Resolution of Optical Isomers

The determination of the configuration of amino acids and the relative proportions of the d and l forms is important in both natural and synthetic contexts. Proteins in living organisms commonly contain only the l-amino acids but d-amino acids occur in antibiotics (e.g. antiamoebin, gramicidin, valinomycin), bacterial cell wall peptidoglycans and in animals and insects. They have also been detected in human urine and blood. On death, the l-amino acids racemize, but so slowly that a racemic mixture is only produced over a geological time scale. The racemization rate is a function of temperature and the structure of each amino acid. Aspartic acid, which has a racemization half-life of about 15 000 years at 20°C, is most commonly used for archaelogocial dating, but there is considerable controversy over the results obtained.

Animal bones and shells and certain sediments contain proteins, for example, collagen and conchiolin. Extraction of the residual protein and determination of the enantiomer ratio of aspartic acid following hydrolysis can, when combined with knowledge of the thermal history of the sample, be used to determine the age of the fossil. Recemization age dating is generally more sensitive and less expensive than the radiocarbon method. Typical examples of the use of this technique have been analysis of Apollo 12 lunar material and dating of the Dead Sea scrolls.

Amino acids also racemize under various conditions such as prolonged acid hydrolysis and during solid-phase peptide synthesis. Chemical procedures such as asymmetric synthesis require proof of enan-tiomeric purity, especially if the product is to be used for pharmaceutical purposes. Configurational analysis of peptide antibiotics and establishing retention of configuration during peptide synthesis are other contexts in which it is important to determine the enan-tiomeric composition of amino acid samples.

Enantiomeric amino acid mixtures are resolved using two approaches. The first is to derivatize (acylate or esterify) with optically active reagents to form diastereoisomers or diastereomers which are resolved on an optically inactive stationary phase. The reagents must be of high optical purity and conversion must be quantitative. The second approach is to de-rivatize with optically inactive reagents, for example the N-TFA isopropyl esters, and then conduct the separation on columns containing optically active stationary phases.

Most amino acid optical isomers result from asymmetry at the a-carbon atom and depend on the presence of an a-hydrogen atom. However, some contain two optically active centres. Thus, the threo and erythro forms of the hydroxy amino acids and isoleucine and allo-isoleucine can be resolved on conventional columns. Similarly, isovaline, which contains one asymmetric centre but no a-hydrogen, has also been resolved. The mechanism has been postulated to depend on the formation of transient dia-stereomeric hydrogen-bonded association complexes but other factors such as dipole-dipole interactions and dispersion forces may also play a role.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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