Ester production from bio-based dicarboxylates via direct downstream catalysis: succinate and 2,5-furandicarboxylate dimethyl esters

Abstract

Two culture broths, one containing succinate produced de novo by Corynebacter glutamicum and the other containing 2,5-furandicarboxylate produced by whole cell biotransformation of 5-(hydroxymethyl)furfural (HMF) by a recombinant Pseudomonas putida, were used for dimethyl ester production. For anion exchange, they were characterized for competing organic anions (i.e., other carboxylates) and inorganic anions (phosphate, sulfate and chloride), which affect capturing of the target dicarboxylate via sorption. For the analysis of the sorption process, independent multicomponent column experiments using mimicked mixtures of the respective target building blocks with organic anions, inorganic anions and actual fermentation broth were performed. In the case of succinate, breakthrough profiles and column capacities showed that α-ketoglutarate, malate and other fermentation impurities reduced sorption capacity. For 2,5-furandicarboxylate, the effect of impurities in sorption was less pronounced, with residual HMF eluting without any apparent ionic interaction. After sorption, upgrading via alkylation from mimicked and bio-based broth was successfully carried out, producing the respective succinate and 2,5-furandicarboxylate dimethyl esters. The yield towards dimethyl succinate was reduced from 0.98 to 0.66 mol ester per mol carboxylate due to the presence of fermentation impurities, which were also esterified in good yields. The final yield of dimethyl 2,5-furandicarboxylate ranged between 0.75 and 0.77 mol ester per mol carboxylate for both pure and raw bio-based sorbed furandicarboxylate. Esterification kinetics correlate well with the acidity of the carboxylates and impurities.

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Introduction

Although many carboxylates can be produced conveniently by fermentation, their recovery is not straightforward.¹ A new option has recently been proposed,² wherein upon sorption of aqueous carboxylates from fermentation broths using quaternary anion-exchange resins, the sorbed carboxylate may be converted in dimethyl carbonate as the solvent and reactant. This may lead to methyl carboxylate and regenerated resin in one step. This integrated direct downstream catalysis has proven to be successful when using synthetic sodium succinate solutions. However, it is not clear if it can be applied with real fermentation broth, containing many impurities, and with other carboxylates.

In this study, the proposed integrated method is investigated using succinate from C. glutamicum broth³ and 2,5-furandicarboxylic acid (FDCA) from P. putida broth.⁴ The resulting dimethyl esters are potentially interesting as building blocks.^5,6 In particular, dimethyl 2,5-furandicaboxylate is a more attractive monomer for the production of polyethylene 2,5-furandicarboxylate (PEF), a drop-in replacement of polyethylene terephthalate (PET), than its diacid precursor.⁶

Materials and methods

Materials

All chemicals used were of analytical grade. A strong anion exchange resin, Dowex Marathon MSA (macroporous), was obtained in the chloride form and converted to the bicarbonate form prior to utilization. Samples of bio-based succinate and 2,5-furandicarboxylate in crude fermentation broth were obtained from Research Centre Jülich³ and Bird Engineering⁴ (now Corbion), respectively. The used succinate broth was produced by a batch fermentation using C. glutamicum BOL-1/pAN6-pycP458S and a similar fermentation protocol and cultivation media as used by Litsanov et al.³ Briefly, both cell cultivation and anaerobic succinate production were combined in a single fermentation via a controlled dissolved oxygen (D.O.) ramp. Cells were grown aerobically over 10 h until 30% D.O. was reached, followed by a linear ramp to anaerobic conditions (0% D.O.) for 6 h. Total cultivation time was 62 h and final succinate titer was 26 g L⁻¹. The harvested broth was centrifuged, filtered, frozen and shipped and subsequently thawed just before its use in these studies. Moreover, a mimicked mixture using the major carboxylate by-products found in a high producing succinate fed-batch fermentation using the strain BOL-3/pAN6-gap was prepared based on the composition reported by Litsanov et al.³ In the case of FDCA, the raw mixture was prepared by dissolving a partially purified FDCA sample with a declared purity of 67%, containing HMF and HMF acid as impurities. Pure and partially purified FDCA were recovered and purified based on existing methods.⁴

