Selective Enzymatic Esterification of Lignin-Derived Phenolics for the Synthesis of Lipophilic Antioxidants

Abstract

Lignin is an abundant and renewable source of phenolic compounds that can be used as natural antioxidants to substitute synthetic, petroleum-based alternatives. The development of lignin depolymerization techniques has improved the accessibility of low-molecular-weight phenolic fractions with enhanced antioxidant activity compared to native lignin. The selective esterification of the aliphatic OH groups in these compounds is necessary in order to increase their compatibility with hydrophobic product matrixes, while preserving their antioxidant capacity. In the present work, lipase was chosen as a selective catalyst for the esterification of the monolignol dihydroconiferyl alcohol (DCA), in order to target the esterification of aliphatic OHs without modifying the aromatic groups. The reaction was studied under solvent-assisted and solvent-free conditions, using different fatty acids and substrate ratios. A product yield of 97% could be obtained after 24 h in a solvent-assisted reaction with 2 molar equivalents of fatty acid, or after 3 h in a solvent-free reaction with 10 molar equivalents of the fatty acid. The esterified monolignol showed relevant long-term radical scavenging activity, comparable to other commercial, petroleum-based antioxidants. Different lignin fractions were also used as substrates for enzymatic esterification with different fatty acids, resulting in esterification degrees of 20–58% (of the total aliphatic OH), depending on the specific combination of fatty acid–lignin fractions.

Lignocellulosic Biomass in Nanopesticides: A Path toward Sustainable Agriculture

“…In the solvent-free medium, an equivalence of 10 mol of fatty acids was obtained. The esterified C8 monolignol showed higher, long-term, radical scavenging activity compared to commercial, nonrenewable, source-based antioxidants …”

“…74−76 The modifications introduced by esterification have the potential to impact the thermal properties of lignin, particularly in industrial processes involving high temperatures, such as the production of biofuels or manufacturing of composite materials. 75,77 Additionally, esterified lignin may change mechanical properties compared with unmodified lignin, which can favor the use of these lignin versions in applications requiring high levels of strength and durability. 74,78 Luo et al ( 2017) presented the incorporation of lignin into different thermoplastic matrices of both fossil and renewable origins.…”

“…The presence of ester groups adds reactivity to the lignin molecule, allowing these groups to participate in specific chemical reactions. This reactivity makes esterified lignin valuable for various applications, such as adhesive production and polymer modification. − …”

“…The esterified C8 monolignol showed higher, long-term, radical scavenging activity compared to commercial, nonrenewable, source-based antioxidants. 75 2.2.2. Lignin Sulfonation.…”

Biocatalytic selective acylation of technical lignins: a new route for the design of new biobased additives for industrial formulations

“…CAL-B is commercially available in different forms (expression in Aspergillus niger (Mustranta et al, 1993) or in yeast (Graber et al, 2003)), either as free enzyme or immobilized on resins (Miletic et al, 2010) in order to allow its easy separation from the reaction mixture by simple filtration. Other groups succeeded with a similar aim of selective acylation of lignins biocatalyzed by CAL-B, involving either ionic liquid as the reaction media (Hulin et al, 2015) or technical lignins that were post-depolymerized through catalytic hydrogenation (Martinez-Garcia et al, 2023). The present work differs in the sense that it involves technical lignin fractions in the solvent MEK, both being industrially common.…”