JH Lh Lh

achiral prochiral chiral

Figure 2.25 Methanol is achiral, by substituting one hydrogen by deuterium it becomes prochiral. Replacing another hydrogen by tritium, the molecule is now chiral. A methyl group substituted with deuterium and tritium is often used in studying the mechanism of biosynthetic reactions stereospecifically removes from ethanol one of the two hydrogen atoms attached to this carbon in the oxidation to acetaldehyde. It is the pro-R hydrogen that is removed (Figure 2.9). We have to imagine the ethanol molecule being held rigidly on the enzyme surface, much like glycerol in Figure 2.23, and being attacked by the co-enzyme from one side. Methanol is not prochiral. Two changes have to be made before it becomes chiral, as in Figure 2.25.

Another example of chiral induction will be found in Chapter 6 in the biosynthesis of terpenes, where isopentenyl pyrophosphate is held on the enzyme surface and the pro-R hydrogen is selectively removed (Figure 6.2). The same pro-R hydrogen is removed in terpene chain extension, as, for example, in the formation of geranyl pyrophosphate (Figure 6.3).

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