In order to describe the configuration of a stereogenic centre, IUPAC recommends the use of Cahn-Ingold-Prelog (CIP) rules yielding to S/R-notations. According to CIP-rules, a priority number is to be assigned to each of the four substituents at a given stereogenic centre. Substituents with higher atomic numbers (Z) are assigned with higher priorities. In the frame of this book, which is focussing particularly on the chirality of amino acids, we will use the most simple chiral amino acid which is a-alanine as an introducing example: The stereogenic center of a-alanine, which is the central carbon atom, is linked to one nitrogen atom (Z = 7), a carbon atom (Z = 6) carrying oxygen (Z = 8), another carbon atom (Z = 6) attaching hydrogen atoms (Z = 1), and one hydrogen atom (Z = 1). Priorities of the substituents are consequently decreasing from the amino- to the carboxyl- over the methyl- to the hydrogen-substituent (Fig. 2.6).
The alanine molecule has now to be arranged in a way that the substituent with the lowest priority, in our case hydrogen (Z = 1), is pointing away from the viewer, as indicated by the dashed line in Fig. 2.6. Now, one describes a circle from the substituent with highest priority to the substituent with lowest priority. If one rotates
2 Stereochemistry for the Study of the Origin of Life 2
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