Chirality, amino acids/sugars

Should we be surprised to find chiral structures in nature Surely we should not, because aU living things are made up of chiral amino acids, sugars, and other components. One major questions is whether the fact that the building blocks of life are chiral controls the overall chirality of the macroscopic system, or whether this macroscopic chirality has a more macroscopic origin. We have already discussed the many... 

There have been many reported chiral stationary phases for use in both packed and capillary gas chromatography. Most of these phases are of the carbonyl-bis-L-valine isopropyl ester, diamide, and peptide phase types. The most common phase is Chirasil-Val from Alltech Applied Science Laboratories (State College, PA). This phase is ideal for the separation of a variety of enantiomers including amino acids, sugars, amines, and peptides. The phase is composed of L-valine-tert-butylamide linked through a car-oxamide group to a polysiloxane backbone every seven dimethylsiloxane units apart. 

As an alternative to resolution, one can start with an enantiomerical-ly pure compound that occurs naturally, and the most common examples of these are amino acids, sugars and terpenes. This group of compounds is known collectively as the chiral pool . The procedure then is to transform the compound of choice from the above group into the desired product by chemical synthesis, with care taken to avoid racem-ization at stereogenic centres. 

This holds true for amino acids, nucleobases, and carbohydrates. Even within one class of the biogenic molecules, two or more independent routes developed. As illustrated by the formose reaction, chiral sugar molecules formed when catalyzed by an appropriate chiral amino acid (to be discussed further below). 

The formose reaction formed chiral sugar molecules when catalyzed by an appropriate chiral amino acid under credible prebiotic conditions. Alternatively, chiral amino acids likely formed via import from meteorites with subsequent transfer of chirality to other molecules. The solid chiral nucleobase cytosine arose by mechanical stirring from solution, Evaporation of ribonucleosides and subsequent hydrolysis led to chiral ribonucleosides. 

Some amino-sugar syntheses have begun with chiral amino-acids. The four isomeric methyl 2,4,6-trideoxy-4-C-methyl-4-trifluoroacetamido-L-hexopyranosides have been synthesised from L-threonine. The L-ribo-isomer (43) was obtained from the known oxazoline derivative (44) by a ten step procedure (Scheme 17). Grignard... 

The question also arises as to where the chiral molecules came from. Were the L-amino acids or the D-sugars selected on the primeval Earth, or are exuaterresuial sources responsible for the homochirality This second possibility is dealt with by hypotheses on the effect of circularly polarised light, of extraterrestrial origin, on chiral molecules in the molecular clouds from which the solar system was formed. One such hypothesis was proposed by Rubenstein et al. (1983) and developed further by others, particularly A. W. Bonner (Bonner and Rubenstein, 1987) both scientists worked at Stanford University. The authors believe that the actual radiation source was synchrotron radiation from supernovae. The excess of one enantiomeric form generated by this irradiation process would have needed to be transported to Earth by comets and meteorites, probably during the bombardment phase around 4.2-3.8 billion years ago. 

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