New Research Rewrites Understanding of Explosive Alkali Metals

Despite being a staple demonstration in many introductory chemistry classes, the classic explanation for the explosive reaction between alkali metals and water has long been incorrect.

Many middle and high school students are familiar with the demonstration. Almost immediately following contact with water, alkali metals such as sodium and potassium produce a brilliant and highly energetic explosive pop. Instructors the world over would often confidently follow by explaining that the  reaction produces hydrogen gas, whose subsequent ignition is responsible for the theatrics.

However, recent research published in Nature Chemistry shows that things are not actually so simple. Although the hydrogen gas may indeed eventually ignite, the initial rapid explosion is caused by something almost entirely unrelated.

In retrospect, it seems obvious that there was something wrong with the orthodox explanation. In order for a reaction to produce an explosion, the reactants would have to mix very effectively in order to react rapidly and release energy suddenly. This is why flour mills are so susceptible to explosive outbreaks of fire; a build-up of finely ground flammable particles suspended in the oxygen-rich air allows any spark to consume an enormous amount of fuel virtually instantaneously.

Alkali metals, on the other hand, are solids. The water can only come into contact with the outer surface, which should result in a brief layer of products preventing it from reaching deeper layers right away. Water isn’t immediately in contact with every metal atom, so at the very least the reaction should proceed more slowly than it does.

In order to investigate this further, researcher Pavel Jungwirth and others set out to scrutinize the reaction with the use of high-speed cameras. Because pure alkali metals tend to accumulate an oxidized layer on their outer surfaces, causing them to be less reactive in water, he used an alloy of sodium and potassium that is liquid at room temperature.

The images captured by the cameras were very telling. The reaction begins less than a millisecond after the droplet contacts the water. At 0.4 milliseconds, spike-like tendrils of metal shoot outward, much too quickly to have been produced by heat. Most interestingly, this spiked droplet develops a never-before-seen aura of dark bluish purple color in the surrounding solution between 0.3 and 0.5 seconds (see supplemental video). This blue color turned out to be the key to understanding what was really going on.

The origin of this mysterious color was confirmed when Jungwirth’s colleage Frank Uhlig recreated the reaction in a quantum-mechanical simulation. This digital analysis revealed that atoms at the surface of the cluster were each stripped of an electron within just a few picoseconds. The electrons then rapidly shoot away from one another and become solvated in the surrounding solution. Free electrons in solution, as many chemists know, appear blue to the naked eye. The loss of these electrons leaves the atoms positively charged, resulting in an incredibly strong repulsive force blowing the cluster apart.

This research represents a feature of science that keeps so many people fascinated by it. Although it may seem like the basics are well-understood, surprises like this frequently come from the most unexpected of places. Scientific knowledge is highly dynamic and constantly evolving, as nature proves time and time again that the richness and complexity of reality rivals the limits of human imagination.

Written by: Aisling Williams

Source

Mason, Philip E., Frank Uhlig, Vaclav Vanek, Tillmann Buttersack, Sigurd Bauerecker, and Pavel Jungwirth. "Coulomb Explosion during the Early Stages of the Reaction of Alkali Metals with Water." Nature.com. Nature Publishing Group, 26 Jan. 2015. Web. 8 Feb. 2015.

Targeting Cancer Cells with Self-Assembling Molecular Cages

Taking inspiration from nature, chemists have developed a new method to destroy bone cancer cells that utilizes artificial extracellular matrices.

Because extracellular matrices provide support and structure to the cells making up many organs and tissues, their artificial production has been very appealing to tissue engineers. Scientists searching for a method to produce them in the lab have mainly focused on self-assembling peptides.

Cancer cells before (left) and after (right) 7 hours of exposure to self-assembling
carbohydrate molecules.

Credit: J. Am. Chem. Soc.

In order to exploit one of the common features of bone cancer cells, Bing Xu of Brandeis University designed such a peptide with one important modification; it is only capable of self-assembly upon removal of a phosphate group. Once it’s gone, the molecules have a hydrophobic and a hydrophilic end, allowing them to aggregate into films like the lipids that form membranes in the body.

This property made them perfect for targeted destruction of certain types of cancer cells, which produce alkaline phosphatase, an enzyme that removes phosphate, in far greater quantities than healthy cells do.

Another researcher, Rein V. Ulijn of the City University of New York’s Hunter College, took it a step further. Because carbohydrates can produce such a rich diversity of structures, he endeavored to use them in a similar way. 

To create a carbohydrate molecule that would self-aggregate, he attached a hydrophilic glucosamine to a hydrophobic aromatic. Then he added a phosphate group that would interfere with the molecules’ mutual attraction to its peers, thereby postponing the formation of a film until the group was cleaved off, hopefully, near a phosphatase-rich cancer cell.

Subsequent tests investigating the effectiveness of the substance against cancer cells yielded optimistic results. The chemical killed 95% of cultured bone cancer cells, while only 15% of healthy control cells perished after 7 hours of exposure. 

