Characterization of catechins in green and black teas using electrochemical methods

U ovome radu ispitivano je voltametrijsko ponašanje galne kiseline (GK) i glavnih katehinskih spojeva ili katehina [epigalokatehin galat (EGCG), epigalokatehin (EGC), epikatehin (EC) i epikatehin galat (ECG)] prisutnih u zelenim i crnim čajevima. Također, razvijena je nova elektroanalitička metoda za karakterizaciju katehina u vodenim ekstraktima odnosno suhom lišću zelenih i crnih čajeva. Podaci o katehinskom profilu zelenih i crnih čajeva dobiveni su primjenom tekućinske kromatografije visoke djelotvornosti u kombinaciji s elektrokemijskim detektorom (HPLCECD). Ustanovljeno je da su glavni katehini u zelenim i crnim čajevima EGCG i EGC, koji čine 84-89 % ukupnih katehina zelenog i 53-89 % ukupnih katehina crnog čaja. Voltametrijska mjerenja [pravokutnovalna voltametrija (SWV) i ciklička voltametrija (CV)] na elektrodi od staklastog ugljika (GCE) pokazala su da je elektrokemijska oksidacija katehina difuzijom kontrolirani kvazireverzibilni proces. Voltametrijska svojstva katehina funkcija su koncentracije katehina, pH i brzine polarizacije. Utvrđeni su optimalni uvjeti i validirana je SWV kao metoda za karakterizaciju katehina u zelenim i crnim čajevima. Analizirani su uzorci zelenih i crnih čajeva primjenom SWV metode na GCE i određen je sadržaj katehina u vodenim ekstraktima čajeva. Koncentracije katehina dobivene SWV metodom gotovo su identične onima dobivenima HPLC-ECD analizom. Stoga, SWV metoda predstavlja vrlo dobru alternativnu metodu analize katehinskih spojeva u ekstraktima zelenih i crnih čajeva u odnosu na zahtjevnu i skupu HPLC analizu. Kao dodatna metoda karakterizacije katehina korištena je abrazivna SWV s anodnim otapanjem uzorka imobiliziranog na površini parafinom impregnirane grafitne elektrode (PIGE). Ova metoda pokazala se kao brza i jednostavna semikvantitativna metoda za detekciju elektroaktivnih katehina u suhom lišću zelenih i crnih čajeva.The voltammetric behaviour of gallic acid (GA) and the main catechins [epigallocatechin gallate (EGCG), epigallocatechin (EGC), spicatechin (EC), and epicatechin gallate (ECG)] found in green and black teas was investigated. In addition, novel electroanalytical procedure for characterization of catechins in tea extracts and dry tea leaves was developed. The concentrations of individual catechins in green and black teas were determined using high-performance liquid chromatography with electrochemical detection (HPLC-ECD). EGCG and EGC were the major catechins in green and black tea samples, representing 84-89% and 53- 89% of total catechins, respectively. Square-wave and cyclic voltammetry experiments showed that the oxidation of catechins on glassy carbon electrode (GCE) is a quasireversible and diffusion controlled process. It has been shown that voltammetric properties of catechins depend on pH, scan rate and catechin concentration. A new SWV method for characterization of catechins has been established, optimized and validated. The proposed method was applied to characterize the catechins in green and black teas. Water extracts of green and black teas were analysed using SWV on GCE. Amounts of catechins in green and black teas obtained using SWV were in good agreement to those obtained using HPLC-ECD. Accordingly, SWV has been shown to be a good alternative compared to more expensive, laborious and time-consuming HPLC methods. In addition, the abrasive stripping SWV of microparticles of tea leaves immobilized on the surface of paraffin-immpregnated graphite electrode (PIGE) was used for characterization of catechins as well. This semiquantitative method enabled a fast and simple screening of catechins in dry leaves of green and black teas