caused by the monolayers was in the order of 4>3>2=1. This order indicated that the cavity of 4 was the most active ones to bind a guest in a solution phase, while the cavities of 1 and 2 was less active. These results indicate that in monolayer film, the positional isomerism of disubstituted-CyDs actually influenced the electrode responses.