ABSTRACT
Helical ribbons with pitch angles of either 11 degrees or 54 degrees self-assemble in a wide variety of quaternary surfactant-phospholipid/fatty acid-sterol-water systems. By elastically deforming these helices, we examined their response to uniaxial forces. Under sufficient tension, a low pitch helix reversibly separates into a straight domain with a pitch angle of 90 degrees and a helical domain with a pitch angle of 16.5 degrees. Using a newly developed continuum elastic free energy model, we have shown that this phenomenon can be understood as a first order mechanical phase transition.
Subject(s)
Models, Theoretical , Molecular Conformation , DNA/chemistry , Fatty Acids/chemistry , Phospholipids/chemistry , Sterols/chemistry , Water/chemistryABSTRACT
The self-assembly of helical ribbons is examined in a variety of multicomponent enantiomerically pure systems that contain a bile salt or a nonionic detergent, a phosphatidylcholine or a fatty acid, and a steroid analog of cholesterol. In almost all systems, two different pitch types of helical ribbons are observed: high pitch, with a pitch angle of 54 +/- 2 degrees, and low pitch, with a pitch angle of 11 +/- 2 degrees. Although the majority of these helices are right-handed, a small proportion of left-handed helices is observed. Additionally, a third type of helical ribbon, with a pitch angle in the range 30-47 degrees, is occasionally found. These experimental findings suggest that the helical ribbons are crystalline rather than liquid crystal in nature and also suggest that molecular chirality may not be the determining factor in helix formation. The large yields of helices produced will permit a systematic investigation of their individual kinetic evolution and their elastic moduli.
