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Hot solutions (>80 °C) of N-dodecanoyl- (d- or l-) serine in toluene or water were cooled to room temperature to give gels and were then vitrified for cryo-TEM investigations. Electron micrographs showed multilamellar aggregates with a repetitive density profile of 33 Å as constituents of tubules and vesicles in toluene as well as of twisted ribbons and tubules in water. In water the aggregates were about 10 times longer than those in toluene (up to 10 μm), and platinum shadowing showed inverse twists for ribbons of d- and l-enantiomers. The racemate gave planar platelets in both solvents. It is concluded that the high dielectric constant of bulk water is needed to stabilize the high surface energy of the edges in thin helical ribbons. The trace amounts of water stabilizing the reverse-micellar structures in toluene, on the other hand, do not have an ordering effect on the headgroup arrangement. Curvature is caused by chirality of the monomers (“chiral bilayer effect”) and is effective in both environments.

Zinc octaethylporphin radical cation is shown to form a diamagnetic dimer in methanol-chloroform, 10:1, at low temperatures. At 293°, the AH of dimerization is -17.5 ± 0.4 kcal/mol-1 and AS is -38 ± 1 cal deg-1 mol-1. The rate constant of dimerization, obtained by the temperature-jump method under these conditions, is 1.3 ± 0.2 X 10s M-1 sec-1, and that of dissociation is 5.0 ± 0.7 X 101 23 sec-1. The stability of the dimer is attributed to a novel -' bond formed by overlap of the half-filled orbitals in the face-to-face dimer of porphyrin rings. The nmr and optical spectra agree with this assignment. The oxidation of zinc octaethylporphin in the solid state and the preparation of the solid dimer containing approximately 10 % of the monomeric radical are also described. Bulk magnetic susceptibility measurements, determination of the elementary cell by X-ray spectroscopy, optical, infrared, and mass spectroscopy, as well as elemental analysis are all consistent with the formulation of this solid as (ZnOEP-Br2)2.

chemical shifts. However, the effect we see is too large to have such an origin.5(5) (a) G.

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In aqueous media substituted protoporphyrins and amphiphilic tetraphenylporphyrins aggregate to fibers, tubules, helical ribbons and sheets. Six different types are presented, characterized by the excitonic shifts in the electronic absorption spectra. On excitation, the aggregates from octopusporphyrin show fluorescence and population of the triplet state. In other aggregates fluorescence, triplet state or both are effectively quenched.

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