Written by: Aisling Williams

Source

Berg, Erika G. "Self-Assembling Carbohydrates Trap Cancer Cells In A Cage." Chemical & Engineering News (2015): n. pag. 20 Jan. 2015. Web. 27 Jan. 2015.

Crystal Nucleation More Nuanced Than Once Thought

The formation of crystals is paramount to the production of an enormous variety of products we use every day. From things as simple as sugar or salt to revolutionary technology involving crystalline metals and silicon, the understanding of the processes of crystal growth has been a staple of scientific progress. However, recent research indicates that nucleation – the process initiating the growth of a crystal – may be much more complex than previously imagined.

The classical model of crystal growth breaks the process down into two major steps. First, ions or molecules come together into a tiny crystalline seed, on whose structure the properties of the emerging crystal will depend. From that point, other solvated ions fall into place, thereby expanding the lattice and growing the crystal. As the crystal grows, the bulk free energy of the mass decreases; yet at the same time, the solid-liquid interface expands, increasing free energy. Nucleation is officially defined as the point at which the crystal reaches the critical size threshold beyond which the energy benefit of growth exceeds the cost.

However, this model is proving to be inadequate in the face of mounting evidence. There seem to be a variety of different mechanisms from which a crystal structure can emerge. For instance, research done in 2002 at MIT involved inducing crystallization in glycine using laser pulses. By altering the polarization of the incident laser, the group produced a variety of different crystal polymorphs. 

According to MIT Chemical Engineering professor Allan S. Myerson, such a phenomenon indicated that the laser must have been acting on some pre-existing structure that was somewhere in between an ordered crystal and completely solvated molecules.

A more recent study in 2014 visually examined the microscopic behavior of calcium carbonate as it formed crystals. Calcium carbonate represented an interesting substance for such a study due to its tendency to form a wide variety of crystal polymorphs, including calcite, aragonite, and vaterite. While material often appeared to nucleate into any of the three directly, sometimes the molecules would aggregate into unstructured blobs which then transformed into aragonite or vaterite. 

This sort of behavior may be important beyond the formation of the initial crystal ‘seedling’, as it is possible for actual crystal growth to depend on the formation of such viscous blobs. Perhaps individual ions or molecules are incapable of adding to the growing crystal in isolation, and need to first form groups to proceed.

The behavior of real systems appears to be extremely diverse. In the words of James J. De Yoreo, who lead the calcium carbonate study, “Think up any mechanism or pathway you want, and there will probably be some system that behaves that way.”

Written by: Aisling M Williams

Sources

Kemsley, Jyllian. "Illuminating Crystal Nucleation." Chemical & Engineering News93.2 (2015): 28-29. CEN RSS. Chemical & Engineering News, 12 Jan. 2015. Web. 13 Jan. 2015.

6th Annual Science Carnival

On November 1^st, the 6^th annual CSUCI-hosted Science Carnival proved once more to be a roaring success.

Being entirely volunteer-run, the carnival remains free to attend and continues to attract guests of all ages. Students between Kindergarten and 8^th grade, many with siblings and parents in tow, flocked to Thurgood-Marshall elementary school to participate in the festivities.

From Biology and Chemistry to Paleontology and Physics, the carnival provided over 70 dazzling science demonstrations and activities to fascinate and inspire a younger generation.

Students admire a demonstration of UV fluorescent chemicals.

Fire isn't always yellow! This volunteer shows a crowd how the color of a flame
can actually depend on what compound is undergoing combustion.

The high-frequency electromagnetism associated with the plasma filaments in a
plasma lamp can induce a nearby fluorescent lightbulb to light up!

This ball python was one of many animals present at the science carnival.

Angular momentum is a physical phenomenon so rarely encountered that even
adults found this demonstration strange and fascinating.

This ping pong ball cannon made it clear
just how powerful air pressure can be.

This apparatus pumps air in and out of a pair of real lungs!

Many guests were surprised by the rich topography
of the microsocopic world.

Live crabs, starfish and many other marine creatures
made this booth particularly popular.

Unprecedented Control Over Organic Synthesis Using Modified Enzymes

Drawing inspiration from the world of biology, researchers have developed a novel approach to solving a long-standing problem in organic synthesis – regioselectivity.

The vast majority of drugs contain nitrogen, driving chemists to search for more efficient and selective methods to form new C-N bonds in substrate hydrocarbons. However, one of the most stubborn obstacles in the way of creating the desired compound is regioselectivity – the preference of a reaction to make or break bonds at particular sites of the substrate molecule over others.

This troublesome habit of nature often makes it very cumbersome to come up with a reaction mechanism that produces a desired molecular structure, at least with anything like a reasonable yield. The traditional approach would be to focus on the substrate molecule, modifying it in such a way as to make some particular carbon more likely to be that which forms the bond. Sometimes this means coming up with weird, exotic molecules that may be expensive or otherwise difficult to obtain.

Rather than manipulating the substrate, it would be much more desirable to have selectivity be determined by the catalyst. This way a chemist could produce different products from the same starting material simply by modifying the catalyst of the reaction.

Regioselectivity is determined by the enzyme catalyst.

