Selective oxidation of allylic alcohols to aldehydes with Bjerkandera adusta.

The green leaf volatiles (GLVs) market is primarily driven by the demand for artificial flavour compounds, which are typically produced via chemical synthesis, involving heavy metal catalysts. However, increasing consumer preference for natural and safer products is driving the industry towards more sustainable production methods. This shift spurred interest in bioproduction techniques such as microbial synthesis, enzymatic processes, and natural extraction methods. Consequently, current research is increasingly focused on the development of greener and more efficient biosynthetic pathways. My master’s thesis work, carried out in the Lab of Prof. Holger Zorn, Justus Liebig University Giessen and supported by the Erasmus+ project, aims to develop an efficient method for oxidation of allylic alcohols to α,β-unsaturated aldehydes, a class of compounds widely recognised for their flavour properties. Building on previous findings, this study leverages, Bjerkandera adusta lyophilisate, that exhibits significant aryl alcohol oxidase (AAO) activity. The methodology emphasises the use of green solvents and sustainable practice, with a focus on minimising reaction time while maintaining high yield and selectivity towards the allylic group. Following optimisation using a model substrate, the protocol was extended to a broader range of aliphatic unsaturated alcohols, including those with conjugated double bonds. The study also evaluates the scalability of the process, the applicability of Pleurotus sapidus and Pleurotus eryngii lyophilisates, and the evaluation of Bjerkandera adusta enzymatic oxidative activity. Optimisation was carried out using (E)-2-octenal as model compound. Key variables included extraction solvent, substrate concentration, buffer pH, and the addition of a co-solvent to the reaction mixture. Product identification and quantification were performed by GC-MS analysis using commercially available calibration standards and 1-adamantanol as internal standard. Enzymes were isolated via Ion Exchange Chromatography and Size Exclusion Chromatography, and oxidative activity was assesed spectrophotometrically. The addition of 10% v/v of organic solvents to the reaction medium significantly enhanced reaction rates and conversions, enabling the selective and sustainable production of (E)-aldehydes while avoiding overoxidation to carboxylic acids. Notably, (E)-2-nonen-1-ol and (E)-cinnamyl alcohol were fully converted into the corresponding aldehydes with yields up to 99%, likely due to their lower volatility compared to (E)-2-octenal. Substrates containing conjugated double bonds, such as (E,E)-2,4-hexadien-1-ol and (E,E)-2,4-decadien-1-ol, also showed good conversions (73–84%) and high product yields. Both nerol and geraniol were oxidised to citral, although nerol exhibited a significantly lower conversion. In the oxidation of (Z)-2-nonen-1-ol, (E)-2-nonenal was the only product detected, suggesting isomerisation precedes oxidation. No product formation was observed for 1-hepten-3-ol, 1-nonen-3-ol, 1-octyn-3-ol, 2-cyclohexen-1-ol. Overall, Bjerkandera adusta lyophilisate presents a promising biocatalyst for the selective and sustainable synthesis of α,β-unsaturated aldehydes supporting the growing demand for greener production methods in the flavour and fragrance industry.