STRUCTURALLY DEFINED REDOX ASSEMBLIES - MOLECULAR-LEVEL VIEW OF PHOSPHOLIPID QUINONE MONOLAYERS AT THE AIR WATER AND GOLD WATER INTERFACES

Langmuir-Blodgett techniques have been employed to prepare well-defined, redox-active monolayers of an anthraquinone-functionalized dipalmitoylphosphatidylcholine, DPPC-AQ, and mixed monolayers of DPPC-AQ and DPPC at the air-water and gold-water interfaces. The pi-A isotherms of the mixed monolayers indicate the formation of a eutectic composition near 25 mol % DPPC-AQ. A comparison of the Langmuir film behavior of DPPC, DPPC-AQ, and the anthracene analogue DPPC-AN indicates that the bulky anthraquinone group reorients from the hydrophobic region of the monolayer in expanded (LE) films to the aqueous subphase in condensed (LC) films. The extended molecular conformation is also evident in supported monolayers ("heads-down" DPPC-AQ on gold) from ellipsometric measurements. The thickness of a condensed Au/DPPC-AQ film is ca. 9 angstrom greater (36 angstrom vs 27 angstrom) than the corresponding DPPC monolayer. Analysis of the ellipsometric data suggests that DPPC-AQ transfers to the Au surface in an extended conformation at all accessible transfer pressures. The cyclic voltammetric response of the supported monolayers immersed in aqueous perchlorate electrolyte is well-defined and nearly ideal. Subtle voltammetric features of the various monolayers in perchlorate electrolyte are enhanced in aqueous chloride and p-toluenesulfonate electrolytes, especially in condensed films. Such behavior is further exaggerated in trilayers. These voltammetric features are explained in terms of the supramolecular architecture of the monolayer.