Tubules, Rods, Fibers, and Related Self-Assembled Structures

6 Tubules, Rods, Fibers, and Related Self-Assembled Structures 

The appearance of tubular myelin-like structures in swollen lecithin was observed by light microscopy well before the systematic investigation of liposomes [351-352]. Similarly, it was also demonstrated some time ago that the addition of calcium ions converted phospholipid liposomes to cochleate cylinders [353]. Subsequent studies have, however, revealed that the system is extremely complex. For example, examination of the phase-transition behavior of synthetic sodium di-n-dodecyl phosphate [(C12H2sO)2PO2Na+ or NaDDP] and calcium di-n-dodecyl phosphate [Ca(DDP)2] showed the presence of many diverse structures [354]. In particular, hydrated NaDDP crystals were shown to form lyotropic liquid-crystalline phases which transformed, upon heating to 50 °C, to myelin-like tubes. Structures of the tubes formed were found 

The realization that tubules may be formed on temperature reduction of polymerized SUVs, prepared from polymerizable diacetylenic phosphatidylcholines (21 where n = 7-16 and m = 5-11), represented a major breakthrough in obtaining the desired supramolecular structure [355-360]. In the initial experiments, 0.4- to 1.0-pm-diameter and 10- to 1000-pm-long tubules were prepared by the gradual lowering of the temperature (to about 38 °Q of 21 (m = 8, n = 9) SUVs [358]. The walk of the tubules had thickness of 10-40 nm and were coated by spiral ripples and helical bilayer strips. Many tubules contained trapped SUVs. Polymerization of the acetylenic moieties greatly enhanced the mechanical and thermal stabilities of the tubules [355-360]. 

More recently, tubules have been produced by precipitation from solution of 21 in an organic solvent at temperatures 10-30 °C below the phase transition of 21 (Tm of 21 (m = 8, n = 9) = 43 °C) that is, via a route which did not involve 

Tubules have also been prepared by swelling thin films of polymerizable diacetylenic phosphatidylhydroxyethanol (choline functionally in 21 is replaced by hydroxyethanol) in aqueous metal ion solutions above the phase transition temperature of the lipid. Various cylindrical structures were observed upon swelling the lipid in the presence of mono- and divalent cations. In contrast, no definable microstructures were noted in the absence of cations [362], 

---