High-value Platform Chemicals Research Articles

Microwave Depolymerization of Lignin via Dynamic Vapor Flow Reaction System: HCOOH as Pretreatment Solvent or Reforming Solvent Vapor.

Different forms of HCOOH in the depolymerization system play an important role in governing the monomeric products from lignin. We reported two strategies for the introduction of HCOOH to enrich the monophenols from kraft lignin by microwave-assisted depolymerization. The reaction of lignin models showed that HCOOH was in favor of the cleavage of C-O bonds (β-O-4 typically) and partial C-C bonds (Cα-Cβ). Subsequently, Microwave-assisted depolymerization of lignin with two strategies was conducted via a designed dynamic vapor flow reaction system. Strategy A with HCOOH as pretreatment solvent showed excellent monophenols enrichment with total mass yields of 193.71 mg/g (lignin basis). Strategy B using HCOOH as reforming solvent vapor significantly increased the monophenols selectivity. It presented unique reforming and upgrading performance by generating catechol (42.59 mg/g, lignin basis) and homovanillic acid (17.58 mg/g, lignin basis). This study provided potential strategies for the efficient conversion of kraft lignin into high-value platform chemicals.

Biological carbon capture from biogas streams: Insights into Cupriavidus necator autotrophic growth and transcriptional profile

Recycling carbon-rich wastes into high-value platform chemicals through biological processes provides a sustainable alternative to petrochemicals. Cupriavidus necator, known for converting carbon dioxide (CO2) into polyhydroxyalkanoates (PHA) was studied for the first time using biogas streams as the sole carbon source. The bacterium efficiently consumed biogenic CO2 from raw biogas with methane at high concentrations (50%) proving non-toxic. Continuous addition of H2 and O2 enabled growth trends comparable to glucose-based heterotrophic growth. Transcriptomic analysis revealed CO2-adaptated cultures exhibited upregulation of hydrogenases and Calvin cycle enzymes, as well as genes related to electron transport, nutrient uptake, and glyoxylate cycle. Non-adapted samples displayed activation of stress response mechanisms, suggesting potential lags in large-scale processes. These findings showcase the setting of growth parameters for a pioneering biological biogas upgrading strategy, emphasizing the importance of inoculum adaptation for autotrophic growth and providing potential targets for genetic engineering to push PHA yields in future applications.

Recent Catalytic Innovations in Furfural Transformation

This review highlights recent advancements in the selective conversion of biomass-derived furfural to high-value platform chemicals over various heterogeneous catalytic systems.

Opportunities and challenges for n-alkane and n-alkene biosynthesis: A sustainable microbial biorefinery

Alkanes and alkenes are high-value platform chemicals that can be synthesized by microorganisms, utilizing organic residues from agri-food industries and municipalities, thereby offering an alternative opportunity in resource recovery. Current research and technological advancements for the biosynthesis of alkanes and alkenes are mainly impeded by low product titers, obstructing the bioprocess upscaling and large-scale applications. Thus, current scientific investigations aim to improve productivity by utilizing natural and engineered metabolic pathways in various microbial chassis to suppress competing metabolic pathways, coupled with bioprocess optimization. Additionally, to reduce costs, research is being conducted on utilizing inorganic carbon sources such as CO2 to promote the green synthesis of alkanes and alkenes. Therefore, this review critically discusses the opportunities and challenges in alkane and alkene biosynthesis, aiming to examine the current technological advancements. In this review, the limitations of five major metabolic pathways for alkane and alkene biosynthesis are thoroughly discussed, highlighting their shortcomings. Additionally, various techniques, including metabolic engineering, autotrophic metabolic pathways, and new non-biosynthetic routes, are investigated as potential methods to enhance product titers. Furthermore, this review offers valuable insights into the economic and environmental aspects of alkane and alkene biosynthesis while also presenting perspectives for future research directions.

