ABA and Its Derivatives: Chemistry and Physiological Functions - Abscisic Acid: Metabolism, Transport and Signaling - page 2

1.1 Structures, Physicochemical Properties,
and Physiological Functions

1.1.1 ABA

ABA( 1 ) has a chiral centre (C-1′). Consequently, two enantiomers, S -( + )-ABA
and R -( − )-ABA (Fig. 1.1 ), are formed during chemical synthesis. All naturally
occurring ABAs have the S -coniguration, and its R counterpart has never been
detected in plants. The physicochemical properties of both isomers of ABA are
identical except for their optical characteristics, such as optical rotation and cir-
cular dichroism. ABA is sufficiently heat-stable to be dissolved in boiling water
without decomposition occurring. Although ABA is relatively stable towards
a broad range of pH values, it is converted to ʳ -lactone ( 2 ) under strong acidic
conditions such as formic acid-hydrochloric acid (Mallaby and Ryback 1972 )
(Fig. 1.2 ). Under alkaline conditions, the hydrogens at C-3′, -5′, -7′, -2, and -6
exchange with deuterium when dissolved in deuterated water (Milborrow 1984 ;
Willows et al. 1991 ; Gray et al. 1974 ) (Fig. 1.2 ). The six hydrogens at C-3′, -5′,
and -7′ are particularly easily exchanged with deuterium, making ABA- d 6 ( 3 ) a
useful internal standard for LC-MS and GC-MS analysis.
ABA is moderately photosensitive due to the dienoic acid side chain and the
ring enone. Irradiation with UV at 365 nm causes photoisomerization at the C-2
double bond to give an equimolar mixture of ABA and 2 E -ABA (2- trans -ABA,
5 ) (Fig. 1.2 ), which is biologically inactive (Todoroki et al. 2001 ). This isomeri-
zation occurs even upon irradiation using fluorescent and tungsten lamps. UV
light at wavelengths shorter than 305 nm decomposes the ABA molecule into
unidentified compounds, in addition to causing photoisomerization (Cornelussen
et al. 1995 ). This photolabile property of ABA limits its applications in agricul-
ture. Consequently, several ABA analogues have been designed and synthesised
to increase the photostability of ABA. For example, compounds 6 - 8 have a rigid
side chain with an aromatic ring (Chen and Mctaggart 1986 ; Kim et al. 1992 ;
Asami and Yoshida 1999 ) (Fig. 1.3 ). Recently, Wenjian et al. ( 2013 ) reported that
2,3-cyclopropanated ABA ( 9 ) is more photostable than ABA (Fig. 1.3 ).
ABA behaves as a weak acid due to the side chain carboxy group. Thus, the lipo-
philicity of ABA depends greatly on pH and is more lipophilic at lower pH. Because

6

8' 9'

5

4

6'

2

5' 3

1'

O H O H

4'

CO 2 H CO 2 H

1

2'

O O

7'

3'

S - 1 R - 1

Fig. 1.1 Chemical formula of S -( + )-ABA ( S - 1 ) and R -( − )-ABA ( R - 1 )