Identification of D-Amino Acids in Tea Leaves.

During manufacturing processes and in the storage period of tea, amino acids may undergo enantiomeric isomerization, converting their L- to D-forms. To examine the hypothesis, a method was developed for the analysis of the enantiomers in tea leaves. After enriched by ion-exchange solid-phase extraction, the enantiomeric pairs were separated by a chiral high performance liquid chromatography (HPLC) and subsequently detected and identified by using a high resolution quadrupole time-of-flight mass spectrometry (QTOF MS). Only L-forms of amino acids were found in fresh tea leaves. A total of 11 D-amino acids were found in 19 tea samples, ranging from trace amount to 43 mu g/g. The results indicated that the enantioisomerization of amino acids occurred in post-harvest tea leaves, and affected by process conditions and storage time.