Chirality and Molecular Order
Naval Research Laboratory
About the Lecture
The importance of chirality, or molecular handedness, has long been recognized in many areas of physics and chemistry. It is well known, for example, that the interactions between chiral molecules change dramatically when one molecule is replaced by its mirror image. This chiral specificity is the basis of a major industry producing chiral drugs. In recent years, researchers have found that chirality also plays other roles—controlling the structure of liquid-crystal phases and the shape of self-assembled supramolecular aggregates. In this talk, we discuss these effects of chirality. We review the wide range of ordered liquid-crystal phases induced by chirality, and consider their significance for liquid-crystal displays. We then present work on self-assembled lipid microstructures developed at the Naval Research Laboratory, and discuss the importance of chirality for these structures. Our overall conclusion is that the concept of chiral order provides one approach for understanding and controlling the structure of materials.
About the Speaker
Jonathan V. Selinger has been a research physicist at the Naval Research Laboratory Center for Bio/Molecular Science and Engineering. Previously, he held research positions at the University of California, Los Angeles and California Institute of Technology. He received a doctorate in physics from Harvard university in 1989, after having received an A.B. Summa Cum Laude and A.M. from the same institution. He is the author of numerous papers regarding chirality and liquid crystal structure.
President Collins called the 2142nd meeting to order at 8:20 p.m. on February 22, 2002. The Recording Secretary read the minutes of the 2141st meeting and they were approved.
The speaker for the evening was Jonathan Selinger, Naval Research Laboratory. The title of his presentation was “Chirality and Molecular Order”.
Optically active compounds which are capable of rotating polarized light were discovered by French scientists in the 1840's. There were two particular acids, tartaric acid and racemic acid, which had the same elemental composition and other physical properties measurable at that time, except that tartaric acid rotated polarized light to the right, and racemic acid did not rotate polarized light. Louis Pasteur observed that crystals of sodium ammonium racemate had two unsymmetric crystal forms one of which matched the crystals of sodium ammonium tartrate. Pasteur separated the two crystal forms by hand, and when he redissolved the separated crystals, those which matched the tartrate salt rotated polarized light in the same way, as did the other crystals but in the opposite direction. Racemic acid was a mixture of equal amounts of left- and right-handed tartaric acid. [The name “racemic” has endured as the general term for such mixtures of equal amounts of right- and left-handed compounds.] This discovery of right- and left-handed forms of chemicals was the first indication that molecules must have a 3-dimensional structure.
In 1893, Lord Kelvin defined the term “chiral” to designate an object that cannot be superimposed on its mirror image; it is asymmetric. An “achiral” object is not chiral; it is symmetric and can be superimposed on its mirror image. This is not an insignificant concept. The sense of smell can distinguish between the asymmetric forms of chiral compounds; right-handed limonene smells like orange, while left-handed smells like lemon. Right-handed carvone smells like caraway, while left-handed carvone smells like mint. The action of some chiral drugs depends on the chirality. One chiral form of Ritalin is pharmacologically active, while its mirror image form is not. One chiral form of thalidomide can prevent nausea, while its mirror image form can cause birth defects.
Liquid crystals are a physical phase intermediate between liquids and crystals that are formed by some compounds. Three forms of liquid crystals are “nematic” in which the layers of molecules are generally aligned with the same orientation, but are not in orderly arrays; “smectic” in which the layers of molecules are in orderly arrays, but are randomly oriented, and “isotropic” in which the layers of molecules are not arrayed or oriented. Although the mirror-image forms of most chiral molecules share the same physical properties, except for their interaction with other chiral molecules or with polarized light, the chirality of some molecules can be manifested in the properties and behavior of the bulk material, in particular chiral molecules that form liquid crystals.
Liquid crystal displays, LCDs, are one economically important application of chirality. Most chiral molecules have an inherent net dipole that interacts with electric fields and the dipole of neighboring molecules. Chiral, liquid crystal molecules do not pack in layers parallel to their neighbors. Instead, each layer packs at a slight twist angle relative to its neighbor, leading to an enhancement of the optical rotation. A cholesteric phase is a nematic phase with a chiral twist between the layers. The size of the twist is such that the pitch between layers with the same orientation is on the order of 1 µm, and as a result, these cholesteric phase liquid crystals are capable of scattering visible light. In LCDs, an applied voltage shifts the dipole tilt of cholesteric, liquid crystal molecules between two polarizing filters, changing the bulk, light scattering behavior of the liquid.
The Naval Research Laboratory has been investigating self-assembling microstructures produced by special chiral lipids. Some of these lipids form cylinders with diameters of about 0.5 µm and lengths from 10 µm to 1 mm. Tubes this size might be used as templates in the fabrication of metal cylinders that could be used for radar absorption, or for controlled release encapsulation. Other cholesteric compounds, like the bile salts that contributed the name to the phase, form intermediate structures like helical ribbons that can condense to tubules. The optical activity signal of the tubules, measured by circular dichroism, is 104 times larger than the signal of the same lipid in solution, confirming that the packing in helical ribbons and tubules depends on the chiral nature of the molecules. The subtle property of chirality can determine the behavior of complex supramolecular systems.
Mr. Selinger kindly answered questions from the floor. President Collins thanked Mr. Selinger for the Society, and welcomed him to its membership. The President made announcements inviting short addresses by Society members, and about the next meeting, parking, and refreshments, and adjourned the 2142nd meeting to the social hour at 9:38 p.m.
Weather: partly cloudy
John S. Garavelli