Biochemistry - The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

The Molecules of Which We Are Made
Principles of Small Molecule Construction
Configuration Description Using RS Notation

In 1956, Cahn, Ingold, and Prelog proposed an unambiguous method for describing the configuration about any chiral center [1]. This approach is particularly useful for classes of compounds that lack a firmly established system for assigning D- and L-configurations. The four groups surrounding the carbon atom (or another central atom) are arranged in order of priority. The priority of each specific group is determined according to a set of rules, the most important of which is that a higher atomic number takes precedence over a lower one. In the illustrative example below, using a D-Alanine molecule, group priorities are designated as a>b>c>d. The highest priority is assigned to the NH₂ group, in which a nitrogen atom is attached to the central atom. To determine the configuration, the observer looks down the axis connecting the central atom to the lowest-priority group, i.e., group d. In this case, The sequence of groups a, b, and c may run either clockwise, as in our example, or counterclockwise.

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To establish group priorities, one first examines the atoms directly attached to the central atom, arranging them in order of decreasing atomic number. Then, if necessary, one proceeds to the next set of atoms and compares their atomic numbers. This is precisely how the priorities of groups b and c in alanine must be determined, since both are linked to the central atom via a carbon atom. If atoms are involved in double bonds, as is the case with the carboxyl group of alanine, they are counted

twice — the second time as fictitious atoms continuing the chain of bonds:

We will not review the complete set of rules here; we merely list the sequence in which the functional groups below are arranged in order of decreasing priority [2]: SH > OR > ОН > NHCOCH₃ > NH₂ > COOR > СООН > ОНО > СН₂OН > С₆Н₅ > СН₃ > ³Н > ²Н > Н. Although the RS system is entirely unambiguous, two closely related compounds belonging to the same configuration in the DL system often have opposite configurations in the RS system. Therefore, the DL system is generally used when determining the configuration of Amino Acids and sugars.

¹ Sometimes the subscripts s or g are added to the letters D and L to indicate which reference compound was used for the configuration standard — Serine or glyceraldehyde (Sec. B.1). Common Threonine is L_s- or D_g-threonine. The configuration of dextrorotatory (+)-tartaric acid is designated as 2(R), 3(R), D_s, or L_g.