Chemists become molecular sculptors, synthesizing tiny, molecular traps

 Using clever but elegant design, University at Buffalo chemists have synthesized tiny, molecular cages that can be used to capture and purify nanomaterials.

Sculpted from a special kind of molecule called a "bottle-brush molecule," the traps consist of tiny, organic tubes whose interior walls carry a negative charge. This feature enables the tubes to selectively encapsulate only positively charged particles.

In addition, because UB scientists construct the tubes from scratch, they can create traps of different sizes that snare molecular prey of different sizes. The level of fine tuning possible is remarkable: In the Journal of the American Chemical Society, the researchers report that they were able to craft nanotubes that captured particles 2.8 nanometers in diameter, while leaving particles just 1.5 nanometers larger untouched.

These kinds of cages could be used, in the future, to expedite tedious tasks, such as segregating large quantum dots from small quantum dots, or separating proteins by size and charge.

"The shapes and sizes of molecules and nanomaterials dictate their utility for desired applications. Our molecular cages will allow one to separate particles and molecules with pre-determined dimensions, thus creating uniform building blocks for the fabrication of advanced materials," said Javid Rzayev, the UB assistant professor of chemistry who led the research.

"Just like a contractor wants tile squares or bricks to be the same size so they fit well together, scientists are eager to produce nanometer-size particles with the same dimensions, which can go a long way toward creating uniform and well-behaved materials," Rzayev said.

To create the traps, Rzayev and his team first constructed a special kind of molecule called a bottle-brush molecule. These resemble a round hair brush, with molecular "bristles" protruding all the way around a molecular backbone.

After stitching the bristles together, the researchers hollowed out the center of each bottle-brush molecule, leaving behind a structure shaped like a toilet paper tube.

The carving process employed simple but clever chemistry: When building their bottlebrush molecules, the scientists constructed the heart of each molecule using molecular structures that disintegrate upon coming into contact with water. Around this core, the scientists then attached a layer of negatively charged carboxylic acid groups.

To sculpt the molecule, the scientists then immersed it water, in effect hollowing the core. The resulting structure was the trap -- a nanotube whose inner walls were negatively charged due to the presence of the newly exposed carboxylic acid groups.

To test the tubes' effectiveness as traps, Rzayev and colleagues designed a series of experiments involving a two-layered chemical cocktail.

The cocktail's bottom layer consisted of a chloroform solution containing the nanotubes, while the top layer consisted of a water-based solution containing positively charged dyes. (As in a tequila sunrise, the thinner, water-based solution floats on top of the denser chloroform solution, with little mixing.)

When the scientists shook the cocktail for five minutes, the nanotubes collided with and trapped the dyes, bringing the dyes into the chloroform solution. (The dyes, on their own, do not dissolve in chloroform.)

In similar experiments, Rzayev and his team were able to use the nanotubes to extract positively charged molecules called dendrimers from an aqueous solution. The nanotubes were crafted so that dendrimers with a diameter of 2.8 nanometers were trapped, while dendrimers that were 4.3 nanometers across were left in solution.

To remove the captured dendrimers from the nanotubes, the researchers simply lowered the pH of the chloroform solution, which shuts down the negative charge inside the traps and allows the captured particles to be released from their cages.

The research on nanotubes is part of a larger suite of studies Rzayev is conducting on bottle-brush molecules using a National Science Foundation CAREER award. His other work includes the fabrication of bottle-brush-based nanomembranes that could be adapted for water filtration, and the assembly of layered, bottle-brush polymers that reflect visible light like the wings of a butterfly do.

Story Source:

The above story is reprinted from materials provided by University at Buffalo.

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Journal Reference:

Kun Huang, Javid Rzayev. Charge and Size Selective Molecular Transport by Amphiphilic Organic Nanotubes. Journal of the American Chemical Society, 2011; 133 (42): 16726 DOI: 10.1021/ja204296v

Note: If no author is given, the source is cited instead.

New way of synthesizing organic chemicals mimics nature

Organic chemists have found a new way of synthesizing multiple complex organic molecules that until now have needed to be synthesized using time-consuming methods. The new strategy, which mimics natural biosynthesis methods, could provide a way to speed up the synthesis of chemicals for use in the laboratory and for testing for possible therapeutic effects.

Chemists can synthesize almost any complex organic chemical in small quantities in the laboratory, but the process can be extremely time consuming and expensive, and difficult or impossible to translate to large-scale production. It is also difficult to synthesize families or related molecules using traditional methods because each has to be synthesized individually.

The new method was developed by researchers from the Merck Center for Catalysis at Princeton University in the U.S., led by organic chemist David MacMillan. The strategy combines two techniques that mimic methods used by living systems to synthesize organic molecules (those based on chains of carbon atoms or carbon rings): organocascade catalysis and collective synthesis. Combining these two techniques for the first time allowed MacMillan and colleagues to considerably speed up the synthesis of a group of six related alkaloids.

The alkaloids the team manufactured were akuammicine, aspidospermidine, kopsanone, kopsinine, strychnine, and vincadifformine.

The new strategy, which the team calls “collective total synthesis,” makes it possible to synthesize useful amounts of related, complex molecules from a “common molecular scaffold,” and is much less time-consuming. The synthesis of strychnine, for example took place in only 12 stages, which is the shortest ever reported.

Traditional methods of synthesizing such chemicals have involved using chemical reactions to produce each individual intermediate and then isolating it and using the intermediate in the next reaction, and repeating this process until the target molecule is finally produced.

MacMillan said the new method represents a new way of thinking for organic chemists. Reducing the number of steps required to synthesize molecules reduces the time required and can also reduce the cost and wastage. Their approach can also produce a number of related molecules at the same time, which could reduce the time and cost of producing and testing chemicals to find the most bioactive drug candidates.

MacMillan and the team suggest the collective total synthesis strategy could be used to manufacture other families of molecules that until now have had to be synthesized individually. They say the approach of combining the two techniques of organocascade catalysis and collective synthesis could be applicable for many other groups of molecules besides alkaloids.

The paper, the first to be published describing this approach to synthesis, is published in Nature.