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128 Chapter 6
O
H
− OH
− O
OH
H
H
O
O O − H H
OH
NaOH
hydride
transfer
+
Figure 6.8 The Cannizzaro reaction.
reaction. Although radical intermediates are in theory possible, attempts to
identify such intermediates have been unsuccessful, so hydride transfer via a
single transition state is the accepted mechanism.
Dehydrogenase enzymes are each speciWc for one of the enantiotopic C-4
protons of NADH: some are speciWc for the 4R hydrogen; others are speciWc for
the 4S hydrogen. StereospeciWcity of an NAD-dependent dehydrogenase can be
readily determined as shown in Figure 6.9. [1- 2 H 2 ]-Ethanol is incubated with
alcohol dehydrogenase and the [4R- 2 H]-NADD product isolated. This is then
incubated with the dehydrogenase of unknown stereospeciWcity, and the incorporation
of 1 Hor 2 H into the reduced product is monitored. The experiment
can be done more sensitively using a 3 H label in a similar way.
While this stereospeciWcity is largely due to the orientation of the cofactor in
the enzyme active site, there are indications that a further stereoelectronic eVect
may also be important. Model studies using dihydropyridine rings contained in
molecules in which the two C-4 hydrogens are diastereotopic have shown that
the environments of the two hydrogens are quite diVerent (see Figure 6.10). This
D
D
OH
alcohol
dehydrogenase
NAD +
H
N
R
D
CONH 2
Figure 6.9 Determination of dehydrogenase speciWcity.
dehydrogenase
R 2 C=O
NAD + + R 2 CHOH or
R 2 CDOH
Ph
N
O
O
NH H H HN
1 H NMR
N
δ H 1.43, 2.12 ppm R
H
H
σ* orbital of this
C H bond aligned
with p-orbitals, promotes
C H cleavage
Figure 6.10 Puckering of the dihydropyridine ring of NADH.