Ship in a bottle catalysts
by David Bradley
Zeolites, porous catalytic minerals are commonly used in refining crude oil and as molecular sieves. Chemists would like to be able to use them for more varied reactions but they come only with pores of a limited range of sizes and shapes. Writing in Chemical Communications (1997, 901) Thomas Bein and Steven Ogunwumi of Purdue University in Indiana describe how they have trapped another catalyst - an asymmetric manganese-containing organic compound - in the pores of a zeolite (EMT) to make a hybrid material.
The hybrid is made using starting materials that can enter the pores of the zeolite but once assembled inside the product cannot get out "like a ship in a bottle", Bein explains. "One of the advantages of encapsulating a catalyst in a zeolite is the convenient recovery of the products from the reactor, compared to reactions in solution," he added.
They have tested their hybrid and found that it produces just excess of one mirror image form of the two possible products (enantiomers) in a so-called alkene epoxidation. In this reaction a bridging oxygen atom is added across an alkene double bond. Epoxides are important precursors to materials from drugs to agrochemicals and making them selectively is important as often one enantiomer of a drug for instance is safer and more effective than the other. For instance, the common painkiller ibuprofen is three times more effective in one form than the other.
choose frozen juice for a vitamin C boost
In research funded not surprisingly by the Florida Citrus Growers scientists have found that frozen concentrated orange juice generally has the highest vitamin C levels compared with other commercial orange and grapefruit juice products, but regardless which one you drink you are probably getting your daily requirement of the vitamin.
Hyoung S. Lee and Gary A. Coates of the Florida Department of Citrus in Lake Alfred, Florida conducted a ten-year study - described in the July 16 issue of the Journal of Agricultural and Food Chemistry - to look at vitamin C levels in some 2299 samples of orange and grapefruit juices collected from 21 Florida processors. They found that 95.7% of the samples provided more than 100% of the daily amount set out by US laws. Overall, an eight ounce serving of frozen concentrated orange juice had the highest vitamin C content (173%), with orange juice from concentrate coming in second (161.2%) and pasteurised orange juice third (138.4%).
worth its salt
Salts are used in almost every cuisine as flavouring agents from the simple table salt of boiled beef and carrots to the monosodium glutamate in a vegetable spring roll. The effects of salt on taste have puzzled most flavour scientists, however, psychophysical studies show that sodium chloride either suppresses or has no effect on flavours rather than enhancing anything, as most people would feel.
Paul Breslin and Gary Beauchamp of the Monell Chemical Senses Center in Philadelphia believe they have found a solution to the paradox (Nature, 1997, 387, 563). They used mixtures of a bitter substance, urea, unpleasantly enough, a sweetener sucrose and almost tasteless sodium acetate to test their idea that salts selectively filter tastes.
Twenty-one volunteers were asked to judge the bitterness of the urea and the sweetness of the sugar of various combinations of solutions of the three ingredients.
The volunteers reported exactly what the scientists predicted that the sodium acetate suppressed the bitterness of the urea much more than enhancing the sweetness of the sucrose. The scientists admit, howf, that the study does not mimic all the possible flavour combinations of food the taste test does show that as well as adding "saltiness" to food (which many people like) salts also filter other tastes suppressing unpalatable bitterness and boosting sweetness.
They hint that this could be why low sodium foods - often recommended for people with high blood pressure - tend to be rather bland tasting.
channel no 5
The surface of every cell in your body is pitted with tiny holes - a worrying thought? It should not be because each hole is formed from a natural channel protein there to control the molecular traffic in and out of the cells controlling sodium, potassium, calcium and chloride ions.
Understanding exactly how they work is, according to George Gokel of the Department of Molecular Biology and Pharmacology at Washington University School of Medicine in St. Louis, Missouri, "One of the hottest areas of modern biology because channels are important in neural transmission as well as in controlling ion concentrations and in communication between cells."
