MULTI-HEME SELF-ASSEMBLY IN PHOSPHOLIPID-VESICLES

The use of weak, intermolecular forces to orchestrate the contruction of multicomponent systems in membranes has significant implications in diverse areas of chemistry, biology, and medicine. We describe here t he construction and characterization of multi-heme molecular ensembles in phospholipid vesicles. A trianionic zinc porphyrin was designed to bind cytochrome c at the membrane surface, while being anchored to a membrane spanning manganese porphyrin in the membrane interior via a t erminal imidazole. The structure of the construct was probed by fluore scence and UV spectroscopy. Cytochrome c formed a stoichiometric 1:1 c omplex with the anionic porphyrin with a high binding constant (K-a ap proximate to 5 x 10(6) M(-1)). The ligation of the imidazole to the ma nganese porphyrin was confirmed by UV spectral changes. Large differen ces in the fluorescence quenching of Zn porphyrins with and without th e terminal imidazole were observed upon their insertion into vesicles containing the Mn porphyrin. These spectroscopic observations were con sistent with the formation of a ligated, ternary system consisting of the Mn(II) porphyrin, the imidazole-tailed zinc porphyrin acting as a bridge, and the surface associated cytochrome c. The nature of the bin ding of cytochrome c at the membrane-water interface was investigated by Langmuir-Blodgett (LB) and differential scanning calorimetric (DSC) techniques. The data obtained suggested that the protein was surface bound with minimal penetration into the membrane. LB studies were also used to probe the orientation of the trianionic porphyrin moiety at t he membrane surface, and an edge-on orientation was inferred from the data. The formation of a stable vesicular system was confirmed by the formation of well-defined DSC thermograms. Phase separation was observ ed at high porphyrin:lipid ratios. Electron transfer from the Mn(II) i n the membrane interior to the surface bound ferricytochrome c was inv estigated, as a probe both for spatial definition of the ensemble and for the elucidation of electron transfer mechanism in the genre of wea kly coupled systems over large distances. Trianionic Zn porphyrins wit h varying tether lengths (12, 8, and 4 carbons) were used. The electro n transfer rate was found to be first order and independent of the tet her length, indicative of medium mediated electron transfer via multip le pathways. Comparison to similar systems in the literature yielded a predicted distance of similar to 23 Angstrom between the Mn and Fe ce nters in DMPC/DPPC vesicles. This distance suggested that the protein was surface bound to the membrane and separated from the Mn porphyrin by the thickness of one leaflet of the phospholipid bilayer. In thinne r DLPC vesicles the predicted increase in the electron transfer rate w as observed. Additionally, electron transfer was observed to be bimole cular in systems where trianionic porphyrins lacking the imidazole tet her were used to recruit the cytochrome c.