关键词: 茶叶, 游离氨基酸, 对映异构体, 气相色谱-质谱联用技术, 五氟丙酸酐

Abstract:

【Objective】 Free amino acids are the main chemical compositions in teas, and they are closely related to the taste quality of tea. In account of the existence of stereogenic centers, most amino acids contain two enantiomers (D and L configurations) with obviously different taste characteristics and biological activities. However, due to the limitation of detection technology, the studies of L-amino acids were focused in the previous work, and the D-amino acids have been rarely studied. Therefore, the study on the enantiomers of free amino acids in teas is very important for deepening the theoretical system of tea chemistry and is beneficial for improving and controlling of tea taste quality. 【Method】 In this study, the separation performance of free amino acid enantiomers (Chiral HPLC, Silanization of MTBSTFA and Esterification PFP acylation) was compared by using three different analytical methods. A total of 15 pairs of amino acids, including alanine, valine, threonine, isoleucine, leucine, proline, serine, cysteine, aspartic acid, methionine, phenylalanine, glutamic acid, tyrosine, lysine and tryptophan, were effectively separated by using esterification- pentafluoropropionic acylation combined with chiral gas chromatography-mass spectrometry (GC-MS). Furthermore, the efficient derivatization, qualitative and quantitative analysis approaches were established, and the distribution regularities of amino acids enantiomers in 11 representative commercial white, oolong and pu-erh teas were investigated. 【Result】 The derivatization conditions were showed as follows: the reaction mixture was esterified at 100oC for 105 min, and then further acylated by pentafluoropropionic anhydride using tetrahydrofuran as the solvent at 100oC for 10 min. Recoveries of the free amino acid enantiomers were ranged from 75.26% to 123.6% (low concentration) and from 81.23% to 121.8% (high concentration), and the corresponding RSDs were ranged from 2.09% to 13.12% (low concentration) and from 1.48% to 10.59% (high concentration). The analysis results indicated that 1-2 enantiomers of most amino acids could be detected, and D-amino acids were frequently distributed in most tea samples, especially D-threonine, D-aspartic acid, D-cysteine, D-phenylalanine, and the mixture of D-theanine, D-glutamic acid and D-glutamine. On the contrary, D-methionine, D-lysine and D-tryptophan were unable to be detected in all samples. As for the detailed content distributions, in addition to the mixture of L-theanine, L-glutamic acid and L-glutamine and L-aspartic acid, L-cysteine (1.48-2.08 mg?g^-1), D-cysteine (1.46-1.49 mg?g^-1) and D-aspartic acid (1.02-1.14 mg?g^-1) presented higher contents in white teas, L-cysteine (1.52-1.70 mg?g^-1), D-cysteine (1.45-1.49 mg?g^-1), L-serine (1.03-1.50 mg?g^-1), L-methionine (1.03-1.52 mg?g^-1), L-tyrosine (1.32-1.35 mg?g^-1) and D-aspartic acid (1.01-1.15 mg?g^-1) were abundant in oolong teas. In pu-erh teas, no significant differences on the contents between the mixture of L-theanine, L-glutamic acid and L-glutamine (1.04 mg?g^-1) and other amino acids, and L-threonine, L-tryptophan, and L-serine presented relatively high content levels ranging from 0.61 to 0.84 mg?g^-1. The content distribution of the total amino acids were presented the following tendency: white tea (up to 40.61 mg?g^-1) > oolong tea (up to 25.43 mg?g^-1) > pu-erh tea (8.01 mg?g ^-1). Moreover, multivariate statistical analysis results indicated that the mixture of L-theanine, L-glutamic acid and L-glutamine, L-tryptophan, L-aspartic acid, L-tyrosine and L-methionine showed significant content differences, and the first three enantiomers were abundant in white teas, and other compounds were rich in oolong teas.【Conclusion】 D-amino acids could be detected in most teas, indicating their potential impact on the taste quality of tea infusion. However, no significant differences on the content distribution of D-amino acids were observed among different kinds of teas used in our study.

Key words: tea, free amino acids, enantiomers, gas chromatography-mass spectrometry, pentafluoropropionic anhydride