Researchers at California Institute of Technology have come up with a way to accomplish just that. By engineering a natural biological enzyme, the team created two artificial variants which steer nitrogen atom transfer of a particular reaction in complementary directions. One enzyme favors ring-closing amination at the α-position of an alkyl substituent on a benzene sulfonyl azide. The other enzyme favors amination at the β-position. Simply by changing the catalyst, they have been able to switch the reaction to favor one product over the other by over 95%.

Synthetic biology may still seem like it belongs in the realm of science fiction to many, but the degree to which these enzymes have controlled the outcome of the reaction is difficult to ignore.

Considering the trouble these types of reactions have given scientists in the past, it may be reasonable to expect these new methods to gain favor in the future. The modification of enzymes for use in organic synthesis, according to the researchers, represents a promising platform for solving long-standing selectivity problems.

Source

Hyster, Todd K., Christopher C. Farwell, Andrew R. Buller, John A. McIntosh, and Frances H. Arnold. "Enzyme-Controlled Nitrogen-Atom Transfer Enables Regiodivergent C–H Amination." Journal of the American Chemical Society. N.p., 5 Nov. 2014. Web. 09 Nov. 2014, 136 (44), pp 15505–15508

Breaking Biofilms with Simple Compounds – A Novel Approach to Fighting Bacteria

The vast majority of chronic bacterial infections involve the formation of biofilms. On their own, individual bacteria tend not to represent much of a threat when met with the average human’s immune arsenal. 

However, some types of bacteria have developed a particularly effective defensive strategy.  By grouping together into slimy aggregate colonies, otherwise known as biofilms, bacteria can make themselves virtually impervious to destruction through conventional means. Once a biofilm forms, an infection may become chronic, or even fatal.

Luckily for us, researchers at the University of Washington, Seattle have developed a new method to turn bacteria’s best defensive strategy against them.

Credit: ACS Sustainable Chem. Eng.

In order to form a biofilm, bacteria secrete polysaccerides and proteins which in turn form an extracellular matrix. As a consequence, their external environment becomes unusually saturated with dissolved salts and sugar, resulting in increased osmotic pressure on the cells. In order to compensate, bacteria fill their internal cytoplasm with small molecules called, unsurprisingly, osmoprotectants.

Researchers have focused their attention on these osmoprotectants. By synthesizing artificial analogs in the lab, they have been able to investigate various compounds that might interfere with their effectiveness. Ethylcoline appeared to be the most promising. It reduced biomass by 70%, and was the only compound tested that produced substantial effects without inhibiting bacterial growth.

Strange as it may seem, the fact that this approach doesn’t directly kill the bacteria is actually what makes it so appealing. By leaving the cells alive, selective pressure is minimized, making it incredibly difficult for the bacteria to adapt. 

Source

Madhusoodanan, J. (2014). Simple Molecules Block Bacterial Biofilms | Chemical & Engineering News. [online] Cen.acs.org. Available at: http://cen.acs.org/articles/92/web/2014/10/Simple-Molecules-Block-Bacterial-Biofilms.html [Accessed 15 Oct. 2014].

Most Complex Molecular Shape “Star of David Molecule” Finally Created in Lab

Over a quarter of a century’s worth of effort has finally come to fruition at the University of Manchester. Affectionately known as the ‘Star of David Molecule’, the beautiful interwoven threads of atoms represent a physical feat that scientists only years ago may have dismissed as impossible.

Image Credit: University of Manchester

The prospect of manually finessing strings of atoms around one another into such a precise configuration seems, even now, like the stuff of science fiction. And yet despite the odds, PhD student Alex Stephans has managed to create the beautiful interlocking molecule, and he did it by taking advantage of a quirk of chemical physics that the microscopic world of biology has known about for billions of years. 

The trick to creating such delicate molecular structures, it turns out, is to allow them to do it themselves. Instead of trying to wrap the interlocking triangles around one another manually, as chemists of the past have tried (and utterly failed) to do, Stephans took advantage of a process known to biologists as self-assembly.

“Nature does the same thing to assemble DNA,” said David Leigh, lead researcher and professor of Chemistry.  "Most have tried to take linear molecules and twist them around each other, but we choose our building blocks very care

fully."

The atoms involved were carefully chosen for their affinities for one another, so that the tiny interlocking triangles would fall into place as the molecule formed, driven by the same subatomic forces that had made such a structure virtually impossible in the past.

Although it may superficially seem like nothing more than a trivial exercise in scientific peacocking, the molecule may have some far-reaching implications. “When you look at viruses, some of their shells have these coatings made of a sort of chainmail of protein, and it's very tough but very light," says Leigh. "So the thinking is that if you could do the same thing with a man-made molecule, you could get those same benefits."

The research team hopes to use the self-assembly method to create even more complex molecules that might someday be used in what they call “molecular chainmail”. A material made out of such structures would be extremely lightweight, flexible, and incredibly strong.

By Aisling M Williams

Works Cited

Feltman, Rachel. "Scientists Create a ‘Star of David’ Molecule — a Step towards Molecular Chainmail." Washington Post. The Washington Post, 22 Sept. 2014. Web. 28 Sept. 2014.