For comparison purposes, a dimethyl 2,5-furandicarboxylate (dmFDCA) standard was synthetized via Fischer esterification adapted from an existing protocol.⁷ Pure FDCA from Bird Engineering (3 g, 19.2 mmol) was reacted with an excess of methanol (32 g, 1 mol) using hydrochloric acid (1.5 mL) as a catalyst. The reaction was carried out under reflux for 18 h. After reaction, the catalyst was neutralized by addition of 50 mL of 0.5 mol L⁻¹ methanolic KOH solution. Methanol was evaporated and the solid product dissolved in chloroform. The solution was filtered and washed with deionized water. Subsequently, traces of water were removed using brine and magnesium sulfate. The filtered solution was evaporated and the solids recrystallized from acetone, obtaining white crystals. Yield: 74%. Purity was >99%, based on the absence of contaminant peaks in HPLC and NMR. ¹H-NMR (400 MHz, CDCl₃, δ, ppm): 3.93 (s, 6H), 7.22 (d, 2H). ¹³C-NMR (400 MHz, CDCl₃, δ, ppm): 52.36 (OCH₃), 118.48 (furan ring C3 and C4), 146.62 (furan ring C2 and C5), 158.31 (C=O).

Fermentation broth characterization

Samples of succinate and FDCA fermentation broths were characterized based on physical appearance and composition of relevant impurities. Succinate broth was obtained free of suspended solids, whereas raw FDCA broth contained cells and other solids which were removed by centrifugation (5000 rpm, 20 min) and a sequence of filtration steps. The broth was diluted using water according to the column size, and thus its theoretical capacity, to be able to analyze the sorption profile. No additional pretreatment was carried out prior to sorption experiments. Further broth characterization involved the determination of organic anionic by-products and inorganic anions from nutrient salts.

Dicarboxylate recovery by column sorption

Dynamic sorption experiments were carried out in a similar way to previous studies.⁸ Briefly, a Bio-Rad column (1 cm internal diameter × 27 cm height) was packed with anion exchange resin, resulting in a 21 mL bed volume corresponding to 16 g wet resin (5.7 g dry resin). Carboxylate solutions were pumped at a 2 mL min⁻¹ flow rate at 25 °C and 1 mL fractions were collected for analysis. Furandicarboxylate sorption was carried out in a shorter column (1 cm internal diameter × 6.2 cm height) with a 5 mL column bed corresponding to a 3.8 g wet resin (1.4 g dry resin).

Ester formation by O-alkylation experiments

Main ester formation experiments were performed as reported previously.² Typically, in a stirred autoclave reactor, 30 g of dimethyl carbonate were added and 1 g of dry loaded resin was held in the solid addition device until reaction temperature (100 °C) was reached. After releasing the resin into the vessel, samples were taken periodically for ester quantification. Ester formation experiments for individual impurities were performed in agitated glass closed tubes, heated using an oil bath. A given amount of resin in the bicarbonate form was loaded batch-wise with carboxylate, then washed and dried, and finally, placed in the agitated tube and reacted with dimethyl carbonate.

Analytical methods

Organic acids present in succinate broth, mimicked mixture and column sorption eluent were determined by an established ion exchange HPLC method.⁸ Concentrations of relevant organic impurities present in FDCA broth and column sorption eluent were determined using a RP-HPLC method based on the method developed by Koopman et al.⁴ Inorganic anions, such as phosphate, sulfate and chloride, were measured spectrophotometrically using respective commercial cuvette tests from Hach-Lange.

