A book on amino acids in the context of the origin of life and life's homochirality is unusual. Professor Meierhenrich has written a very detailed scenario on the attempts of scientists to propose hypotheses taking into account the stereochemical information given by the analysis of organic molecules. One key tool refers to the famous statement of Louis Pasteur in his lectures: "Optical activity is a signature of life".
The author presents many facets of the chemical and physical processes involved on earth or in space and he enters into a fascinating area of ongoing scientific debate. At times the diversity of the topics involved might be demanding for undergraduates or newcomers to the field, but the book, while challenging, illuminates the way science is progressing among a wide diversity of techniques, scientific areas, and hypotheses.
When I started preparing my Ph.D. thesis at the College de France in Paris under the supervision of Dr. Jean Jacques, I was attracted by the possibility of deducing the shape of molecules, including their absolute configuration, from experimental data, either spectroscopic or chemical. Organic chemistry remained strongly connected to pharmaceutical chemistry, where many compounds of biological interest are asymmetric and optically active. The manipulation of molecular models as Dreiding models (there were no computers at that time) allowed for analysing the three-dimensional behaviour of many rigid or mobile structures. Conformational analysis of cyclohexane and polycyclic systems such as steroids have been recently rationalized by D. H. R Barton and O. Hassel (Nobel prize winners in 1969). The prediction of the steric course of reactions that create a new asymmetric center became possible and was based on simple concepts. After my Ph.D., I was involved in a fruitful collaboration with Prof. Alain Horeau, Head of the Organic Chemistry Laboratory at the College de France. He proposed a chemical method to establish the absolute configuration of an asymmetric center of secondary alcohols RCHR(R')OH as well as deuterated primary alcohols RCHD(R)OH. I was associated with the latest developments of the Horeau's method. Later, I started, always in association with A. Horeau, to approach the difficult problem of efficiently creating an asymmetric center in an achiral molecule (asymmetric synthesis). For that purpose, we used chiral auxiliaries temporarily connected to a substrate or a reagent. Especially successful was an asymmetric synthesis that provided almost enantiomerically pure aspartic acid.
When I became scientifically independent, I launched two completely different projects simultaneously in Orsay. One was the exploration of asymmetric catalysis using chiral transition metals. The other was to revisit the old question of the use of a chiral physical agent such as circularly polarized light (CPL), already suggested by Pasteur and Le Bel in 1860-1874. In asymmetric catalysis, we made important discoveries in the 1970s by devising chiral ligands (C2-symmetric chiral diphosphines) for rhodium complexes. These new catalysts allowed preparation of a-amino acids by asymmetric hydrogenation with up to 80% enantiomeric excess (e.e.), values which were the highest at that time. Nowadays, some enantioselective catalysts are used in industry to prepare chiral compounds with 99% e.e.
Our second project was more in line with the topic of the present book. The presence of circularly polarized light, a chiral agent, has been suggested to be present when the prebiotic soup started to produce biomolecules. Pasteur, Le Bel, van't Hoff, Cotton, Kuhn, and many others considered this possibility seriously, but it was W. Kuhn, who, in 1929-1930, first performed a partial asymmetric photolysis of a racemic compound (N,N-dimethyl a-azidopropionamide). The recovered material was optically active, although of very small e.e. This case of kinetic resolution was not followed by other examples until 1974, when we clearly showed that kinetic resolution of camphor can be carried out with CPL and afforded recovered camphor with 20 % e.e. at high conversion. The key factor for a successful experiment is to select a compound with a good g factor (g = Ae/e). Later several groups established that CPL can also be used to perform similar experiments on amino acids. In 1971, we described the first example of an asymmetric synthesis by using an achiral substrate and CPL. A photolytic cyclization gave rise to hexahelicene, which was slightly enriched in one enantiomer. This was easy to measure with a polarimeter because hexahelicene has a huge specific rotation. With regard to the question of whether CPL is able to produce optically active compounds, we learned that the answer is "yes". But what is its prebiotic significance? This question still remains unanswered.
We were also involved in a third project that may be of interest for prebiotic chemistry: the amplification of a small enantiomeric excess by catalytic or stoichiometric methods. In fact, we showed in 1986 that it is possible to use an enantioim-pure catalyst to create a product of higher enantiomeric excess than the one of the catalyst. This effect was named the "nonlinear effect" and it can be useful in auto-catalytic reactions. In 2007, we established experimentally, as proposed by I. Ugi in 1977, that some racemic reagents can increase the e.e. of partially resolved samples thanks to a selective destruction of some racemic content in the samples.
The present book discusses many ways to initiate or develop optical activity from achiral or racemic materials. The outlined space exploration and the study of chi-ral molecules are certainly of great interest to better understand the origin of ho-mochirality of biological systems. The underlying problem, however, may remain unsolved for a long period of time, if not forever. The scientific subject is intellectually appealing, and it has great potential for attracting young people towards chemistry as well as the physical and natural sciences.
Orsay, France May 2008
"Now, if you'll only attend, Kitty, and not talk so much, I'll tell you all my ideas about Looking-glass House. First, there's the room you can see through the glass - that's just the same as our drawing room, only the things go the other way. I can see all of it when I get upon a chair - all but the bit behind the fireplace. Oh! I do so wish I could see that bit! I want so much to know whether they've afire in the winter: you never can tell, you know, unless our fire smokes, and then smoke comes up in that room too - but that may be only pretence, just to make it look as if they had afire. Well then, the books are something like our books, only the words go the wrong way; I know that, because I've held up one of our books to the glass, and then they hold up one in the other room."
"How would you like to live in Looking-glass House, Kitty? I wonder if they'd give you milk in there? Perhaps Looking-glass milk isn't good to drink - But oh, Kitty! now we come to the passage. You can just see a little peep of the passage in Looking-glass House, if you leave the door of our drawing-room wide open: and it's very like our passage as far as you can see, only you know it may be quite different on beyond. Oh, Kitty! how nice it would be if we could only get through into Looking-glass House! I'm sure it's got, oh! such beautiful things in it!
Let's pretend there's a way of getting through into it, somehow, Kitty. Let's pretend the glass has got all soft like gauze, so that we can get through. Why, it's turning into a sort of mist now, I declare! It'll be easy enough to get through"-She was up on the chimney-piece while she said this, though she hardly knew how she had got there. And certainly the glass was beginning to melt away, just like a bright silvery mist.
In another moment Alice was through the glass, and had jumped lightly down into the Looking-glass room. The very first thing she did was . . .
Lewis Carroll in
Through the Looking-Glass, and What Alice Found There
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