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(
∼
1.4 kcal mol
−
1
) (Todoroki and Hirai
2000a
,
b
) (Fig.
1.4
), the shape of the ABA
molecule when bound to proteins such as receptors, transporters, and catabolic
enzymes cannot be estimated based on the preferred form of the uncomplexed
ABA molecule. Todoroki et al. (
1996
) demonstrated that the biologically active
conformation of ABA is a half-chair with the pseudoaxial side chain, based on the
biological activity of cyclopropane analogues of ABA. Furthermore, the crystal
structures of the ABA receptor proteins PYR/PYL/RCAR (PYL) bound to ABA
revealed that ABA adopts a half-chair with the pseudoaxial side chain in the bind-
ing pocket (Melcher et al.
2009
; Miyazono et al.
2009
; Nishimura et al.
2009
;
Santiago et al.
2009
; Yin et al.
2009
) (Fig.
1.4
). However, this does not guarantee
that the conformation of ABA in other binding proteins, including transporters and
catabolic enzymes, is also a half-chair with the pseudoaxial side chain.
Many reports have demonstrated that both the
S
- and
R
-isomers of ABA show
similar hormonal activities in several assay systems (Lin et al.
2005
), suggesting
that plants have a mechanism which permits the
R
-isomer, which is not naturally
occurring, to mimic the endogenous hormone. The ABA molecule, although a
chiral molecule, is relatively symmetric about the plane constructed by C-5, C-1′,
and C-4′ (Fig.
1.5
). When the structural formula of
R
-ABA is drawn on paper, the
orientation of the side chain and the hydroxy group at C-1′ of
S
-ABA is usually
exchanged. However, this method of drawing does not accurately represent the
pseudo-symmetric property of the ABA molecule. Instead, the molecule should be
flipped to have the side chain and hydroxy group in the same orientation, as in the
case of
S
-ABA. By doing so, a large structural difference between the enantiomers
2+
2
+
&2
+
&2
+
2
2
S
1
R
1
Fig. 1.5
Structural comparison of enantiomers of ABA
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