Methyl esters of succinate and other carboxylates from succinate fermentation broth and the mimicked mixture were determined by gas chromatography as reported before,² using anisole as the internal standard and commercial methyl ester standards from Sigma. dmFDCA was determined using the same RP-HPLC method as for the acid, properly adjusting the running time and using the synthesized diester, as described above, as the quantification standard. The identity of dmFDCA produced by alkylation was confirmed by ¹H-NMR and ¹³C-NMR as previously described.^7,9

Results and discussion

Fermentation broth characterization and definition of mimicked mixtures

Succinate fermentation broth³ had a translucent purple to brown appearance without the presence of any solids. The color is attributed to the presence of protocatechuic acid (3,4-dihydroxybenzoic acid), used as a micronutrient for C. glutamicum. The broth containing 2,5-furandicarboxylate had very dark brown color and contained solids in suspension, which were effectively removed by centrifugation and filtration.

Succinate and FDCA raw broths may contain other carboxylates that may decrease the sorption capacity towards the target dicarboxylate, which indicates the importance of their identification and quantification prior to recovery using sorption. As mentioned in the previous section, the reported final composition of a fed-batch cultivation for a similar succinate fermentation³ was chosen to compose the mimicked mixture used in the current study. Table 1 summarizes the composition of the three cases.

Table 1 Composition of a representative succinate fermentation final broth and the mimicked mixture used in sorption experiments. All the broths were adjusted to neutral pH

Component	Reported composition^3		Mimicked mixture, g L^−1	Diluted fermentation broth, g L^−1
	mM	g L^−1
Succinic	1135	134.0	10.0	10.0
Pyruvic	6	0.53	0.04	0.12
Acetic	20	1.20	0.09	0.10
α-Ketoglutaric	35	5.11	0.38	0.05
Malic	33	4.42	0.33	—
Fumaric	13	1.51	0.11	0.02

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In the case of 2,5-furandicarboxylate, the final broth contained several HMF-related compounds co-produced during the biotransformation (Table 2). The partially purified sample, used in this case as the equivalent mimicked mixture, and broth were diluted as done in the succinate case. Given the very low concentrations of residual furanic compounds in the partially purified compounds, all of them were below quantification limits after dilution and thus reported as not detected.

Table 2 Composition of a representative raw and diluted FDCA fermentation broth^a

Component	Reported composition^b		Mimicked mixture,^c g L^−1	Diluted biotransformation broth, g L^−1
	mM	g L^−1
a n.d.: not detected. Limit of detection < 0.5 g L^−1.b Data provided by Bird Engineering.c Prepared using a partially purified FDCA sample of 67%.d 5-Hydroxymethyl-2-furancarboxylic acid.e 5-Formyl furoic acid.
FDCA	472.42	73.74	9.3	10.0
HMF acid^d	142.10	4.53	n.d.	0.60
FFA^e	140.09	2.92	n.d.	n.d.
HMF	12.11	8.34	n.d.	1.10

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Inorganic anions, present from unconsumed salts in the medium, will influence the carboxylate sorption equilibrium. For succinate production, most of the salts used in the medium were sulfate salts, especially ammonium sulfate as the nitrogen source. Phosphates and chlorides were expected in lower amounts. Whole cell bioconversion of HMF to FDCA was done using culture conditions based on a described protocol,⁴ in which a phosphate buffer system was used and ammonium sulfate as the nitrogen source. Other micronutrient salts contribute additional sulfate and chloride anions. The concentrations of such inorganic anions in the raw broths were determined for each case and presented in Table 3. The levels encountered for both cases are in line with the values expected on the basis of the initial medium composition.

Table 3 Main inorganic anions present in succinate and FDCA final broths

Fermentation broth	PO4^3−, g L^−1	SO4^2−, g L^−1	Cl^−, g L^−1
Succinate	0.88 ± 0.02	11.53 ± 0.07	0.09 ± 0.01
FDCA	2.09 ± 0.02	5.30 ± 0.30	0.22 ± 0.01

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Upgrading of bio-based succinate via direct downstream catalysis

Part I: dynamic sorption. Sorption studies were done using the mimicked mixture and diluted fermentation broth. Although the succinate titers were low, the molar ratio between the different components was maintained, resulting in an experiment that is intended to reflect the real concentrated case in terms of component separation.

