Characterization of phenolic compounds in green and red oak-leaf lettuce cultivars by UHPLC-DAD-ESI-QToF/MS using MS^E...

Lettuce (Lactuca sativa) is one of the most popular leafy vegetables in the world and constitutes a major dietary source of phenolic compounds with health-promoting properties. In particular, the demand for green and red oak-leaf lettuces has considerably increased in the last years but few data on their polyphenol composition are available. Moreover, the usage of analytical edge technology can provide new structural information and allow the identification of unknown polyphenols. In the present study, the phenolic profiles of green and red oak-leaf lettuce cultivars were exhaustively characterized by ultrahigh-performance liquid chromatography (UHPLC) coupled online to diode array detection (DAD), electrospray ionization (ESI), and quadrupole time-of-flight mass spectrometry (QToF/MS), using the MS^E instrument acquisition mode for recording simultaneously exact masses of precursor and fragment ions. One hundred fifteen phenolic compounds were identified in the acidified hydromethanolic extract of freeze-dried lettuce leaves. Forty-eight of these compounds were tentatively identified for the first time in lettuce, and only 20 of them have been previously reported in oak-leaf lettuce cultivars in literature. Both oak-leaf lettuce cultivars presented similar phenolic composition, except for apigenin-glucuronide and dihydroxybenzoic acid, only detected in the green cultivar; and for luteolin-hydroxymalonylhexoside, an apigenin conjugate with molecular formula C₄₀ H₅₄ O₁₉ (monoisotopic MW = 838.3259 u), cyanidin-3-O-glucoside, cyanidin-3-O-(3″-O-malonyl)glucoside, cyanidin-3-O-(6″-O-malonyl)glucoside, and cyanidin-3-O-(6″-O-acetyl)glucoside, only found in the red cultivar. The UHPLC-DAD-ESI-QToF/MS^E approach demonstrated to be a useful tool for the characterization of phenolic compounds in complex plant matrices.