Usefulness of 1H-NMR in the study of the bioactive compounds of Hop (Humulus Lupulus L.)

The cultivation of the hop plant (Humulus lupulus L.) has a long history. Since ancient times, hop have been used in folk medicine for its claimed anti-inflammatory, antiseptic, antidiuretic, (an)aphrodisiac, hypnotic, sedative, and stomachic properties. Moreover it is one of the main ingredients used in brewing. Today, a wide range of over-the-counter preparations containing hop extracts or hop-derived products is available on the market, in particular for use in the phytotherapy of sleep disorders or pain relief and in postmenopausal therapy [1]. Moreover it is demonstrated that moderate beer drinking can be good for health and for the prevention of different chronic diseases [2]. Much of attention is addressed to the polyphenolic content, and specific compounds, such as xanthohumol and 8-prenylnaringenin, have been identified as multipotent bioactive compounds. Moreover, increasing evidence reveals that the so-called hop bitter acids, which represent up to 30% of the total bitter-tasting compounds of hops, exhibit interesting effects on human health. In the present work, we focused on this group of hop secondary metabolites and evaluated the usefulness of 1H-NMR in performing a phytochemical characterization of the main hop compounds. For this reason 1H NMR spectra of different extracts of hop were recorded using a Bruker FT-NMR AVANCE III HD 600 MHz spectrometer. It was possible to identify signals belonging to the main components of hop in the regions ranging between 6.5 to 8.5 ppm and 1.5 to 3 ppm, using 13C, JRes and heteronuclear experiments such as HSQC and HMQC. The identified compounds were tentatively quantified using ERETIC 2 tool. This tool is based on PULCON method which is based on the principle of reciprocity which indicates that le length of a 90° degrees pulse is inversely proportional to the NMR signal intensity. Pyridoxine hydrochloride was selected as external standard compound and the quantification was performed using a simple non- presaturated 1H-NMR sequence. The obtained results further demonstrate the potential of the NMR technique in the characterization of health beneficial compounds from natural sources.