Cation recognition by self-assembled monolayers of oriented helical peptides having a crown ether unit

Cation recognition of self-assembled monolayers (SAMs) of helical peptides having a crown ether unit was investigated by the impedance spectroscopy and cyclic voltammetry. Lipo-(Ala-Aib)₈-Ala-Cr and Boc-Glu(Cr)-(Ala-Aib)₈-Lipoa (Lipo, Lipoa, and Cr represent lipoic acid, lipoamide, and amidobenzo-18-crown-6, respectively) were synthesized and the helix SAMs were prepared. The peptides having a crown ether unit formed SAMs oriented nearly vertically to the substrate. The capacitance of the Lipo-(Ala-Aib)₈-Ala-Cr SAM changed specifically with the addition of cations, and the binding constants of the SAM were larger than those of the crown ether in aqueous solution because of a large dipole moment of the helical peptide. In the case of the Boc-Glu(Cr)-(Ala-Aib)₈-Lipoa SAM, the cation binding to the SAM showed a drastic decrease in the peak current of the cyclic voltammetry around 1O^-5M of K⁺ ion. In either capacitance measurement or cyclic voltammetry, the helical peptide SAM played an important role in the sensitive response to cations.