Scientists know what they do but not a lot about how they accomplish it and how they control which ions pass which way in the channel. Gokel and his team have devised a simple synthetic channel which they hope will work in the same way as a protein channel so they can use chemical methods to analyse form and function.
They recently reported results in Chemical Communications (1997, 1145) that show they can indeed mimic natural channels using ring-shaped molecules - macrocycles - stacked together.
The long-term goal is to develop new drugs based on artificial channels. For example, if a channel mimic could be made to get into a cell wall selectively, perhaps by adding a targeting peptide, fatty acid, or steroid it would effectively punch a hole in the cell, which might cause a deleterious flood of ions either in or out. If the cell is a cancer cell then it would be killed therefore potentially halting tumour growth.
Gokel points out that cystic fibrosis is intimately linked to chloride ion transport and that making a chloride-conducting channel could be used to redress the balance in patients although this is a very distant goal.
Gokel's team is currently trying to control selectivity; since publishing in Chem Comm they have achieved about 3:1 transport through the channel of potassium over sodium and are now altering the size of the macrocycles with further success.
ever so sensitive
A super-sensitive detector for brain chemicals has been designed and built by David Parker and Ritu Kataky at Durham University. The sensor is based on a ring of sugar molecules known as a cyclodextrin (CD)
and can detect quantities of the neurotransmitter acetylcholine down to 10-14 molar. More exciting , says Parker, is that it can be used to "watch" changes in the concentration of molecules that inhibit the enzyme that breaks down acetylcholine after it has passed on its signal. These enzyme inhibitors include various organophosphorus pesticides and also chemical warfare agents such as Sarin.
The sensor works by trapping the target molecule in the central cavity of the cyclodextrin rings, which are held in a thin plastic membrane. The presence of a molecule in the CD cavity then triggers a series of chemical changes in a ferrocene molecule also embedded in the membrane that is then detected by an electrochemical connection through an enzyme. Parker says the sensing unit is simply a disposable, coated screen printed electrode costing less than a pound each.
The team reported results earlier in the year in Chemical Communications (1997, 141) and having patented the work, are now considering exploiting its use in pesticide analysis. The work was funded by the BBSRC and the EPSRC in the UK.
the root of all evens?
Conscientious readers might like to check back to a previous issue of elemental discoveries to read up on the odd-even disparity found in the organic world by Desiraju and colleagues. They claimed to have found a far larger number of even numbered carbon compounds than odd and were at a loss to explain the result. Several readers came up with suggestions as to what might be going on and indeed a follow up paper was published in Nature, 1997, 385, 782.
The various explanations seemed to hinge on ideas such as there being more natural products than non-natural molecules (based on acetate units for instance) and benzene and hexose sugar rings and the like. Desiraju, however, is not convinced by any of them having delved a little deeper. He and his colleagues are fairly sure that the disparity is not of biosynthetic origin. "If the acetate origin of organic compounds were paramount, the even-odd effect should taper off with time as synthesis became increasingly diverse," explains Desiraju, "this, however, is not the case." He adds that if it were the origin there would be a preference for compounds with 4n carbon atoms as compared to those with 2n, say after C20 or so but this again does not happen.
He admits that for limited classes of natural products where the acetate route is important, say steroids, the even-odd effect may be rationalised but not for the entirety of organic compounds. It might also be worth looking again at the illustration we used. It is just as easy to have an extra methyl group to upset the numbers.
Suspecting that money might be the root of all evens, they did an analysis of the Aldrich and Acros catalogues by cost. There is no preference for the evens!
We look forward to further suggestions...
Finally, this is not exactly chemistry but because I play guitar this piece of research caught my eye.
Engineers at Cornell University in New York have used electron-beam lithography, to build what they say is the world's smallest guitar. With a Fabry-Perot interferometer they carved a piece of crystalline silicon no larger than a single cell into a guitar to demonstrate the potential for making microscale mechanical devices. The strings are about 4000 times thinner than a human hair but make no sound. I bet they still cannot get it to stay in tune when they use the whammy bar!