In this study, the chemical compositions and acetylcholinesterase (AChE) inhibitory activitiy of the volatile oil from the bark of Peltophorum dasyrachis Kurz ex Bakar (yellow batai) were evaluated. As a result, 68 compounds, accounting for 88.0% of the total oil, were identified. The main characteristic constituent in P. dasyrachis was isolated by silica gel column chromatography and found to be a sesquiterpenoid, (+)-(S)-ar-turmerone (1). In the AChE inhibitory assay, the volatile oil showed potent inhibitory activity with the IC(50) value of 83.2 +/- 2.8 microg/mL. Among the volatile oil components and characteristic sesquiterpenoids, (+)-(S)-ar-turmerone (1) and (+)-(S)-dihydro-ar-turmerone (2) were potent compounds, inhibiting AChE in a dose-dependent manner, with IC(50) values of 191.1 +/- 0.3 and 81.5 +/- 0.2 microM, respectively. (+)-(S)-Dihydro-ar-turmerone (2), in particular, was found to be the most potent AChE inhibitor. Also, bisabolane-type sesquiterpenoid derivatives, (+)-(7S,9S)-ar-turmerol (3), (+)-(7S,9R)-ar-turmerol (4), (+)-(7S,9S)-dihydro-ar-turmerol (5), (+)-(7S,9R)-dihydro-ar-turmerol (6), (+)-(S)-ar-curcumene (7), and (+)-(S)-dihydro-ar-curcumene (8), were synthesized and tested for their AChE inhibitory effect, and their structure-activity relationships were evaluated. All sesquiterpenoids exhibited AChE inhibitory activity. The order of AChE inhibitory potency by bisabolane-type sesquiterpenoids was as follows: ketones > alcohols > hydrocarbons. Furthermore, the inhibition kinetics analyzed by Dixon plots indicated that (+)-(S)-ar-turmerone (1) is a competitive inhibitor, with a K(i) value of 882.1 +/- 2.1 microM, whereas (+)-(S)-dihydro-ar-turmerone (2) is a non-competitive inhibitor.
