SUPRAMOLECULAR AGGREGATES OF AZOBENZENE PHOSPHOLIPIDS AND RELATED-COMPOUNDS IN BILAYER ASSEMBLIES AND OTHER MICROHETEROGENEOUS MEDIA - STRUCTURE, PROPERTIES, AND PHOTOREACTIVITY

A number of phosphatidyl choline derivatives containing hans-azobenzen e 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 hav e been investigated by microcalorimetry, dynamic light scattering, cry o-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 azoben zene phospholipids (APL's) give evidence for several different structu res, including relatively large plates in at least one case. The azobe nzenes show strong evidence of ''H'' aggregate formation both in the p ure and mixed dispersions. The aggregation number has been estimated f or several of the APL's and found to be typically 3 or a multiple ther eof. On the basis of simulations and studies with similar stilbene pho spholipids as well as on the strong induced circular dichroism signals observed for the aggregate, we infer a chiral ''pinwheel'' unit aggre gate structure similar to that found for several aromatics. The azoben zenes in the aqueous dispersions have been found to photoisomerize to give cis-rich photostationary states; the cis-azobenzenes show no evid ence for aggregation and no induced circular dichroism. The cis-azoben zenes 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 fluores cent dye carboxyfluorescein (CF) under conditions where the CF fluores cence is very low due to self-quenching. By varying the APL/host phosp holipid ratio the azobenzene can be aggregate, monomer, or dimer. In c ases where the azobenzene is monomer or dimer, irradiation produces co mplete isomerization but little ''leakage'' of CF from the vesicle int erior; In contrast, where the azobenzene is predominantly aggregate, i rradiation results in both photoisomerization and reagent release. Tha t photoisomerization in the latter case can result in ''catastrophic'' destruction of the vesicle can also be shown by cryo-transmission ele ctron microscopy.