Analytical pyrolysis – A tool for revealing of lignin structure-antioxidant activity relationship

Highlights

• High radical deactivation indexes of technical lignins–polyphenols of natural origin.

• Importance of lignins OH phenolic bond dissociation enthalpy.

• Py-GC/MS and chemometrics for revealing of lignins antioxidant activity descriptors.

• Structure–activity relationship of polymeric lignins and related monomeric phenols.

• Model for the rationalization and prediction of lignins antioxidant activity.

Abstract

Systematic and thorough investigation of the structure and antioxidant properties of different technical lignins, high molecular polyphenols of natural origin, which are formed in large amounts as by-product of pulp-and-paper industry and other lignocellulosics chemical processing, was made. Py-GC/MS was the main method used to quantitatively determine the structural descriptors needed for rationalization of lignin antioxidant activity. Size-exclusion chromatography (SEC), electron paramagnetic resonance (EPR) spectroscopy and wet chemistry tests were also used as complementary methods. Antioxidant properties of lignin samples under study were evaluated as their capacity to scavenge the 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radicals. The use of Py-GC/MS in combination with some other experimental and chemometric methods allowed for the first time quantitatively characterize lignin structure-antioxidant activity relationship, which can significantly favor the rational use of lignin.

Introduction

Various materials, food products, means for human health care, animal feed etc., undergo oxidative degradation. In order to protect their properties and prolong their shelf life antioxidants have to be added to almost all oxidizable organic substrates. On June 2013 the world antioxidant market was 1.5 billion USD [1]. The market for antioxidants is expected to grow by 2–4% in a year. In a society oriented to the bio-economy, the resources should be used in a manner that allows them to be replenished by natural systems, as well as pollution in biological systems must be avoided [2]. There is an increasing public awareness regarding synthetic antioxidants application not only for food but also for stabilization of polymeric materials. Due to their biodegradability and significantly lower toxicity, antioxidants of natural origin, in particular polyphenols, can be a good alternative to the synthetic ones [3], [4], [5] and a possible replacement for the synthetic antioxidants in plastics with natural ones is an attractive task for the scientific community [6], [7]. However, most of the naturally originated antioxidants are not efficient enough for the protection of plastics and composites due to their lower molecular mass and lesser thermal stability in comparison with synthetic analogs [8]. On the contrary, bio-renewable polymeric polyphenol lignin is a suitable natural antioxidant for use in polymers due to its lower sensitivity to high temperatures and higher molecular mass in comparison with the low molecular weight natural antioxidants which are proposed for the stabilization of polymeric materials, e.g., α-tocopherol [9], [10]. Lignin antioxidant activity is explained by radical-scavenging properties of its phenolic moieties [11], [12], [13], [14], [15], [16], [17], [18].

Unlike most natural polymers, lignin molecules are highly complicated due to their natural variability. Lignin in situ is a three dimensional amorphous phenolic polymer composed of phenylpropanoid units (PPU), which are bound together through different types of interunit linkages such as β-O-4-aryl ether bonds, β-β-resinol, β-5 phenylcoumaran, β-1-(1,2-diarylpropane), 5-5-biphenyl and 4-O-5-diaryl ethers. Lignin PPU units differ by the number of methoxyl groups in the phenyl ring [19], [20], [21]. The H-structure (4-hydroxy phenyl) does not have methoxyl groups, the G-structure (guaiacyl) has one and the S-structure (syringyl) has two methoxyl groups. The heterogeneity and diversity of lignin increases in the process of its isolation from phytomass, particularly on industrial scale. Lignins, isolated from plant lignocellulosic materials by chemical or biochemical processing (mostly co-products of pulp and paper industry or fuel ethanol production) are named “technical lignins” [22], [23], [24], [25], [26]. Technical lignins require valorization since more than 50 million tons of these naturally occurring polyphenolic polymers are generated annually [27] and are mainly burned to produce heat and/or electricity within paper mills and biorefineries [28].

Application of lignin for production of effective naturally originated antioxidant can be considered as a promising avenue for technical lignins valorization. Its antioxidant potential is verified for a variety of polymeric composite materials, animal feed, and biological systems [11], [12], [13], [14], [15], [16], [17], [18]. However, despite the well proved antioxidant properties, the absence of rationalized conceptions about lignin structural features and physical–chemical properties influencing its antioxidant activity hinders the realization of lignin potential in the area of stabilization of materials and food/feed products against oxidative damage. Structure–activity relationship is crucial to identify the most optimal antioxidants and to determine the most promising directions for upgrading of technical lignins in order to produce effective antioxidants for various application systems.

The few studies devoted to the structure-antioxidant activity of lignins do not take sufficient number of structural descriptors into account (mainly OH phenolic and methoxyl groups are considered). Our previous studies, made with some types of technical lignins [29], [30], [31] allowed establishing additional descriptors, needed for the rationalization of lignins antioxidant activity and have shown the similarities between the mechanisms of action of relatively low and high molecular plant polyphenols.

The aim of the present work was obtaining of qualitative and quantitative structure–activity relationship, which would be valid not only for specific group of technical lignins, but for all types of lignins. For the realization of this aim it was necessary to develop methodology, which combines analytical and chemometric methods and to make investigations, using the large number of lignin samples isolated from various kinds of plant biomass using a variety of industrial and pilot plants methods and laboratory procedures, as well.

Analytical Pyrolysis (Py-GC/MS/FID) was used as the main analytical tool to obtain data needed for quantitative comparison of structural characteristics of the broad variety of lignins [32], [33], [34], [35], [36]. A method for processing of Py-GC/MS/FID results, which allows characterizing lignins structural properties related to their antioxidant activity, was developed. As a result, the equation, which quantitatively describes the structure-antioxidant activity of lignins, was obtained for the first time.