Phospholipid-linked quinones-mediated electron transfer on an electrode modified with lipid bilayers

Phospholipid-linked naphthoquinones separated by spacer methylene groups (C(n)), PE-C(n)-NQ (n=0, 5, 11), were synthesized to investigate the quinone-mediated electron transfers on a glassy carbon (GC) electrode covered with phospholipids membrane. The PE-C(n)-NQ could be incorporated in lipid bilayer composed of phosphatidylcholine and exhibited characteristic absorption spectral change corresponding to their redox state, quinone/hydroquinone. The cyclic voltammogram of PE-C(n)-NQ-containing lipid bilayer modified on a GC electrode indicated a set of waves corresponding to the consecutive two-electron and two-proton transfer reduction of the quinone moiety. The peak currents of PE-C(n)-NQ as a function of temperature showed a sharp break point in the current-temperature behavior, reflecting the gel-fluid phase transition. The shape of the cyclic voltammograms changed with the pH of the buffer solution. Below pH 6 the first step of the reduction of quinone was a monoprotonation of quinone, whereas above pH 10 the first step of the oxidation was a monodeprotonation of hydroquinone. This indicates that reaction sequences of quinone/hydroquinone were different with the change of the pH. These results showed that the PE-C(n)-NQ exhibited electron transfer associated with proton transfer in the lipid membranes, depending on the diffusivity of the redox species in the membrane and pH. Interestingly, less effect of the number of methylene of the spacer group on the peak currents was observed. Comparison of manganese porphyrin-mediated electron transfer that depends on the spacer methylene lengths [M. Nango, T. Hikita, T. Nakano, T. Yamada, M. Nagata, Y. Kurono, T. Ohtsuka, Langmuir 14 (1998) 407] is made.