Insights into co-solvent roles in oxidative depolymerization of lignin to vanillin and lactic acid

Efficient low-boiling-point co-solvent systems were developed for the oxidative depolymerization of lignin to high-value platform chemicals, mainly vanillin and lactic acid, using molybdenum (Mo)-based solid oxide catalysts and hydrogen peroxide (H2O2) oxidant. The co-solvent, designed with the matching Hansen solubility parameter (HSP) values to lignin, facilitated efficient solvation of lignin. Various factors, including co-solvents, catalyst supports, H2O2 concentrations, and active metals, were detailly investigated. Optimal yields of total liquid products (EA oils: 19.8 %) were obtained in water-tetrahydrofuran (H2O-THF) co-solvent systems, increased by 61.0 % compared to that of the sole aqueous solution, employing a MoO3/Al2O3 catalyst. Additionally, the oxidized lignin (O-lignin) or EA oils was analyzed using UV–vis, CHN, GPC, and FT-IR techniques. Results indicated that the co-solvent system remarkably inhibited the repolymerization of lignin fragments, and enhanced the oxidative cleavage of lignin side chains. Lastly, a hypothetical mechanism explained the reaction pathway of lignin depolymerization in co-solvent systems.

Touch wood: Conversion of lignin to dicarboxylic acids using biochar-based solid-acid photo-Fenton catalysts

The depolymerization of lignin into dicarboxylic acids (DCA) by the Fenton oxidation process is a promising approach to valorising lignin. However, Fenton oxidation is challenging due to the demand for acidic reaction conditions and homogeneous Fe2+ catalysts, which cannot be recovered. Herein, we fabricated a biochar-based solid-acid catalyst (SBC) from the pyrolysis of rice straw biomass followed by H2SO4 treatment. This novel catalyst achieved an enhanced catalytic performance in the activation of H2O2 under circumneutral pH, visible-light irradiation and Fe-free conditions. Under optimum conditions, a cumulative yield of 40.1 μ mol. gcat−1. h−1 of malonic, maleic, fumaric, succinic and gallic acid was obtained using 40 mM H2O2, 1 g/L catalyst loading and 4 h reaction time, which is comparable to yields reported in the literature under elevated temperatures and pressures. The activity of SBC stems from the synergy of oxygenated surface functional groups and persistent free radicals (PFRs), which can chemisorb and subsequently activate H2O2 and dissolved O2 to yield •OH and 1O2. Addition of NaN3 scavenger for 1O2 enhanced the cumulative DCA yields to 282.4 μ mol. gcat−1.h−1, suggesting that 1O2 is the main reactive oxygen species in the reaction. Finally, the SBC catalysts remained stable at the said reaction conditions and could be recycled without significant deactivation. After five cycles, the lignin conversion and cumulative DCA yields decreased by 19.7% and 23% respectively. These results show that Fe-free, visible-light Fenton-like catalytic oxidation of lignin with acid-modified biochars is a promising way to produce high-value platform chemicals.

Oxidative depolymerization of lignin to vanillin and lactic acid in an aqueous solution

A heterogeneous molybdenum (Mo)-based oxide catalyst system in combination with hydrogen peroxide as an oxidant was developed for oxidative depolymerization of lignin to yield high-value platform chemicals in a nearly neutral aqueous solution. The impact of various reaction conditions, including the ratios of H2O2 and/or MoO3/Al2O3 catalyst to lignin, reaction temperature and duration, and lignin concentration, were investigated in a batch system or a continuous flow fixed-bed system. The structural changes between dealkaline lignin and the recovered lignin (O-lignin and M−lignin) were analyzed using UV–vis, FT-IR, and GPC techniques. The combination of MoO3 and H2O2 resulted in a superior peroxo complex, promoting the formation of vanillin and lactic acid. The medium and weak acid feature of MoO3/Al2O3 catalyst induced the highest vanillin yield (5.6%) and the selectivity (86.9%) of main platform products. However, the highest yield of lactic acid (3.4%) was observed on MoO3/SiO2 with more weak acid sites. Hypothetical formation mechanisms for vanillin and lactic acid were proposed based on the depolymerization of lignin model compounds.

Repurposing of waste PET by microbial biotransformation to functionalized materials for additive manufacturing.

Plastic waste is an outstanding environmental thread. Poly (ethylene terephthalate) (PET) is one of the most abundantly produced single-use plastics worldwide, while its recycling rates are low. In parallel, additive manufacturing is a rapidly evolving technology with wide-ranging applications. Thus, there is a need for a broad spectrum of polymers to meet the demands of this growing industry and address post-use waste materials. This perspective article highlights the potential of designing microbial cell factories to upcycle PET into functionalized chemical building blocks for additive manufacturing. We present the leveraging of PET hydrolyzing enzymes and rewiring the bacterial C2 and aromatic catabolic pathways to obtain high-value chemicals and polymers. Since PET mechanical recycling back to original materials is cost-prohibitive, the biochemical technology is a viable alternative to upcycle PET into novel 3D printing materials, such as replacements for acrylonitrile butadiene styrene (ABS). The presented hybrid chemo-bio approaches potentially enable the manufacturing of environmentally friendly degradable or higher-value high-performance polymers and composites and their reuse for a circular economy.