Fig. 1 and 2 show the elution profiles of the carboxylates as breakthrough curves for both cases. Normalized outlet concentrations allow a better analysis of the system behavior. As expected, monocarboxylic acids (acetate and pyruvate) are less retained than dicarboxylic acids. As ion exchange is the main interaction mechanism present, selectivity rules according to anion valence apply. In the mimicked case (Fig. 1), succinate breakthrough occurred at approximate 76 mL and showed a narrow mass transfer zone saturating the column until 208 mL. Later in the run, α-ketoglutarate and fumarate eluted slowly and did not reach the feed concentration by the end of the experiment after 640 mL (30 bed volumes, BV, 5.3 h). The elution profiles of the dicarboxylates are consistent with trends observed in several studies,¹ wherein the selectivity order was established as fumarate > succinate ≥ malate for strong anion exchangers.

Fig. 1 Multicomponent column sorption experiments for succinate using mimicked mixture (a). Normalized profiles for each component are presented in (b). The elution profiles were constructed by overlaying two sets of data from independent experiments.

Fig. 2 Multicomponent column sorption experiments for bio-based succinate using diluted fermentation broth (a). Normalized profiles for each component are presented in (b). The elution profiles were constructed by overlaying two sets of data from independent experiments. Profiles for malate and acetate present in the fermentation broth were not determined because they were not properly resolved by HPLC.

Acetate, pyruvate and succinate showed chromatographic peaking, resulting in elution concentrations higher than feed concentrations. The reason for this can be attributed to the high affinity towards α-ketoglutarate and fumarate, which compete for exchange sites already occupied, resulting in desorption of those species. In a longer run, it is expected that the inlet and outlet concentrations would be equal, meaning that the column is exhausted and in equilibrium with the feed solution.

Similar behavior was observed in the dynamic sorption runs with fermentation broth presented in Fig. 2. An earlier succinate breakthrough at 25 mL was observed, reaching succinate saturation after 132 mL. All other species started to elute faster when compared to the mimicked mixture. A cause for this could be the presence of inorganic anions competing for exchange sites. Chromatographic peaking was less pronounced in this case, resulting in less succinate desorption.

As all the carboxylates adsorb to a certain extent, the column capacity towards succinate was reduced. Sorption capacities for each component were evaluated by integral analysis at succinate saturation and at the end of the run. The results of these calculations are presented in Table 4.

Table 4 Sorption capacities for organic anions at succinate saturation and at the end of the column run

Component	Sorption capacity (g carboxylate per g dry resin)
	Mimicked mixture		Fermentation broth
	Succ. sat.	End	Succ. sat.	End
Succinate	0.210	0.184	0.120	0.108
α-Ketoglutarate	0.013	0.031	0.009	0.002
Malate	0.009	0.012	—	—
Fumarate	0.008	0.011	5.0 × 10^−4	0.001
Pyruvate	5.0 × 10^−4	5.0 × 10^−4	0.0016	0.003
Acetate	4.0 × 10^−4	4.0 × 10^−4	—	—

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In both studied cases, a reduction in succinate capacity compared with the single component case (0.24 g succinate per g dry resin) was observed. For the fermentation broth, the decrease was as much as 50%. During such a run, darkening of the resin was noticed as an indication that other species might be interfering. Although it is not clear whether such colored compounds are bound by ion exchange, as they were only desorbed by acid treatment (not by salt displacement), they are likely to influence the sorption by fouling the resin, impeding accessibility to functional sites.

