Abstract:

Based on the charge-transfer complexation between tetrachlorobenzoquinone and theanine, a spectrophotometric method was developed for the determination of theanine contents in tea and tea beverage. Pure theanine could be obtained by 80% ethanol extraction and then copper salt precipitation, followed by recrystallization in absolute ethanol, and the extraction yield of theanine was over 84.6%. After theanine aqueous solution and tetrachlorobenzoquinone ethanolic solution was left to react at pH 8－9 and 50℃ for 50 min, a purplish red complex with the mole ratio of theanine to tetrachlorobenzoquinone of 2:1 was obtained. Its stability constants k (293 K) and Gibbs free energy ΔrGm (293 K) were 3.9×103 and －20.14 kJ/mol, respectively, and at working wavelength (λ = 331 nm), its apparent molar absorptivity coefficient was 1.98×104 L/ mol·cm, which obeys Beer's law in the range of 5×10-6－9×10-5 mol/L with the linear equation, A = 0.028×10-6 C － 0.02 (r = 0.9958). When the method was applied in the determination of theanine content in commercial tea and tea beverage, the results were consistent with those determined by HPLC with a recovery of 98.7%－102.6% and a relative standard deviation of 1.2%－4.2%. Altogether, this method is easy-operating and highly sensitive, and can provide accurate results.

Key words: tea, tea beverage, theanine, tetrachlorobenquinone, charge-transfer reaction, spectrophotometry