Redox and structural properties of quinone-functionalized phosphatidylcholine liposomes
作者:Min D. Liu、Dale H. Patterson、Claude R. Jones、Charles R. Leidner
DOI:10.1021/j100157a065
日期:1991.2
Quinone-functionalized liposomes can be prepared by the sonic dispersion of a phosphatidylcholine anthraquinone (DPPC-AQ) with simple phospholipids such as DPPC (at 52-degrees-C) and DOPC (at room temperature). These small (ca. 25-30-nm diameter), unilamellar liposomes typically contain 4-12 mol % phospholipid quinone, which can be reduced and reoxidized by solution reagents. S2O4(2-) rapidly and completely reduces the liposome-bound quinones to the hydroquinone form; Fe(CN)6(3-) 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 S2O4(2-) reduction of DPPC-AQ/DOPC, k(obs) = k1k2[S2O4(2-]/(k-1 + k2[S2O4(2-)]), 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 (72-98% Q(outer)) 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-1 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.