Part II: ester formation by O-alkylation. After loading using either the mimicked mixture or fermentation broth, the resin was used in alkylation experiments with dimethyl carbonate as the solvent and alkylating agent. The reaction mechanism is expected to follow the reaction stoichiometry proposed previously,² wherein sorbed carboxylates are methylated in the presence of water yielding methyl or dimethyl ester in the case of mono and dicarboxylates, respectively. An additional feature of the reaction is the regeneration of the anion exchange resin to the bicarbonate form and methanol formation. Fig. 3 shows the obtained yield profiles for both cases. In the first instance (Fig. 3a), dimethyl succinate, dimethyl fumarate and methyl acetate were produced in good yields. Dimethyl succinate formation kinetics resembles the case in which succinate is present as the only counter-ion in the resin, seen in previous studies.² In the case of the sorbed succinate from fermentation broth (Fig. 3b), only partial conversion towards dimethyl succinate was found. The reason for this incomplete conversion is not clear and might be related to other species being sorbed, and their influence on the catalytic activity of the quaternary ammonium site. Therefore, the conversion of other carboxylates was determined and interestingly, a high yield of methyl acetate was determined and traces of dimethyl fumarate were observed.

Fig. 3 Formation of esters (dmSucc, dimethyl succinate; dmFum, dimethyl fumarate; mAcet, methyl acetate) from sorbed carboxylate species using final loaded resin from mimicked mixture (a) and fermentation broth (b).

Dimethyl α-ketoglutarate, dimethyl malate and methyl pyruvate were not detected as products in any of the experiments. To elaborate a feasible hypothesis about this observation, experiments wherein the resin was loaded solely with the carboxylate impurities were carried out. Table 5 shows that, as expected, esters of succinate (control experiment), acetate and fumarate were produced in good yield. Esters of ketoglutarate and pyruvate were not detected and led to a dark coloration of the resin after reaction, indicating a possible decomposition of those carboxylates at the tested reaction conditions. Interestingly, malate was mainly converted to dimethyl fumarate rather than the expected dimethyl malate. Malic acid is an alpha-hydroxy acid that can undergo dehydration at the alpha carbon, yielding fumaric acid. Although it is known that esters are better substrates for such a reaction, it occurs in the presence of an acid as the catalyst. The mechanism of the reaction in our particular case is not understood. It was noted that such a reaction might be of industrial interest if the same mechanism would prevail in the case of lactate, for which methyl acrylate would be obtained. However, preliminary experiments pointed to the formation of oligomers of unclear composition rather than methyl acrylate.

Table 5 Resin loading and ester yield for carboxylate impurities tested individually. Reaction conditions 100 °C, 500 rpm, 10 h

Component	Resin loading, g per g dry resin	Ester yield, mol ester per mol sorbed carboxylate
a Yield for dimethyl fumarate based on sorbed malate.
Succinate	0.16	1.03
Acetate	0.11	1.00
Fumarate	0.19	0.44
Malate	0.18	0.10 (0.33^a)
Pyruvate	0.17	0.00
α-Ketoglutarate	0.23	0.00

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The observed reaction yields and rates are correlated. Their magnitude can be partly attributed to the strength of the interaction, carboxylate-quaternary amine, being higher for the more acidic carboxylic acids (Table 6). The presence of other impurities and even the carboxylate concentration in the resin bead could also be affecting factors.

Table 6 Acid dissociation constant of selected carboxylates¹⁰

Acid name	pKa,1	pKa,2
Acetic	4.75	—
Succinic	4.16	5.61
α-Ketoglutaric	3.90
Malic	3.40	5.11
Fumaric	3.03	4.44
Pyruvic	2.50	—
Furandicarboxylic^11	2.69	4.13

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Upgrading of bio-based 2,5-furandicarboxylate via direct downstream catalysis

