Direct chiral separation of abscisic acid by high‐performance liquid chromatography with a phenyl column and a mobile phase containing γ‐cyclodextrin

Abscisic acid (2‐cis,4‐trans‐abscisic acid) is a plant hormone that has an asymmetric carbon atom. We tried to separate the enantiomers of native abscisic acid by HPLC using a phenyl column and a chiral mobile phase containing γ‐cyclodextrin. The optimum mobile phase conditions were found to be 0.8% (w/v) γ‐cyclodextrin, 4% (v/v) acetonitrile, and 20 mM phosphate buffer (pH 6.0). It was found that (R)‐abscisic acid was earlier detected than (S)‐abscisic acid. Since γ‐cyclodextrin is hardly retained on a phenyl column, it was suggested that (R)‐abscisic acid formed a more stable complex with γ‐cyclodextrin than the (S)‐abscisic acid. Abscisic acid in an acacia honey sample was successfully enantioseparated with the proposed method and only (S)‐abscisic acid was detected. A biologically inactive 2‐trans,4‐trans‐abscisic acid, which was prepared by irradiation of abscisic acid with a light‐emitting diode lamp at 365 nm, was partially enantioseparated by the proposed method. Since the irradiation of (S)‐abscisic acid‐induced cis‐to‐trans isomerization to produce one 2‐trans,4‐trans‐abscisic acid enantiomer, it is reasonable that racemization did not proceed during the cis‐to‐trans isomerization. (S)‐Abscisic acid and probably (S)‐2‐trans,4‐trans‐abscisic acid were detected in a honey sample, where the peak area of (S)‐abscisic acid was 7 times larger than that of (S)‐2‐trans,4‐trans‐abscisic acid.