Stability and Photoisomerization of Stilbenes Isolated from the Bark of Norway Spruce Roots.

Harri Latva-Mäenpää,Riziwanguli Wufu,Pekka Saranpää,Daniel Mulat,Tytti Sarjala,Kristiina Wähälä

doi:10.3390/molecules26041036

Harri Latva-Mäenpää, Riziwanguli Wufu + Show 4 more

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https://doi.org/10.3390/molecules26041036

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Abstract

Stilbenes or stilbenoids, major polyphenolic compounds of the bark of Norway spruce (Picea abies L. Karst), have potential future applications as drugs, preservatives and other functional ingredients due to their antioxidative, antibacterial and antifungal properties. Stilbenes are photosensitive and UV and fluorescent light induce trans to cis isomerisation via intramolecular cyclization. So far, the characterizations of possible new compounds derived from trans-stilbenes under UV light exposure have been mainly tentative based only on UV or MS spectra without utilizing more detailed structural spectroscopy techniques such as NMR. The objective of this work was to study the stability of biologically interesting and readily available stilbenes such as astringin and isorhapontin and their aglucones piceatannol and isorhapontigenin, which have not been studied previously. The effects of fluorescent and UV light and storage on the stability of trans stilbenes were assessed and the identification and characterisation of new compounds formed during our experiments were carried out by chromatographic (HPLC, GC) and spectroscopic techniques (UV, MS, NMR). The stilbenes undergo a trans to cis isomerisation under extended UV irradiation by intramolecular cyclisation (by the formation of a new C-C bond and the loss of two hydrogens) to phenanthrene structures. The characterised compounds are novel and not described previously.

Highlights

Stilbenes or stilbenoids are expected to show many future applications as drugs and functional ingredients or preservatives in food products or improving material properties
The stability of trans isomers of the isolated isorhapontin, astringin, piceid, piceatannol, resveratrol and isorhapontigenin in methanol as well as that of crude spruce bark extract dissolved in methanol was monitored for 2 weeks under two different conditions: (1) light protected: solutions of isolated stilbenes were stored in a freezer (−20 ◦C) with protection from light and (2) light unprotected: solutions of isolated stilbenes were placed in a laboratory hood and exposed to continuous fluorescent light
HPLC–UV analysis of trans-resveratrol revealed that irradiation with fluorescent light reduced the trans-resveratrol peak and a new one appeared with a longer retention time [21]

Summary

Introduction

Stilbenes or stilbenoids are expected to show many future applications as drugs and functional ingredients or preservatives in food products or improving material properties. The best known stilbenoid compound is resveratrol, but there are other interesting stilbenoids such as astringin and isorhapontin, which are derived from forest biomass and are potential starting materials for new products. Stilbenes are secondary metabolites and play an important role in the tree defence mechanism [2]. They provide protection against ultraviolet (UV) light and attacks by fungal pathogens and other microorganisms [3,4,5] and they show potential to prevent pathologies associated with oxidative stress [6]. Stilbenoid glucosides are mainly localized to the inner part of the bark in the functional phloem [7] and they can be converted to their aglucones by enzymatic hydrolysis [8]

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