Research Overview

Many fields in molecular biology, the physical sciences, and materials engineering consider a detailed understanding of the fundamental mechanisms by which synthetic and biological molecules self-assemble to be essential if scientific and technological progress is to be made. In recent years, 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 enable entirely new avenue for the measurement of forces on the biological scale. Given the importance of these structures, I am working on experimentally probing the kinetics and energetics of helix formation. I am also working on theoretical predictions and experimental measurements of helix elastic properties.

In the early nineties, helical ribbons were discovered in model bile systems and were shown to be metastable intermediaries in the process of cholesterol crystallization. These helical ribbons formed with one of two distinctive pitch angles: either 11 degrees or 54 degrees. Until recently, the formation of these particular types of helical ribbons was thought unique to the model bile systems. To examine this idea, I have studied a series of enantiomerically pure systems composed of a bile salt or nonionic detergent, a phosphatidylcholine or a fatty acid (or mixture of fatty acids), and a steroid analog of cholesterol in water. In almost all systems studied, helical ribbons with the same two distinctive pitch angles found in model bile systems were observed. The majority of the helical ribbons were right-handed, although some left-handed helices were found. Additionally, a small number of helices with pitch angles between 30 and 47 degrees were discovered in some systems. All of the helical ribbons, notwithstanding their pitch, had diameters in the range between 3 and 70 mm, with lengths varying between 5 and ~200 mm. These findings raise serious doubts concerning the validity of present theories concerning the formation of helical ribbons, indicating that the structures may be crystalline rather than liquid crystal in nature and that molecular chirality may not be the driving force behind the formation of the helices.

My colleagues and I have also performed experiments to determine the elastic properties of low pitch helical ribbons formed in Chemically Defined Lipid Concentrate (CDLC). During the relaxation experiments, these helices exhibited Hookean behavior over a large range of extensions. These results have been bolstered by preliminary measurements of the force versus extension curve using precalibrated silicon cantilevers as force probes. The forces involved in deformation of low pitch helices to up to 200% of their original axial length have been found to be in the 0.25-1.0 nN range. This Hookean behavior and the appropriate force range makes helices ideal for use as biological force probes since they behave like perfect springs even under deformations of several hundred microns. In addition to this surprisingly Hookean behavior, my we have discovered a novel tension-induced straightening transition, impossible for ordinary steel springs. When helices are extended beyond a critical value, part of the helix unwinds leaving distinct helical and straight sections existing in equilibrium with each other. When the tension is removed, the helix recovers fully and shows no sign of damage, even after repeated experimentation. The discovery of this phase transition has provided us with a wealth of information including the determination of possible microscopic symmetries of the ribbons as well as providing us with a means for measuring some of the other physical parameters relevant to current models for these structures. Probing these fascinating elastic properties is currently the best hope for more fully illuminating the microscopic nature of the helical ribbons and the driving force(s) behind their formation. My colleagues and I are currently working on two new theories: a new crystalline model that describes the existence of helical ribbons, and a phase transition theory that describes the tension-induced phase separation of low pitch helices into straight and helical parts.

Publications

1. B. Smith, Y. V. Zastavker, G. B. Benedek, "The Tension-Induced Straightening Transition of Self-Assembled Helical Ribbons", Phys. Rev. Lett. 87: 278101-(1-4) (2001).
   
   
2. Y. V. Zastavker, N. Asherie, A. Lomakin, J. Pande, J. M. Donovan, J. M. Schnur, and G. B. Benedek, "Self-Assembly of Helical Ribbons", Proc. Antl. Acad. Sci. USA 96:7883 (1999).
   
   
3. M. J. Maryanski, Y. V. Zastavker, and J. C. Gore, "Radiation Dose Distributions in Three-Dimensions from Tomographic Optical-Density Scanning of Polymer Gels II: Optical Properties of the BANG Polymer Gel", Phys. Med. Biol. 41: 1 (1996).
   
   
4. A. Husmann, D. S. Jin, Y. V. Zastavker, T. F. Rosenbaum, X. Yao, and J. M. Honig, "Dynamical Signature of the Mott--Hubbard Transition in Ni(S,Se)₂", Science 274: 1874 (1996).
   
   
5. D. S. Jin, A. Husmann, Y. V. Zastavker, T. F. Rosenbaum, X. Yao, and J. M. Honig, "Probes of Quantum Critical Behavior at the Anderson - Mott Transition", Phys. Rev. Lett. (in press).

Talks and Posters

September, 2002 -- "Biological Self-Assemblies: Microsprings", Invited Colloquium Lecture Presentation, Bryn Mawr College, Bryn Mawr, PA

May, 2002 -- "The Status of Women in Physics -- What, Why, and How to Change", Brown Bag Lunch Lecture, Wellesley College, Wellesley, MA

April, 2002 -- "The Status of Women in Physics -- What, Why, and How to Change", NARST Annual Conference Poster, New Orleans, LA

July, 2000 - "DNA is Not the Only Helix in Town or a Story of Crafty Microsprings", "Research Experiences for Undergraduates", Wellesley College, Wellesley, MA

June, 2000 - "Self-Assembly of Helical Structures", "Biomaterials and Complex Fluids Workshop", University of Massachusetts, Amherst, MA

April, 1999 - "Formation of Helical Ribbons as a General Phenomenon", Office of Naval Research, Washington D.C.

October, 1999 -- Poster , Physics Department Poster Session, MIT, Cambridge, MA

September, 1999 -- Poster , "Materials Day", Materials Processing Center, MIT, Cambridge, MA

November, 1997 -- Poster , Physics Department Poster Session, MIT, Cambridge, MA

October, 1997 -- Poster , "Biomaterials and Complex Fluids Workshop", Brandeis University, Waltham, MA