Concept explainers
Thioesters
Thioesters have the general formula . They resemble their oxygen counterparts
(oxoesters) in structure and reactivity more than other
chlorides, acid anhydrides, and amides. Thioesters can be prepared from thiols by reaction with acyl chlorides or acid anhydrides in much the same way as oxoesters are prepared from
alcohols.
The preparation of thioesters by Fischer esterification is not very effective, however, because
the equilibrium is normally unfavorable. Under conditions in which ethanol is converted to ethyl
benzoate to the extent of
This, and numerous other observations, indicates that
Like chlorine, sulfur is a third-row element and does not act as an electron-pair donor to the carbonyl group as well as oxygen.
Thioesters and oxoesters are similar in their rates of nucleophilic acyl substitution, except with
amine nucleophiles for which thioesters are much more reactive. Many biological reactions involve nucleophilic acyl substitutions referred to as acyl transfer reactions. The thioester acetyl coenzyme A is an acetyl group donor to alcohols,
Melatonin, a hormone secreted by the pineal gland that regulates circadian rhythms, including wake–sleep cycles, is biosynthesized by a process in which the first step is an enzyme-catalyzed transfer of the acetyl group from sulfur of acetyl coenzyme A to the
Acetylcholine is a neurotransmitter formed in nerve cells by the enzyme-catalyzed reaction
of choline with acetyl coenzyme A.
What is the most reasonable structure for choline?
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