Chemistry Reference
In-Depth Information
Scheme 4.2 Diels-Alder cycloaddition of tetrachlorothiophene diox-
ide (4) and N-ethyl maleimide (5) yields a high energy intermediate (6)
that decomposes spontaneously by elimination of sulfur dioxide. The
initially formed product is subsequently oxidized in situ. Transition
states for cycloaddition and chelotropic SO
2
elimination closely re-
semble the hexachloronorbornene derivative 3 used as a hapten to elicit
antibody 1E9.
Antibodies raised against neutral bicyclic compounds that mimic the boat-like geome-
try of the Diels-Alder transition state have achieved significant catalytic effects, including
large rate accelerations, multiple turnovers, and control over both reaction pathway and
absolute stereochemistry [2]. For example, the hexachloronorbornene derivative
3
is an
excellent mimic of the transition state for the [4+2] cycloaddition between tetrachlorothio-
phene dioxide (
4
) and N-ethylmaleimide (
5
) (Scheme 4.2) [22]. Antibodies that bind this
compound catalyze the Diels-Alder reaction efficiently and experience minimal product
inhibition because large structural changes ensue when the initially formed product (
6
)
eliminates sulfur dioxide and becomes oxidized [23]. One of these antibodies, 1E9, attains
an effective molarity (EM) in excess of 1000
M
[22]. The EM represents the diene (or
dienophile) concentration needed to convert the uncatalyzed reaction into a pseudo-un-
imolecular process as fast as the reaction at the antibody active site. As such, it is a mea-
sure of the advantage the antibody derives frompreorganizing the substrates for reaction.
By comparison, EMs seldom exceed 20
M
for other antibodies [24-26], ribozymes [27],
and even natural enzymes [28] that promote Diels-Alder reactions.
Structural studies indicate that 1E9's efficiency results from an unusually snug fit
between the protein and the cycloaddition transition state [22]. Approximately 86% of
the hapten is buried upon binding, with no interfacial cavities discernable between
protein and ligand. This tight packing is achieved through extensive van der Waals
contacts and
-stacking interactions. In addition, the side chain of Asn
H35
at the
base of the pocket provides a hydrogen bond to the buried succinimide carbonyl group.
Such interactions are ideally suited to preorganize the diene and dienophile for pro-
ductive reaction, and to stabilize the transition state enthalpically [29].
The immunological origins of 1E9 have been traced to a family of highly restricted
germline antibodies that bind diverse hydrophobic ligands [30]. A precise fit to the
p
