Tyrian purple (also known as royal or imperial purple) is the oldest known commercial pigment and still one of the most expensive dyes, often associated with the wardrobes of clergy and royalty. It is a brominated derivative of indigo, a natural dye that has been used since 4000 BC. Moreover, just recently, the therapeutic value of indigoids for the treatment of several disorders was discovered. The manufacturing of indigo derivatives by the existing chemical routes has become increasingly uninteresting due to the use of aggressive reagents, expensive starting materials and high-energy costs. Thus, both dyestuff manufacturers and the pharmaceutical industry are interested in the development of gentle preparation methods of indigoids from simple precursors. Here, we describe a simple enzymatic method for the one-step synthesis of Tyrian purple and other indigo derivatives with fungal peroxygenases (UPO, EC 1.11.2.1). The reaction does not require complex co-substrates and works well in phosphate buffers with H₂O₂ (<0.1 wt%) and less than 5% (v/v) acetonitrile as co-solvent. We demonstrate the scaling up of the reaction to 10 Liters and established thereupon an environmentally friendly combined synthesis and in-situ dyeing process, further simplifying the manufacturing of vat-dyed fabrics. Eventually, we screened a number of halogen-substituted indoles in the search for novel indigo derivatives, which may be of interest for pharmaceutical and/or dyeing purposes.

Due to the increased interest in durable, sustainable and recyclable fabrics, natural fibres are currently experiencing a renaissance. This study investigated controlled anaerobic water retting of flax as a potential process step in a future biorefinery, allowing the simultaneous production of high-quality fibres, stalks, organic acids or biomethane. Anaerobic water retting of flax resulted in the formation of a retting effluent containing 1.745 ± 605 mg L⁻¹ acetic acid, 1.051 ± 354 mg L⁻¹ butyric acid, 145 ± 38 mg L⁻¹ propionic acid and 190 ± 96 mg L⁻¹ caproic acid. Furthermore, 38.0 ± 7.2 mL g oDM⁻¹ hydrolysis gas with 41.6–54.2% CO₂ and 45.3–58.1% H₂ was produced. Anaerobic digestion tests resulted in a specific biomethane potential range of 221 ± 14 to 261 ± 13 mL CH₄ g COD⁻¹ for the retting effluent. The average yield of fibres and stalks from flax was 22% and 51%, respectively. The flax fibres obtained were mainly composed of cellulose (75.1% of dry matter). Hemicellulose and lignin accounted for 7.1 ± 0.3% and 1.9 ± 0.3%, respectively. The tensile strength and Young’s modulus of the fibres were 354 ± 130 MPa and 35 ± 8 GPa, respectively. The annual product yields for a cultivation area of 1 ha of flax were estimated at 0.67 t of fibres, 1.55 t of stalks and 117 m³ biomethane.