Towards redox active liquid crystalline phases of lipids: a monoolein/water system with entrapped derivatives of ferrocene

The phase and electrochemical behavior of the aqueous mixtures of monoolein (MO) and synthetic ferrocene (Fc) derivatives containing long alkyl chains -(Z)-octadec-9-enoylferrocene (1), (Z)-octadecen-9-ylferrocene (2), and ferrocenylmethyl (Z)-octadec-9-enoate (3)-were studied. At low hydration, the reversed micelles (L-2 phase) and cubic Q(230) phase of MO can accommodate relatively high amounts ( > 6 wt.%) of the Fc-derivative 2, whereas at high hydration, the pseudoternary cubic phase Q(224) is destabilized even at about 2 wt.% of this Fc. Increasing the Fc-derivative content induces L-alpha L-2 and L-alpha --> reversed bicontinuous cubic phase (Q(II)) H-II transitions depending upon hydration. A rough study of the MO system containing compounds I and 3 indicates very similar phase behavior to that of the MO/2/H2O system. Compound 2 apparently has no effect on the lipid monolayer thickness in the pseudoternary L-alpha, H-II and Q(II) liquid crystalline phases of MO. Within a 3D-structure of the Q224 phase, derivatives 1-3 exhibit electrochemical activity on the gold electrode. The one-electron redox conversion processes are electro-chemically quasi-reversible and controlled by diffusion. The values of apparent diffusion coefficient (D-app) and heterogeneous electron-transfer rate constant (k(s)) of Fcs are significantly lower in the cubic,phase matrix when compared to the acetonitrile solution. By contrast, the MO H-II phase with entrapped Fc-derivatives does not exhibit electrochemical activity on the electrode surface. It is suggested that the diffusional anisotropy and/or localized aggregation of compounds 1-3 within a 2D-structure of the H-II phase account(s) for the latter observation. (C) 2003 Elsevier Science Ireland Ltd. All rights reserved.