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Non-Enzymatic Biological Catalysis 267
SH
O
O
O
SH
O
O
O
catalyses
RCH
NO 2
RCH
O O O
SH
COO
EtOH
+
NH 3
NO 2
O
O
O
SH
CO 2 Et
+
+ HO
NH 3
Ar
S − O
O
O
H
H
O
H
O
O
H
R
O
O
selectivity for D-amino acids
explained by positioning of R
group away from aromatic spacer
SH
Figure 12.13 Crown ether catalytic binding site.
O
O
O
O
+ H 2 N NH HN NH +
2 + H 2 N
NH +
NH HN
2
O
O
NH 2 H 2 N
P
NH 2
H 2 N
P
O O−
O
−O O
O
OH
O 2 N
Figure 12.14 Anionic binding site.
By attaching a blocking agent to a hydroxyl group on one face of the
bucket, a more hydrophobic cavity is obtained that is more eVective at binding
aromatic molecules. One such blocked cavity accelerated the hydrolysis of a
meta-substituted phenyl acetate, shown in Figure 12.15, by 3300-fold over the
rate of uncatalysed hydrolysis. This can be rationalised, as shown in Figure
12.15, by the participation of the free hydroxyl groups on the rim of the cavity,
which come into close proximity with the meta-substituent.