Lignan Project

Introduction

Lignin is an abundant, complex organic polymer that acts as one of the principle components of plants. Lignin plays a role in the structure of plant cell walls, works to move water through the plant to critical areas and helps to prevent water evaporation. Lignins account for a large percentage of overall plant mass, at around 20-25%. We know that lignin does not act alone in it’s structural duties, commonly forming complexes with polymeric carbohydrates. Difficulty in isolating lignin from these lignin-carbohydrate complexes means that synthetic lignins are commonly used for research purposes. The basic monomers that make up the lignin polymer are p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol (Figure 1.). These three monomers are phenylpropane units that differ in their substitutions at the 3 and 5 positions. In the case of lignin nomenclature, the side chain attachment to the aromatic ring is considered position C-1 and the phenol group considered to be position C-4. These monomeric alcohols undergo oxidative coupling with each other and with a growing polymer end unit. This oxidation process results in radicals delocalized at positions C-1, C-3, C-5, C-β, and O-4.1 The most frequently formed bonds are β-O-4 bonds.

The coupling of two lignin monomers, usually at their β carbons, results in lignan, a nonstructural component of plants that play a role in plant defense. These lignans show important biological activity that the lignin polymer does not. One interesting medical application of lignan is found in its interactions with breast cancer. It has been shown that both flaxseed and sesame seed lignans and their metabolites have the potential to act as competitive inhibitors of estrogen and prevent the binding of estrogen to estrogen receptors on breast cancer tumors.,4 These effects are possible on the basis that many lignans share structural features with the human estrogen estradiol, including hydrophobic benzene rings and alcohol groups that are capable of interaction with the binding site of the estrogen receptor.4,5 Lignan also displays antioxidant properties, showing the ability to scavenge hydroxyl radicals.,6 Lignan’s medical applications make it an interesting topic for further study, but an issue lies within the commercial availability of lignan. Lignan itself is not commercially available except as an analytical standard, and analytical standards can range upwards of $300 USD for 10 mg of product. For an undergraduate research lab, this presents serious problems in terms of budget and funding. In order to study the chemistry of lignans, we need a way to isolate and purify lignans in a cost effective manner.