Competitive Reactions during Ethanol Chain Elongation Were Temporarily Suppressed by Increasing Hydrogen Partial Pressure through Methanogenesis Inhibition.

Organic waste streams can be converted into high-value platform chemicals such as medium-chain carboxylic acids (MCCAs) using mixed microbial communities via chain elongation. However, the heterogeneity of waste streams and the use of complex microbial communities can lead to undesirable reactions, thus decreasing process efficiency. We explored suppressing excessive ethanol oxidation to acetate (EEO) by increasing the hydrogen partial pressure (PH2) through hydrogenotrophic methanogenesis inhibition by periodically adding 2-bromoethanesulfonate (2-BES) to an MCCA-producing bioreactor to reach 10 mM of 2-BES upon addition. The bioreactor was fed with pretreated food waste and brewery waste containing high concentrations of short-chain carboxylic acids and ethanol, respectively. While 2-BES addition initially reduced EEO, some methanogens (Methanobrevibacter spp.) persisted and resistant populations were selected over time. Besides changing the methanogenic community structure, adding 2-BES also changed the bacterial community structure due to its impact on PH2. While we demonstrated that PH2 could be manipulated using 2-BES to control EEO, methods that do not require the addition of a chemical inhibitor should be explored to maintain optimum PH2 for long-term suppression of EEO.

Trends in in-silico guided engineering of efficient polyethylene terephthalate (PET) hydrolyzing enzymes to enable bio-recycling and upcycling of PET

Polyethylene terephthalate (PET) is the largest produced polyester globally, and less than 30% of all the PET produced globally (∼6 billion pounds annually) is currently recycled into lower-quality products. The major drawbacks in current recycling methods (mechanical and chemical), have inspired the exploration of potentially efficient and sustainable PET depolymerization using biological approaches. Researchers have discovered efficient PET hydrolyzing enzymes in the plastisphere and have demonstrated the selective degradation of PET to original monomers thus enabling biological recycling or upcycling. However, several significant hurdles such as the less efficiency of the hydrolytic reaction, low thermostability of the enzymes, and the inability of the enzyme to depolymerize crystalline PET must be addressed in order to establish techno-economically feasible commercial-scale biological PET recycling or upcycling processes. Researchers leverage a synthetic biology-based design; build, test, and learn (DBTL) methodology to develop commercially applicable efficient PET hydrolyzing enzymes through 1) high-throughput metagenomic and proteomic approaches to discover new PET hydrolyzing enzymes with superior properties: and, 2) enzyme engineering approaches to modify and optimize PET hydrolyzing properties. Recently, in-silico platforms including molecular mechanics and machine learning concepts are emerging as innovative tools for the development of more efficient and effective PET recycling through the exploration of novel mutations in PET hydrolyzing enzymes. In-silico-guided PET hydrolyzing enzyme engineering with DBTL cycles enables the rapid development of efficient variants of enzymes over tedious conventional enzyme engineering methods such as random or directed evolution. This review highlights the potential of in-silico-guided PET degrading enzyme engineering to create more efficient variants, including Ideonella sakaiensis PETase (IsPETase) and leaf-branch compost cutinases (LCC). Furthermore, future research prospects are discussed to enable a sustainable circular economy through the bioconversion of PET to original or high-value platform chemicals.

Furfural electrovalorisation using single-atom molecular catalysts

The selective electrovalorisation of furfural towards drop-in fuel precursors on single-atom molecular catalysts at ambient conditions via a possible radical coupling pathway.

