Novel Fatty Acid Esters of p-Coumaryl Alcohol in Epicuticular Wax of Apple Fruit

Abstract

Hexane extracts of epicuticular wax from cv. Gala apples were noted to have an unusual, broad absorbance maximum at ∼258 nm, which led us to isolate and identify the primary UV-absorbing compounds. Column and thin-layer chromatography yielded a fraction that gave a series of paired, 260-nm-absorbing peaks on C₁₈ HPLC. These were shown to be a family of phenolic fatty acid esters, for which retention times increased with increasing fatty acid chain length, and paired peaks were esters of two related phenolics with the same fatty acid moiety. Alkaline hydrolysis of the esters released two water-soluble phenolics separable by C₁₈ HPLC. Electrospray ionization mass spectrometry gave a molecular mass of 150 for both, and ¹H NMR plus UV absorbance spectra identified them as E and Z isomers of p-coumaryl alcohol. Alkaline cleavage of the fatty acid esters in the presence of methanol or ethanol resulted in partial derivatization of E-p-coumaryl alcohol to the corresponding γ-O-methyl or O-ethyl ether. Gradient HMQC NMR of the HPLC-purified stearate ester of E-p-coumaryl alcohol indicated that fatty acid esterification occurs at the γ-OH rather than at the 4-OH on the phenyl ring. This is the first report of fatty acid esters of monolignols as a natural plant product. Keywords: p-Coumaryl alcohol; fatty acid esters; monolignol; epicuticular wax; apple fruit; Malus domestica