Self-assembly of helical ribbons from chiral amphiphiles

Abstract

The study of the self-assembly of helical structures has been motivated by their newly found biological and technological importance. In many systems, helical ribbons are precursors to the formation of tubules, which may be used in the controlled release of drugs or as templates for micron scale electronic components. Used as springs, helical ribbons open up an entirely new avenue for the measurement of forces on the biological scale. Given the importance of these structures, a series of experiments to probe the kinetics and energetics of helix formation has been performed. Theoretical interpretation and experimental measurements of helix elastic properties have also been performed. It was shown that the formation of helical ribbons of pitch angles of 11 and 54ʻ, previously thought to be a property unique to model bile systems, is a general phenomenon of quaternary sterol systems composed of a bile salt or nonionic detergent, a phosphatidylcholine or a (mixture of) fatty acid(s), and a steroid analog of cholesterol in water. The majority of helical ribbons were right-handed; but some left-handed helices have been found. Additionally, a small number of helices with pitch angles between 30 and 47ʻ were found in some systems. The elastic properties of the low pitch helical ribbons in Chemically Defined Lipid Concentrate were studied via relaxation experiments and measurements of force versus extension curves using silicon cantilevers as force probe. The helices exhibited linear behavior over a large range of extensions (up to 200% of helix original axial length). The forces involved in the deformation of low pitch helices have been found to be in the 0.25-1.0 nN range making them ideal for use as biological force probes.

(cont.) Additionally, a novel tension-induced reversible straightening transition of the helical ribbons has been observed: when a helix is extended beyond a critical value, part of it unwinds leaving separate straight and helical sections in equilibrium with each other. Probing these fascinating elastic properties is currently the best hope for more fully illuminating the microscopic nature of helical ribbons and the driving force behind their formation.