Toward a facile depolymerization of alkaline lignin into high-value platform chemicals via the synergetic combination of mechanocatalysis with photocatalysis or Fenton process

Even though lignin is very rich in aromatic compounds, it remains the least exploited fraction of biomass due to its recalcitrant nature. Consequently, prevailing efforts to fully unlock its potential are still far from reaching due to the use of harsh conditions and energy-intensive processes which are not environmentally friendly. Herein, a facile approach for coupling acid-catalyzed mechanocatalysis (MC) via ball milling with TiO2 photocatalysis (PC)/Fenton for depolymerization of alkaline lignin was investigated. Mechanocatalysis (MC) involved prior acid impregnation (AI) followed by ball milling. A direct comparison of pretreatments (AI, BM, MC) coupled with oxidation processes was preliminarily undertaken. The untreated lignin (UL) served as the control. MC/PC showed the highest yield of major aromatic products. Vanillin, homovanillic acid, acetovanillone, guaiacol, and vanillic acid were major aromatic building blocks identified and quantified. Further optimization revealed a total monomer yield of 22.77 wt%. SEM and FTIR analysis were done with the latter revealing the existence of pre-oxidation reaction in the BM and MC lignin prior to PC. The proposed synergetic process could offer an advantage over prevailing robust systems used for lignin depolymerization.

A consolidated bioprocess design to produce multiple high-value platform chemicals from lignocellulosic biomass and its technoeconomic feasibility

5-Hydroxymethyl furfural (HMF) and furfurals are DOE-listed platform chemicals that can be derived from the renewable carbon in the lignocellulosic biomasses and have the potential to replace petroleum-derived alternatives. High substrate cost and use of expensive solvents limit the economic feasibility of bio-based HMF production on an industrially relevant scale. The study presents an experimental optimized condition that maximizes the chemical-free production of HMF and furfurals without lowering the yield of total fermentable sugars from Saccharum bagasse. Hydrothermal pretreatment at 210 °C for 15 min yielded approximately 10%, 12%, and 46% of HMF, furfurals, and fermentable sugars per gram of dry biomass, respectively. Additionally, the study proposes a consolidated bioprocess model to produce and recover four high-value bioproducts i.e., HMF, furfurals, ethanol, and acetic acid based on the experimental results and evaluates its technoeconomic feasibility considering HMF as the main product. The minimum selling price (MSP) of HMF was estimated to be 930.6 USD/t which is competitive with its petroleum-derived precursor alternative p-xylene (1,113 USD/t). The sensitivity analysis performed for the process parameters suggests that pretreatment cost and revenues from coproducts immensely influence the MSP of HMF. The preliminary technoeconomic analysis performed on the consolidated bioprocess design indicates that additional revenue streams from diversified coproducts in biorefineries aid in lowering the MSP of high-value bioproducts.

Microwave-assisted deep eutectic solvents/dimethyl sulfoxide system for efficient valorization of sugar bagasse waste into platform chemicals: A biorefinery approach for circular bioeconomy

The exploitation of lignocellulosic biomass (LB) such as sugar bagasse waste in biorefineries is the most cost-effective and favourable sustainable approach to producing essential platform chemicals, materials, and energy environmentally benignly. Herein, a microwave-mediated deep eutectic solvents (DESs)/dimethyl sulfoxide (DMSO) system for efficiently processing LB waste into platform chemicals was proposed thereof. Under optimized appropriate diverse parameters such as solvent varieties, catalyst dosage, DMSO addition, reaction time and temperature, the proposed catalytic system (i.e., microwave mediated DESs/DMSO system) has demonstrated significant yields of 5-hydroxymethylfurfural (5-HMF), furfural (FF) and levulinic acid (LevA) of 31.29 %, 28.38 % and 35.65 %, respectively. These favourable results were obtained at the reaction temperature of 140 °C for 40 min. The anticipated catalytic system's activation energy (Ea) was found to be 29.11 kJ/mol. Hence, a practical, inexpensive and sustainable process with the potential of high-value platform chemicals, explicitly for a sustainable strategy in a circular bioeconomy was proposed.

