Biologic systems chiral molecules

Biological systems depend on specific detailed recognition of molecules that distinguish between chiral forms. The translation machinery for protein synthesis has evolved to utilize only one of the chiral forms of amino acids, the L-form. All amino acids that occur in proteins therefore have the L-form. There is, however, no obvious reason why the L-form was chosen during evolution and not the D-form... 

In this chapter, we have surveyed a wide range of chiral molecules that self-assemble into helical structures. The molecules include aldonamides, cere-brosides, amino acid amphiphiles, peptides, phospholipids, gemini surfactants, and biological and synthetic biles. In all of these systems, researchers observe helical ribbons and tubules, often with helical markings. In certain cases, researchers also observe twisted ribbons, which are variations on helical ribbons with Gaussian rather than cylindrical curvature. These structures have a large-scale helicity which manifests the chirality of the constituent molecules. 

Metalaxyl and most of its active analogues are chiral molecules. Chirality is caused by the asymmetric carbon atom in the alkyl side chain of the alanine moiety. The two optically pure enantiomers S (+) and R (-) differ widely in their biological activity both in vitro and in vivo. In all experiments, the R (-) enantiomer was more active than its antipode S (+) (22, 24, 30). The main characteristics of metalaxyl have been discussed in detail by several authors (J, 21, 28, 29, 32> 38). Of particular value is the rapid uptake of metalaxyl by the plant tissue, especially under the wet conditions that favor foliar Oomycete diseases. Acylalanines are easily translocated in the vascular system of the plant after foliar, stem or root treatment (35, 47). The predominant route of transport is the transpiration stream, thus apoplastic (12, 35). Symplastic transport occurs but is much less evident (35, 47). In potatoes treated by foliar sprays of metalaxyl concentrations (0.02-0.04 ppm), Bruin et al. (SO were able to demonstrate protection of harvested tubers from late blight. 

Two enantiomers of one molecule may be the same compound, but they are clearly different, though only in a limited number of situations. They can interact with biological systems differently, for example, and can form salts or compounds with different properties when reacted with a single enantiomer of another compound. In essence, enantiomers behave identically except when they are placed in a chiral environment. In Chapter 45, we will see how to use this fact to make single enantiomers of chiral compounds, but next we move on to three classes of reactions in which stereochemistry plays a key role substitutions, eliminations, and additions. 

The purpose of this article is to provide an overview of the different types of chemical and biological catalysis currently available to the pharmaceutical industry in the process area. In other words, these transformations can be performed at scale. The types of catalysts that have been used are given together with systems that show potential for future application. The chemocatalytic area has addressed the synthesis of aromatic and heterocyclic compounds, which are common classes in pharmaceutically active compounds, whereas biocatalyst applications tend to be aimed toward the production of chiral molecules. 

Note that all of these families of molecules exhibit chirality s in some, and generally most, of tlreir members. Also, biological systems chose a single chirality for each family (e.g., L-amino acids, D-sugars)... 

Proteins are complex polymers of high molecular weight that are composed of chiral subunits (L-amino adds). These biopolymers play a variety of different roles in a biological system, including the uptake and transport of drug substances. One of the key aspects of the uptake and transport mechanisms is the reversible binding of small molecules to the protein and this process is often stereospectfic. 

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