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Non-destructive spectroscopic methods


Chiroptical methods and mass spectrometry
Carl Djerassi Scientist
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These are methods which tell you something about the stereochemistry of a chemical structure. And what do I mean by that? Stereochemistry is basically the three-dimensional picture of... of molecules, rather than the two-dimensional ones, which I’ve shown you right over here. And furthermore, what is very important in chemistry, the chirality, the handedness, of the molecule was it left-handed or right-handed, because most molecules can exist in two forms. Organic ones, left- and right-handed, and we call them dextro- and levorotatory. And in other words they are... you can see right here, they do not fit into the two hands, they are mirror images, but they are not really identical, you know, I mean it’s not... if you superimpose it, it’s not identical it’s a mirror image. It turns out that virtually all biologically-active molecules are optical... are... exist in mirror images, but only one of them is biologically active. So it’s extremely important to be able to tell, and we call that determining the absolute configuration of a compound, whether it’s left-handed or right-handed. Now, what does left- or right-handed mean? We use, really as an index how it transmits, or... or absorbs left or right circularly polarised light, and therefore it’s called left or right, levo or dextro.

So I was very interested in working also on methods to determining that. Because, again if you want to know a... the composition of that empty room, you don’t really want to know the chair and just draw a picture, a two-dimensional one, you want to know the depth of it, you want to know, of course, again the chirality of it, even the position of it here, both relatively and absolutely, and I worked very much on methods. In the first book I ever published, called Optical Rotatory... Optical Rotatory Dispersion, that was the technique, applications to organic chemistry. And that was the first method, and that was... had a big impact in stereochemistry and the applications of that, and we were particularly interested in natural products, because we always wanted to determine that for each natural product.

And the second one, the method in which we started work, is mass spectrometry. Mass spectrometry is really a method where you use a minute amount of material, really an invisible amount of material, and at that time... now you can do it in many other ways... you bombard it with electrons, and what you’re really doing is you demolish it, absolutely demolish it, because you hit it with an awful lot of energy, and so I take you and I fragment... Well, let’s- rather than commit murder on you, let’s assume you are just a beautiful sculpture, and I really want to know what that sculpture looks like, rather than the room, and now I’m taking something in it... the sculpture. And what I really do is, as in archaeology, you find segments of an arm, a leg, a head, and so on, and if you find enough of them, as an archaeologist, and it shows again what I would have liked to done at one time, you can start putting it together. And if you are very successful you put the whole thing again together from the pieces that you find originally, and sometimes you need only a few pieces to tell an awful lot, and that’s certainly true in palaeontology, where people discover a lot from just certain skeletons. So, in mass spectrometry you destroy that substance, but you don’t destroy it into just... into just ash, which of course wouldn’t help you. In other words, if you just convert it all into carbon, hydrogen, oxygen, or whatever atoms it contains, then it tells you nothing, but into segments, and the ideal one is, they do it in such a way that if it’s a sculpture it happens to cut off, ideally let's say, the head here, and arm here, a leg there, so it’s very easy to put it together. That, of course is usually much too easy, it doesn’t work that way, so instead you lose also a ear, and in some cases a piece of a nose, some teeth, and therefore all the different places, but you learn how to put them together. When you see that ear really fits over here, and that head really fits to that torso, and that torso really to those legs, and before you know it you can start putting this together, and that’s really what mass spectrometry in a way is. But what is so great about it is you need infinitesimal amounts of material, and I would say, with... I was not the first one to use mass spectrometry, but one of the first, but I probably used it more extensively and published more than, probably anyone else.

I published with two of my post doctorate fellows, one an Austrian, Herbert Budzikiewicz, and one an Englishman, Dudley Williams, who just recently retired as a Professor of Chemistry at Cambridge. We published four or five books together, just on mass spectrometry, and I’ve probably published something in the order of, maybe 300 papers alone in that field, on applications of mass spectrometry. And some of this was very sophisticated work, because of the way you could only develop... really develop the language of interpretation for these mass spectra, and some rules... the rules of the game, so to speak... interpretive rules, and we could only do this by labelling known compounds with isotopes. With deuterium, or carbon 13, and... or oxygen isotopes, and a few time nitrogen isotopes, so I also got involved with a lot of isotope chemistry, but these are stable isotopes, not radioactive ones, so therefore there’s not a question of hazards, but it’s... these are labelling devices. I really won’t go further on this, except saying that was an enormous contribution, and that I did when I first came into Stanford, because all the instruments were expensive, and I was able to get a mass spectrometer, which until then was really only used by men in the petroleum industry for the analysis of very simple petroleum constituents, and it was a petroleum chemist who introduced mass spectrometry, which otherwise was a physical instrument, because the instruments themselves, whether it's ultraviolet, infrared, mass spectrometry, they were obviously developed by physicists developing... implementing physical phenomenon in instrumental terms, but having not the foggiest idea what the application of this would turn out to be. And these were usually then applications that only the user would develop, and I was one of these users, and particularly, I think, did this rather well, and in these particular areas.

Austrian-American Carl Djerassi (1923-2015) was best known for his work on the synthesis of the steroid cortisone and then of a progesterone derivative that was the basis of the first contraceptive pill. He wrote a number of books, plays and poems, in the process inventing a new genre, 'science-in-fiction', illustrated by the novel 'Cantor's Dilemma' which explores ethics in science.

Listeners: Tamara Tracz

Tamara Tracz is a writer and filmmaker based in London.

Tags: Stanford, mass spectrometry, stereochemistry, Chirality, Optical Rotatory Dispersion, Herbert Budzikiewicz, Dudley Howard Williams

Duration: 7 minutes, 17 seconds

Date story recorded: September 2005

Date story went live: 24 January 2008