Comprehensive study on plasma chemistry and products in [formula omitted] pulsed discharges under Martian pressure

With the development of Mars exploration, the In-Situ resource utilization on Mars has been drawn significant attention in recent years. The abundant CO2 in the Martian atmosphere makes it possible to decompose CO2 to produce high-value platform chemicals by plasma technology. In this comprehensive study, the discharge characteristics, products distribution, and reaction pathways in CO2 pulsed discharges under Martian pressure are investigated, and the effects of pulsed waveforms on discharge characteristics and produced species are also discussed. The simulation results show that electrons and CO2+ ions are the main components of charged particles in the discharge region; CO and O are mainly produced by the electron impact dissociation and electron impact attachment reactions with ground state and vibrationally excited state of CO2 during the power-on phase and consumed by the ion–neutral reactions. The recombination reaction of electrons and CO2+ ions is very important for the formation of O2. The simulation data also give the main generation and consumption pathways of vibrationally excited CO2 molecules, and indicate that the vibrational energy of excited CO2 molecules can lower the energy barrier of the reaction and facilitate the decomposition of CO2. In addition, the experimental and computational data also suggest that the pulsed waveforms could affect the discharge characteristics and plasma chemistry in CO2 pulsed discharges under Martian pressure, the densities of product species in CO2 plasma increase with the pulse rise rate and pulse rise duration of pulsed voltage. This study provides an enhanced understanding of discharge characteristics and plasma chemistry in the CO2 pulsed discharges under Martian pressure, and suggests optimized ways to utilize the CO2 on Mars.

A genome-scale metabolic model of Cupriavidus necator H16 integrated with TraDIS and transcriptomic data reveals metabolic insights for biotechnological applications.

Exploiting biological processes to recycle renewable carbon into high value platform chemicals provides a sustainable and greener alternative to current reliance on petrochemicals. In this regard Cupriavidus necator H16 represents a particularly promising microbial chassis due to its ability to grow on a wide range of low-cost feedstocks, including the waste gas carbon dioxide, whilst also naturally producing large quantities of polyhydroxybutyrate (PHB) during nutrient-limited conditions. Understanding the complex metabolic behaviour of this bacterium is a prerequisite for the design of successful engineering strategies for optimising product yields. We present a genome-scale metabolic model (GSM) of C. necator H16 (denoted iCN1361), which is directly constructed from the BioCyc database to improve the readability and reusability of the model. After the initial automated construction, we have performed extensive curation and both theoretical and experimental validation. By carrying out a genome-wide essentiality screening using a Transposon-directed Insertion site Sequencing (TraDIS) approach, we showed that the model could predict gene knockout phenotypes with a high level of accuracy. Importantly, we indicate how experimental and computational predictions can be used to improve model structure and, thus, model accuracy as well as to evaluate potential false positives identified in the experiments. Finally, by integrating transcriptomics data with iCN1361 we create a condition-specific model, which, importantly, better reflects PHB production in C. necator H16. Observed changes in the omics data and in-silico-estimated alterations in fluxes were then used to predict the regulatory control of key cellular processes. The results presented demonstrate that iCN1361 is a valuable tool for unravelling the system-level metabolic behaviour of C. necator H16 and can provide useful insights for designing metabolic engineering strategies.

Catalytic conversion of sugars and biomass to furanic biofuel precursors by boron-doped biochar in ionic liquid

In this study, the production of furfural and 5-hydroxymethylfurfural (HMF) from sugars e.g. xylose and glucose, and agricultural wastes e.g. corncob, rice straw and wheat straw were examined in the presence of boron-doped biochar catalysts, boric acid and an ionic liquid, 1-ethyl-3-methylimidazolium chloride ([emim]Cl). Boron-doped trapa shell biochar (BTB) and coconut shell biochar (BCB) catalysts were synthesized via the wet impregnation method. Fourier transform infrared spectra of biochar catalysts showed a band for BO stretching vibrations confirming successful boron impregnation. Furfural and HMF yields from glucose reached 32% and 40%, respectively. One-pot hydrolysis-dehydration conversion of corncob, rice straw and wheat straw resulted in yields of up to 12% and 21% for furfural and HMF, respectively. In addition, catalyst reusability up to 4 cycles was demonstrated with less than 5% variation in HMF yield. Overall, the results of the study offer new insights and great potential in the conversion of waste biomass to high value platform chemicals for energy applications.

