Recyle, Reuse

A quick flip through my students' problem sets tells me more about them than just their ability to do quantum mechanics. My greener students make good use of the reverse side of the mountain of pages their colleagues print out every day. The paper drafts and announcements on the back side often catch my eye and offer a window into a student world I don't often get to see. Of course, recycling sheets for writing is hardly a new phenomenon. Palimpsests are parchment or vellum pages that have been erased by various methods and reused. The earlier, generally chemical, methods of erasure left faint traces of the original writing on the sheets. As methods improved, and relied more on mechanical means, such as sanding with pumice, the erasure became more complete.

In my quantum chemistry class we talked about fluorescence today. A common example of fluorescence is the odd luminescence of white t-shirts under a black light. The black light is a source of UV light, which excites some of the molecules in the detergent residue (yep - that bright white shirt is not quite as clean as you think!). The molecules then re-emit light at a slightly lower energy, which happens to be in the visible, and that we perceive as an eerie glow. The glow is present even in daylight, but the amount of visible radiation emitted through fluorscence is so much smaller than what is in incident sunlight that it swamps out the effect. But it does make your whites look subtly brighter, which is why detergent companies include "brighteners" in their formulations.

So what do white shirts have to do with palimpsests? X-rays are just another form of light (albeit very high energy light) and can cause fluorescence, too. Iron in the ink is the source of the fluorescence. Researchers at Stanford have recently uncovered not only an Archimedes manuscript hidden underneath a 13th century Byzantine prayer book, but also a text by Hyperides, a contemporary of Aristotle. The discovery of this text extends the known works of Hyperides by 20%!

A Request to Readers

In March I'm giving a talk at the American Chemical Society National Meeting in Chicago. To prepare for the talk, I'd like to know more about my audience beyond what I can get from technorati and StatCounter. If you're a regular reader, can you tell me what is interesting about this blog? Do you learn anything from it?

Thanks!

Half-awake, half-life

I had a moderate allergic reaction to peanuts last night. I took diphenhydramine (Benadryl) and the hives had subsided by this morning. Lecturing on perturbation theory was more challenging. I felt like I was walking in a fog. Which got me to wondering, just what was the half-life of Benadryl? Benadryl has a relatively long half-life, between 8 and 10 hours. A typical 50 mg dose leads to a peak blood level of around 80 nanograms/ml. Most people feel drowsy at blood levels around 30 nanograms/ml. Assuming first order kinetics apply to the breakdown/elimination of Benadryl, a 30 nanogram/ml is not unlikely 10 to 15 hours later. Which would certainly explain my fogged state this morning! But not so foggy as to be unable to work the kinetics....

The Sticking Point - or Weird Words of Science 11: Eutectic

It would be an understatement to say that my youngest son is not looking forward to his annual flu shot. Last week, the Philadelphia Inquirer had a piece on helping kids cope with the pain of immunizations. Son was not particularly impressed with the advice, but he noticed that they refered to a cream which could be applied to diminish the pain of injections. "What is it, Mom?" "EMLA, I think." "EMLA?" "A eutectic mixture of local anesthetics..." Somewhere around mixture, I think I lost him!

What is a eutectic mixture? Eutectic comes from the Greek eutektos for "easily melted"(any resemblence to tectonic is, I believe, purely accidental - tectonic also comes from the Greek, but for building, not melting!). An eutectic mixture is one in which the melting point of the mix is lower than the melting point of either of the components. The binary phase diagram has a "eutectic point". EMLA is a mixture of equal weights of lidocaine and prilocaine, made into an emulsion.

It's apparently quite effective, but requires a lead time of several hours (and the foresight to ask the pediatrician for a prescription!).

"What was to be demonstrated" needed to be demonstrated!

A student in my office hours today asked me what the term QED meant at the bottom of a page, and got a (very short) lesson in Latin. Quod erat demonstrandum, "what was to be demonstrated", is a translation of the Greek hoper edei deixai used by Euclid to close a proof. Modern mathematical publications often substitute other symbols, including a  or simply note: proven.

Orion brandy anyone?

I'll admit to being a Trekkie at some time in my life, but Dr. McCoy's stash of Saurian brandy aside, there is alcohol in interstellar space. More than 120 molecules and ions -- including ethanol -- have been identified by radioastronomers in interstellar space. The transitions between different molecular rotational states give rise to very specific lines, which can be used as molecular fingerprints.

