Evaluation of the Free Radical Scavenging Capacity of Fresh and Fermented Pu-Erh Tea

Dried tea leaves ( Camellia sinensis ) contain various antioxidant molecules depending on their origin and preparation method. Computational methods are used to calculate the thermodynamic properties of their phenolic compounds as well as their free radical scavenging capacity. Extracts from young Puerh tea shoots and fermented cakes originated from tea forests in China's Yunnan province were analysed. The more a weak O-H bond dissociation enthalpy (BDE) is, the greater the capacity of an O-H to give its hydrogen and act as an antioxidant. The theoretic calculations on the main phenolic compounds present (gallic acid, catechin, epicatechin, epigallocatechin (EGC), epigallocatechin gallate (EGCG), epicatechingallate (ECG), and theaflavin) demonstrate the importance of the catechol and galloyl nuclei. Moreover, theaflavin contains a trihydroxy-annulene nucleus with a ketone forming a hydrogen bonds stabilising network. Superoxides and singlet oxygens are toxic energetic radicals involved in skin ageing, effects of UV light and pollution. Comparing the BDE values and kinetic constants on superoxides gives the following ranking: EGCG>ECG ≈ EGC>catechin ≈ epicatechin. The theoretic assessment of the kinetic constants of the scavenging capacity of singlet oxygen indicates similar and very high values (10 7 -10 8 M -1 .s -1 ) for the compounds, which suggest they are effective at scavenging these radicals. Extracts from young leaves rich in particularly active catechol and galloyl fragments, are able to scavenge the main reactive oxygen species (hydroxyl, alkoxyl, peroxyl, superoxide, singlet oxygen) at a faster rate. The high concentration of theaflavin in young leaf extract increases its superoxide scavenging. Extracts from fermented tea cakes richer in catechin oligomers (average degree of polymerisation 3-4), explains the enzyme inhibition and detoxifying activities. The use of one of the two extracts or a combination enables to modulate a potential skin care action.