Production of Gaseous Olefins from Syngas over a Cobalt-HZSM-5 Catalyst

Light olefins (C2–C5) are high-value platform chemicals used to produce plastics, lubricants, coating material, and surfactants, and can be intermediates to produce transportation fuels. In this study we report on the development of a Co-based catalyst for direct conversion of fossil as well as biomass derived syngas into light olefins that enables high conversion and olefin/paraffin (O/P) product ratio, minimal selectivity to CO2 (< 1%) and wax formation (C14+ selectivity < 1%). Over a Co/HZSM-5 bi-functional catalyst 38% CO conversion with C2–C5 olefin selectivity of 48%, and O/P ratio of 4.0 was achieved at 240 °C, 17 bar, and 1.5 L/(g–h). In addition to a gaseous olefin-rich stream a C6–C13 mixed olefin/paraffin liquid byproduct was also produced with 25% selectivity. Both Co loading and the HZSM-5 acidity levels were optimized to maintain the balance between the hydrogenation and the cracking activity to generate high levels of C2–C4 olefins. A suite of catalyst characterization tools were used to correlate the physical and chemical property of the catalyst to the performance of the catalyst towards syngas conversion and C2–C4 olefin selectivity. Direct conversion of syngas into specialty chemicals (light/gaseous olefins) as well as liquid hydrocarbons over cobalt-HZSM-5 catalysts.

Reactive extraction for in-situ carboxylate recovery from mixed culture fermentation

Biorefinery wastewaters can be fermented to produce carboxylates which are high-value platform chemicals. However, the major challenges in this fermentation are limited product yields and productivities faced due to product inhibition and difficulty in carboxylate separation and recovery from fermentation broths. To mitigate the above problems, process optimization via integrated fermentation-separation i.e. in-situ product recovery (ISPR) systems can be considered. As a first step towards development of such coupled carboxylate bioprocesses, this study aimed to provide a detailed analysis of extraction behaviour of a wide array of extractants and diluents for C2-C6 carboxylates in synthetic solutions and real effluent from acidogenic fermentation. Compared to physical extraction without extractant, a 75–85 % increase was achieved when using reactive extraction (RE) and the difference was more pronounced for short chain carboxylates, particularly at pH 4.5. Distribution coefficients and extraction efficiencies increased with increasing extractant concentration and reached an equilibrium at molar ratio of 1:2. Aliquat 336 and tri-octylphosphine oxide solved in methyloctanoate emerged as the best RE systems and yielded high extraction efficiencies of 11.5 and 29.5 (acetic acid) to almost 100 (caproic acid) respectively. Testing with real fermentation effluent demonstrated similar high extraction yields as observed on synthetic solutions. Potential toxicity of RE on acidogenic fermentation was also investigated which suggested the application of an external ISPR configuration for these coupled bioprocesses.

The White-Rot Basidiomycete Dichomitus squalens Shows Highly Specific Transcriptional Response to Lignocellulose-Related Aromatic Compounds.

Lignocellulosic plant biomass is an important feedstock for bio-based economy. In particular, it is an abundant renewable source of aromatic compounds, which are present as part of lignin, as side-groups of xylan and pectin, and in other forms, such as tannins. As filamentous fungi are the main organisms that modify and degrade lignocellulose, they have developed a versatile metabolism to convert the aromatic compounds that are toxic at relatively low concentrations to less toxic ones. During this process, fungi form metabolites some of which represent high-value platform chemicals or important chemical building blocks, such as benzoic, vanillic, and protocatechuic acid. Especially basidiomycete white-rot fungi with unique ability to degrade the recalcitrant lignin polymer are expected to perform highly efficient enzymatic conversions of aromatic compounds, thus having huge potential for biotechnological exploitation. However, the aromatic metabolism of basidiomycete fungi is poorly studied and knowledge on them is based on the combined results of studies in variety of species, leaving the overall picture in each organism unclear. Dichomitus squalens is an efficiently wood-degrading white-rot basidiomycete that produces a diverse set of extracellular enzymes targeted for lignocellulose degradation, including oxidative enzymes that act on lignin. Our recent study showed that several intra- and extracellular aromatic compounds were produced when D. squalens was cultivated on spruce wood, indicating also versatile aromatic metabolic abilities for this species. In order to provide the first molecular level systematic insight into the conversion of plant biomass derived aromatic compounds by basidiomycete fungi, we analyzed the transcriptomes of D. squalens when grown with 10 different lignocellulose-related aromatic monomers. Significant differences for example with respect to the expression of lignocellulose degradation related genes, but also putative genes encoding transporters and catabolic pathway genes were observed between the cultivations supplemented with the different aromatic compounds. The results demonstrate that the transcriptional response of D. squalens is highly dependent on the specific aromatic compounds present suggesting that instead of a common regulatory system, fine-tuned regulation is needed for aromatic metabolism.