Quinone-functionalized liposomes can be prepared by the sonic dispersion of a phosphatidylcholine anthraquinone (DPPC-AQ) with simple phospholipids such as DPPC (at 52 "C) and DOPC (at room temperature). These small (ca. 25-30-nm diameter), unilamellar liposomes typically contain 4-1 2 mol % phospholipid quinone, which can be reduced and reoxidized by solution reagents. S20,2-rapidly and completely reduces the liposome-bound quinones to the hydroquinone form; Fe(CN)65 rapidly reoxidizes the hydroquinone. BH4-reduces only those quinones situated on the outer surface of the liposome. The rates of redox reactions of the liposome-bound quinones depend on temperature and solution reactant but not on mol % DPPC-AQ in the liposomes or the quinone concentration. The rate law for S202-reduction of DPPC-AQ/DOPC,indicates the presence of two kinetically distinct forms of DPPC-AQ. Comparison with the corresponding homogeneous rate constant suggests the identities of the two forms. Manipulation of the transmembrane, distribution of DPPC-AQ (72-9895 Q,,d is demonstrated by using various phospholipid compositions. Liposomes prepared from the anthracene-functionalized phospholipid DPPC-AN provide a measure of the permeability of the functionalized liposomes.'H NMR spectroscopy (500-MHz) reveals the structure and composition of the quinone-functionalized DPPC liposomes.Nuclear Overhauser experiments reveal that the anthraquinone "head group" of DPPC-AQ in DPPC liposomes resides near the hydrophobic-hydrophilic interface and does not extend into solution. Assimilation of these various results leads to a simple, consistent description of these novel biomimetic systems. Applicability of these and related systems as simple, chemical models for respiratory and photosynthetic energy transduction is discussed.The phospholipid bilayer that comprises biological membranes* is an attractive target for biophysical and biomimetic studies. The ease with which liposomes (closed bilayer membranes) can be (1) Present address: Mail Zone U3E. Monsanto Agricultural Co., 800 N. prepared has aided significantly in these endeavors. Numerous chemical and physical studies4 of liposomes have revealed their(2) (a) Fendler, J. H .
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