[NH2:1][C@H:2]([C:8]([OH:10])=[O:9])[CH2:3][CH2:4][C:5](=[O:7])[NH2:6].[CH2:11](N)[CH3:12]>B([O-])([O-])[O-]>[NH2:1][C@H:2]([C:8]([OH:10])=[O:9])[CH2:3][CH2:4][C:5]([NH:6][CH2:11][CH3:12])=[O:7]0.3 M glutamine and 1.5 M ethylamine were allowed to react at 30° C. for 22 hours in borate buffer (Na2B4O7—NaOH, pH 11) in the presence of 0.3 U glutaminase, to give 225 nmol of L-theanine. In addition, a by-product glutamic acid was 20 nmol. The purification of theanine from the reaction mixture was carried out by subjecting the reaction mixture to column chromatography using Dowex 50×8 column and Dowex 1×2 column, and thereafter treating the resulting product with ethanol.
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