The lines which helped identify ethanol are rotations around the carbon-carbon single bonds, rather like little propellers turning. The lines arise from vicinity of the Orion Nebula (a mere 1500 light years away), which can be seen just under Orion's belt.

Isotope Counts

In my quantum chemistry class we are exploring molecular vibrations. The characteristic frequency of vibration depends on the masses of the atoms in the vibrating bond. Not every student in my class is a chemist, a fact that was driven home when I assigned a problem asking students to compute the fundamental vibrational frequency of an molecule with a deuterium atom.

Most elements have several naturally occuring isotopes - forms of the element which have the same number of protons,but varying numbers of neutrons. For example, the most abundant form of the element carbon has a mass number (the sum of the number of protons and neutrons) of 12. One percent of carbon atoms, however, have an extra neutron and a mass number of 13. Carbon-14 has two extra neutrons, and is radioactive. Chemists often use isotopic substitution as a way to "tag" molecules (particularly if the isotope is radioactive).

Deuterium is an isotope of hydrogen - much of its chemical behavior is exactly the same as that of hydrogen,it just weighs more, twice as much to be precise. Unlike other elements, where the isotopes are designated by their usual name plus their mass number, isotopes of hydrogen get their own names. Deuterium has a mass number of 2, tritium a mass number of (surprise) 3. Tritium is radioactive and has a half-life of around 12 years.

In 1955, in his novel "The Mouse that Roared", Irish writer Leonard Wibberly coined "quadium" for hydrogen-4, which at the time had not been made. Since then, hydrogen-4 has been created. It is a fleeting species, its half-life is just over 10^-22 seconds!

Strands of Life

The Nobel prize in medicine and physiology today went to two American scientists, Andrew Fire and Craig Mello, for their work on gene silencing and double-stranded RNA. When we think double-stranded, we often think of another pair of Nobel laureates (Watston and Crick) and a related molecule, DNA. RNA indeed is typically single-stranded, and uses a modified set of bases relative to DNA, subsitituting uracil for thymine. (Wikipedia has a nice diagram.)

The double-stranded version, dubbed RNAi, interferes with the decoding of genes in cells, hence the "gene-silencing" tag.

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A colleagues hazards that Nobel winners are getting younger every year. Is it because the time between discovery and award is shrinking or is it that younger scientists are making more critical discoveries?

Girls Don't Like Hard Science

John Tierney had an op-ed piece in the NY Times on Tuesday about the recent National Academy of Sciences report on bias toward women in science. He dismisses their findings of bias, and pins the reason for the underrepresentation of women in research universities on "they don't want to". Most girls, he opines, like the soft sciences, because they are concrete and people oriented, while boys prefer the abstract and "things". Perhaps, but you can be movivated by the concrete, be people oriented and still do "hard science". Check out the letters in response to his piece (full disclosure, one of the letters is mine). Martha Pollack's response regarding engineering was wonderful - people oriented hard science exists.

Royal Purple Molecules

My quantum mechanics class had a problem last week aimed at figuring out the color of a porphyrin molecules. Porphyrins are nitrogen-containing ring shaped chelating molecules (here is a picture) and are ubiquitous in biological systems. An iron bound to a porphyrin is the heme in hemoglobin, when a magnesium is bound, it is a key piece of chlorophyll.

The color of porphyrin should not be a mystery, as long as you know some Greek. The name comes from the Greek for purple, and indeed these compounds have vivid red-violet hues.

I asked the students to compute the energy needed to excite one of porphyrin's 18 pi electrons from the highest occupied level to the lowest unoccupied level, assuming that they could model the compound as 18 independent electrons trapped in a square 1000 pm on a side. The answer in the back of the book gave the absorbtion wavelength of 588 nm, which is precisely what you would expect for a purple compound (absorbing visible yellow light). It seemed too good to be true, for such a simple model to give such a good anwer and it was! There is an error in the answer, and the actual value is not in the visible at all, suggesting that the porphyrin is colorless!

The problem was an apt one for me to be grading this morning, as I was waiting to donate some of my own hemes in the form of whole blood at the college's blood drive.