Twisting molecules by brute force: A top-down approach

Molecules that are twisted are ubiquitous in nature, and have important consequences in biology, chemistry, physics and medicine. Some molecules have unique and technologically useful optical properties; the medicinal properties of drugs depend on the direction of the twist; and within us -- think of the double helix -- twisted DNA can interact with different proteins.

This twisting is called chirality and researchers at Case Western Reserve University have found they can use a macroscopic blunt force to impose and induce a twist in an otherwise non-chiral molecule.

Their new "top-down" approach is described in the Dec. 2 issue of Physical Review Letters.

"The key is that we used a macroscopic force to create chirality down to the molecular level," said Charles Rosenblatt, professor of physics at Case Western Reserve and the senior author on the paper. Rosenblatt started the research with no application in mind. He simply wanted to see if it could be done -- essentially scientific acrobatics.

But, he points out, since antiquity chirality has played a role in health, energy, technology and more -- but until now, chirality always has been a bottom-up phenomenon. This new top-down approach, if it can be scaled up, could lead to custom designed chirality -- and therefore desired properties -- in all kinds of things.

Rosenblatt worked with post-doctoral researcher Rajratan Basu, graduate student Joel S. Pendery, and professor Rolfe G. Petschek, of the physics department at Case Western Reserve, and Chemistry Professor Robert P. Lemieux of Queen's University, Kingston, Ontario.

Chirality isn't as simple as a twist in a material. More precisely, a chiral object can't be superimposed on its mirror image. In a "thought experiment," if one's hand can pass through a mirror (like Alice Through the Looking Glass), the hand cannot be rotated so that it matches its mirror image. Therefore one's hand is chiral.

Depending on the twist, scientists define chiral objects as left-handed and right-handed. Objects that can superimpose themselves on their mirror image, such as a wine goblet, are not chiral.

In optics, chiral molecules rotate the polarization of light -- the direction depends on whether the molecules are left-handed or right-handed. Liquid crystal computer and television screen manufacturers take advantage of this property to enable you to clearly see images from an angle.

In the drug industry, chirality is crucial. Two drugs with the identical chemical formula have different uses. Dextromethorphan, which is right-handed, is a cough syrup and levomethorphan, which is lefthanded, is a narcotic painkiller.

The reason for the different effects? The drugs interact differently with biomolecules inside us, depending on the biomolecules' chirality.

After meeting with Lemieux at a conference, the researchers invented a method to create chirality in a liquid crystal at the molecular level.

They treated two glass slides so that cigar-shaped liquid crystal molecules would align along a particular direction. They then created a thin cell with the slides, but rotated the two alignment directions by approximately a 20 degree angle.

The 20-degree difference caused the molecules' orientation to undergo a right-handed helical rotation, like a standard screw, from one side to the other. This is the imposed chiral twist.

The twist, however, is like a tightened spring and costs energy to maintain. To reduce this cost, some of the naturally left-handed molecules in the crystal became right-handed. That's because, inherently, right-handed molecules give rise to a macroscopic right-handed twist, Rosenblatt explained. This shift of molecules from left-handed to right-handed is the induced chirality.

Although the law of entropy suggests there would be nearly identical numbers of left-handed and right-handed molecules, in order to keep total energy cost at a minimum, the right-handed molecules outnumbered the left, he said.

To test for chirality, the researchers applied an electrical field perpendicular to the molecules. If there were no chirality, there would be nothing to see. If there were chirality, the helical twist would rotate in proportion to the amount of right-handed excess.

They observed a modest rotation, which became larger when they increased the twist.

"The effect was occurring everywhere in the cell, but was strongest at the surface," Rosenblatt said.

Scientists have built chirality into optical materials, electrooptic devices, and more by starting at the molecular level. But the researchers are not aware of other techniques that use a macroscopic force to bring chiralty down to molecules.

The researchers are continuing to investigate ways this can be done.

Story Source:

The above story is reprinted from materials provided by Case Western Reserve University.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Rajratan Basu, Joel Pendery, Rolfe Petschek, Robert Lemieux, Charles Rosenblatt. Macroscopic Torsional Strain and Induced Molecular Conformational Deracemization. Physical Review Letters, 2011; 107 (23) DOI: 10.1103/PhysRevLett.107.237804

New top-down strategy of identifying proteins could lead to early detection of disease

A team led by Northwestern University chemical biologist Neil Kelleher has developed a new "top-down" method that can separate and identify thousands of protein molecules quickly. Many have been skeptical that such an approach, where each protein is analyzed intact instead of in smaller parts, could be done on such a large scale.

The promise of a top-down strategy is that the molecular data scientists do collect will be more closely linked to disease.

"Accurate identification of proteins could lead to the identification of biomarkers and early detection of disease as well as the ability to track the outcome of treatment," Kelleher said. "We are dramatically changing the strategy for understanding protein molecules at the most basic level. This is necessary for the Human Proteome Project -- the mapping of all healthy human proteins in tissues and organs -- to really take off."

Kelleher is the Walter and Mary E. Glass Professor of Molecular Biosciences and professor of chemistry in the Weinberg College of Arts and Sciences. He also is director of the Proteomics Center of Excellence and a member of the Robert H. Lurie Comprehensive Cancer Center of Northwestern University.

Kelleher says his approach is conceptually simple. "We take proteins -- those swimming around in cells -- and we measure them," he said. "We weigh proteins precisely and identify them directly. The way everyone else is doing it is by digesting the proteins, cutting them up into smaller bits called peptides, and putting them back together again. I call it the Humpty Dumpty problem."

The new strategy, Kelleher says, solves the "protein isoform problem" of the "bottom-up" approach where the smaller peptides often do not map cleanly to single human genes. The study will be published Oct. 30 by the journal Nature.

The top-down method can accurately identify which gene produced which protein. The bottom-up method is only 60 to 90 percent accurate in identifying proteins precisely.

"We need to define all the protein molecules in the human body," Kelleher said. "First, we need a map of healthy protein forms, which will become a highly valuable reference list for understanding damaged and diseased forms of proteins. Our technology should allow us to get farther down this road faster."

In the first large-scale demonstration of the top-down method, the researchers were able to identify more than 3,000 protein forms created from 1,043 genes from human HeLa cells.

Their goal was to identify which gene each protein comes from -- to provide a one-to-one picture. They were able to produce this accurate map of thousands of proteins in just a few months.

The researchers also can produce the complete atomic composition for each protein. "If a proton is missing, we know about it," Kelleher said.

One gene they studied, the HMGA1 gene associated with premature aging of cells, produces about 20 different protein forms.

Kelleher's team developed a four-dimensional separation system that uses separations and mass spectrometry to measure the charge, mass and weight of each protein as well as how "greasy" a protein is. The software the researchers developed to analyze the data during years of work prior to the study proved critical to the success of the top-down method.

"If you want to know how the proteins in cancer really work and change, top-down mass spectrometry is getting to the point where it can be part of the discussion," Kelleher said.

"Analyzing the entire set of proteins expressed in a cell presents a continuing and significant technical challenge to the field of proteomics," said Charles Edmonds, who oversees proteomics grants at the National Institute of General Medical Sciences of the National Institutes of Health. "By combining multiple fractionation technologies with mass spectrometry, Dr. Kelleher and colleagues have demonstrated more than an order of magnitude improvement in proteome coverage. This is a great start."

More information: The title of the paper is "Mapping Intact Protein Isoforms in Discovery Mode Using Top-Down Proteomics."

Provided by Northwestern University (news : web)