Novel fatty acid esters of p-coumaryl alcohol in epicuticular wax of applefruit

Hexane extracts of epicuticular wax from ev. Gala apples were noted to have an unusual, broad absorbance maximum at similar to 258 nm, which led us to isolate and identify the primary UV-absorbing compounds. Column and thin-l ayer chromatography yielded a fraction that gave a series of paired, 260-nm -absorbing peaks on C-18 HPLC. These were shown to be a family of phenolic fatty acid esters, for which retention times increased with increasing fatt y 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 C-18 HPLC. Electrospray ionizatio n mass spectrometry gave a molecular mass of 150 for both, and H-1 NMR plus U-V absorbance spectra identified them as E and Z isomers of p-coumaryl al cohol. Alkaline cleavage of the fatty acid esters in the presence of methan ol or ethanol resulted in partial derivatization of E-p-coumaryl alcohol to the corresponding gamma -O-methyl or O-ethyl ether. Gradient HMQC NMR of t he HPLC-purified stearate ester of E-p-coumaryl alcohol indicated that fatt y acid esterification occurs at the gamma -OH rather than at the 4-OH on th e phenyl ring. This is the first report of fatty acid esters of monolignols as a natural plant product.