A number of phosphatidyl choline derivatives containing
trans-azobenzene units in the fatty ester
backbone
have been synthesized and studied in aqueous dispersions both pure and
in the presence of saturated and unsaturated
phospholipids. The structures of the assemblies formed have been
investigated by microcalorimetry, dynamic light
scattering, cryo-transmission electron microscopy, and reagent
entrapment. While many of the mixed phospholipid
dispersions give evidence for the formation of small unilamellar
vesicles, the aqueous dispersions of pure azobenzene
phospholipids (APL's) give evidence for several different structures,
including relatively large plates in at least one
case. The azobenzenes show strong evidence of “H” aggregate
formation both in the pure and mixed dispersions.
The aggregation number has been estimated for several of the
APL's and found to be typically 3 or a multiple
thereof. On the basis of simulations and studies with similar
stilbene phospholipids as well as on the strong induced
circular dichroism signals observed for the aggregate, we infer a
chiral “pinwheel” unit aggregate structure similar
to that found for several aromatics. The azobenzenes in the
aqueous dispersions have been found to photoisomerize
to give cis-rich photostationary states; the
cis-azobenzenes show no evidence for aggregation and no
induced circular
dichroism. The cis-azobenzenes can be isomerized back
to the trans either by irradiation or by thermal paths.
Mixed
aqueous dispersions of trans-APL's with saturated or
unsaturated phospholipids can be prepared which entrap the
fluorescent dye carboxyfluorescein (CF) under conditions where the CF
fluorescence is very low due to self-quenching.
By varying the APL/host phospholipid ratio the azobenzene can be
aggregate, monomer, or dimer. In cases where
the azobenzene is monomer or dimer, irradiation produces complete
isomerization but little “leakage” of CF from
the vesicle interior. In contrast, where the azobenzene is
predominantly aggregate, irradiation results in both
photoisomerization and reagent release. That photoisomerization in
the latter case can result in “catastrophic”
destruction of the vesicle can also be shown by cryo-transmission
electron microscopy.
There are three parameters in Eq. (2) which determine the behavior of the model and these can, within limits, be determined without reference to surface order. We took J 2 Mi as -0.2, quite close to the preferred value of -0.25 suggested in Ref. 7. To illustrate best the contrast between results from the bulk and from the surface, we adjusted J ± so that the calculated points for the bulk order parameter agree with experiment 4 up to the experimental transformation temperature. This gave J 1 /k = 23TK, a value about 7% higher than would have been obtained by the procedure of Ref. 7. Finally we let AE = 2.5^, corresponding to a constant average surface composition.In Fig. 4 we show the average of the order parameters for the two topmost Cu-Au layers, for the two boundary conditions of the surface model, together with the measured surface order and the calculated bulk order as a function of temperature. It appears from the figure that the difference between the observed behavior of the longrange order parameter at the surface and that in the bulk can be explained by a simple model involving only nearest-and next-nearest-neighbor interactions which are the same for both surface and bulk. Furthermore, it is evident from the analysis that consideration of a region of accommodation between surface and bulk is a necessary Considerable interest exists in examining the properties of both organic and inorganic structures which are highly anisotropic and can be visualized as containing one-dimensional chainsmetallic "spines"-surrounded by a matrix whose properties are highly important in determining the transport properties of that system. The or-feature of any model meant to describe the phenomenon of long-range order at a crystal surface.The temperature dependence of the electrical conductivity and Seebeck coefficient for the polymer chain system (SN) X has been measured along the chain axis from 4.2 to 300°K. The data show the system to be metallic over the entire temperature range studied, with a small conductivity maximum at ~33°K.
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