HPLC Determination of Phenolic Acids, Flavonoids and Juglone in Walnut Leaves

A high-performance liquid chromatographic method with gradient elution and diode-array detection was developed to quantify free phenolic acids (gallic, vanillic, chlorogenic, caffeic, syringic, p-coumaric, ferulic, sinapic, salycilic, elagic and trans-cinnamic), flavonoids (catechin, epicatechin, rutin, myricetin and quercetin) and juglone in walnut leaves. Chromatographic separation was performed on a Hypersil Gold C18 column (5 µm particle size, 250 × 4.6 mm) and detection was conducted at three different wavelengths (254, 278 and 300 nm) according to the absorption maxima of the analyzed compounds. Validation procedures were conducted and the method was proven to be precise, accurate and sensitive. The developed method has been applied to analyze walnut leaves samples from nine different cultivars, with the same agricultural, geographical and climatic conditions. The experimental results revealed high concentrations of myricetin, catechin hydrate and rutin, and low concentrations of quercetin and epicatechin aglycones. Ellagic acid was established as the dominating phenolic acid of walnut leaves, followed by trans-cinnamic, chlorogenic and caffeic acids. Juglone content varied between 44.55 and 205.12 mg/100 g fresh weight. Significant differences were detected among cultivars for the concentration levels of phenolics.