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1′-deoxy-ABA (
32
), 1′-fluoro-ABA (
33
) (Todoroki et al.
1995a
,
b
,
c
), 1′-
O
-methyl-
ABA (
34
) (Rose et al.
1996
), 4′-deoxo-ABA (
35
) (Oritani and Yamashita
1974
),
1′,4′-
cis
/
trans
-diol-ABAs (
36
and
37
) (Walton and Sondheimer
1972
), and
trans
-
4′-alcoxy-ABA (
38
-
40
) (Asami et al.
2000
;
2002
) are weaker than that of ABA
by a factor of 10-100. The
cis
isomer of 4′-alcoxy-ABA is much less potent than
the
trans
isomer. Interestingly, some ABA analogues containing a larger functional
group than ABA's functional group at C-4′ (
41
-
44
) (Fig.
1.10
) retain ABA activ-
ity, albeit weaker than ABA's by a factor of 10-100 (Kohler et al.
1997
; Asami
et al.
1997
; Kitahata et al.
2005
). Because these compounds are substituted using
a hydrozone linker at C-4′, it is possible that ABA is released by hydrolysis of the
analogue in plant cells. On the other hand, the activity of 4′-octoxy-ABA (
39
) and
4′-benzyl-ABA (
40
) (Asami et al.
2002
) cannot be derived from released ABA and
must be due to the analogue, since an ether bond is more resistant to hydrolysis
than a hydrazone. The significance of this will be discussed later.
The two methyl groups at C-3 in the side chain and at C-2′ in the ring (C-6
and C-7′, respectively) are more important for activity than the geminal methyl
groups at C-6′ in the ring (C-8′ and C-9′). The C-6 and C-7′ methyl groups may
contribute to specific recognition rather than to nonspecific hydrophobic interac-
tions. The modification of C-8′ or C-9′ sometimes results in an increase in activ-
ity. In particular, ABA analogues, in which C-8′ is replaced by a trifluoromethyl
(
45
) or an acetylenyl (
46
), and where C-9′ is replaced by a propagyl (
47
), are the
most potent analogues tested to date in bioassays (Todoroki et al.
1995a
,
b
,
c
;
Cutler et al.
2000
) (Fig.
1.11
). 5′
ʱ
,8′-Cyclo-ABA (
48
), which contains a direct
linkage between C-5′ and C-8′, is also remarkably effective in some bioassays
(Todoroki et al.
1996
). The amplified activity of these analogues may depend par-
tially on their long lifetime in plant cells, since modification of C-8′ or its neigh-
bouring groups makes these compounds resistant to CYP707A enzymes (Cutler
et al.
2000
). Nevertheless, since recombinant Arabidopsis ABA 8′-hydroxylase
CYP707A3 is not inhibited by the 8′-acetylenic analogue of ABA (Ueno et al.
2005
), another factor may be involved. 2
E
-ABA (
5
) is also inactive, probably
because the 2
E
-coniguration greatly affects the orientation of the C-1 carboxy
group. The C-2′ double bond in the ring can be changed to a single bond with only
a slight loss of activity if C-7′ is
cis
to the side chain (
49
), that is, when the ring
prefers a half-chair conformation with the pseudoaxial side chain (Fig.
1.11
). This
agrees with the finding that the ring conformation is a significant factor for activ-
ity. As described above, the active conformation of ABA is a half-chair with the
pseudoaxial side chain.
1.2.2 For Binding to PYL Proteins
The affinity of ABA analogues to PYL proteins has rarely been measured
directly or indirectly. We can only speculate on their affinity on the basis of
the crystal structure of the PYL-ABA complex, where ABA is almost entirely
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