ABA and Its Derivatives: Chemistry and Physiological Functions - Abscisic Acid: Metabolism, Transport and Signaling - page 3

CO 2 H

OH

O

5

h ν

HCO 2 H/HCl

OH

O

CO 2 H

O

O O

S - 1 2

NaOD/D 2 O

CD 3

D
D

D
D

D

NaOD/D 2 O

O H

OH

CO 2 H CO 2 H

> 2 months

C D 3

O CD 3

O

D D

3 4

Fig. 1.2 Acidic and alkaline reactions and photoisomerization of ABA

the undissociated form of ABA is cell-membrane-permeable, the in vivo distribu-
tion of ABA changes depending on the pH conditions (Wilkinson et al. 1998 ; Jiang
and Hartung 2008 ). This means that the movement of ABA within plants does not
always require specific transport proteins; nevertheless, some ABA transporters
seem to be involved in physiological processes mediated by ABA (Kang et al. 2010 ;
Kuromori et al. 2010 ; Seo and Koshiba 2011 ; Kanno et al. 2012 ).
The shape of the ABA molecule depends largely on the conformation of the
cyclohexenone ring. The crystal structure of ABA shows that the ring adopts a
slightly distorted sofa conformation with the pseudoaxial side chain (Ueda and
Tanaka 1977 ; Schmalle et al. 1977 ) (Fig. 1.4 ). The preferred form in solution,
revealed by NMR and CD analyses, is a half-chair with the pseudoaxial side chain
(Milborrow Milborrow 1984 ; Willows and Milborrow 1993 ; Koreeda et al. 1973 ;
Harada 1973 ). Theoretical conformational analyses revealed that the minimum-
energy conformer is a half-chair with the pseudoaxial side chain (Todoroki et al.
1996 ) (Fig. 1.4 ). Because the cyclohexenone ring can invert into another half-
chair with the pseudoequatorial side chain by paying only a small energy penalty