Part I: dynamic sorption. Similar to the succinate case, column loading experiments were carried out using three different 2,5-furandicarboxylate feeds. Fig. 4a shows the sorption comparison between breakthrough curves for the pure FDCA²⁻, that in the mimicked mixture based on a partially purified sample and that in the diluted bioconversion broth. A certain reduction can be observed in FDCA loading capacity as the feed mixture complexity is increased. Table 7 summarizes the calculated capacities for the three cases, in which a maximum capacity of 0.30 g FDCA²⁻ per g dry resin is achieved in the pure case, dropping 17% in the case of the diluted bio-based FDCA. Such reduction in capture capacity is less pronounced than in the succinate case (50%); however, it cannot be justified by the sorption of the main furanic-related impurities. Fig. 4b shows normalized breakthrough curves for FDCA, HMF acid and HMF. HMF acid was initially sorbed but almost fully desorbed by competition with FDCA, which is seen as chromatographic peaking. HMF eluted at the empty bed volume and was not sorbed at the run conditions, indicating no major ionic or hydrophobic interactions between the compound and the resin.

Fig. 4 2,5-Furandicarboxylate breakthrough curves from different mixtures (a) FDCA breakthrough from pure, mimicked mixture and diluted biotransformation broth. (b) Normalized multicomponent breakthrough of FDCA and related impurities in diluted biotransformation broth.

Table 7 Sorption capacities for FDCA and related impurities at the end of the column run for the three cases studied

Component	Sorption capacity (g carboxylate per g dry resin)
	FDCA pure	Mimicked mixture	Diluted bioconversion broth
FDCA	0.30	0.29	0.25
HMF acid	—	—	1.0 × 10^−4
HMF	—	—	Not sorbed

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The reduction seen for the diluted broth is likely to be caused by sorption of competing inorganic anions and fouling of the resin by other compounds not determined in this study. These sorption results are promising for FDCA and may indicate a greater feasibility of ion exchange sorption as primary recovery.

Part II: ester formation by O-alkylation. As demonstrated before, succinate and other carboxylates can be upgraded to esters via O-alkylation. However, it was also seen that stronger carboxylates are alkylated at a slower reaction rate. To demonstrate the feasibility of this transformation for FDCA, a more acidic dicarboxylic acid, alkylation experiments with dimethyl carbonate were carried out in a similar manner as for succinate, using FDCA-loaded resins from the three mentioned mixtures. The reaction stoichiometries for the sorption and upgrading of FDCA are assumed to be analogous to those suggested for the succinate case and are presented in Fig. 5a and b, respectively. Briefly, during sorption at neutral pH, the divalent furandicarboxylate anion will be captured, occupying two exchange sites and subsequently alkylated producing the respective dimethyl ester. As a result of both processes, the resin is regenerated to the bicarbonate form with stoichiometric amounts of the respective bicarbonate salt and methanol as by-products. As discussed in previous studies, such by-products can be recycled within an integrated process comprising fermentation and upgrading.²

Fig. 5 Proposed overall stoichiometry for the formation of dmFDCA via direct downstream catalysis: (a) FDCA sorption stoichiometry. (b) Production of dmFDCA by O-alkylation using dimethyl carbonate as the alkylating agent. PS is polystyrene resin and Q is quaternary ammonium.

After initial short reaction trials, the reaction product was analyzed in RP-HPLC by comparing the retention time of the compound, which corresponds well with the dmFDCA standard prepared by Fischer esterification. Further identity verification was done using NMR after work up of the product of direct downstream catalysis.

Sorbed 2,5-furandicarboxylate from the three cases evaluated was used to determine the rate of formation of its dimethyl ester. As can be seen in Fig. 6, alkylation of FDCA from a purified mixture yielded 0.77 mol dmFDCA per mol FDCA after 40 h. Compared to succinate, the reaction kinetics are very slow, in agreement with the discussed pK_a reasoning in the previous section. In contrast to the results seen in the case of succinate, ester formation kinetics and yields were not significantly decreased when raw bio-based solutions were used, suggesting the importance of optimizing fermentation conditions towards residual salt concentration and by-products. Higher temperatures would make the process more attractive for this carboxylate, resin stability being a major hurdle for this improvement.

Fig. 6 Dimethyl furandicarboxylate (dmFDCA) formation yield from sorbed FDCA from pure solution, mimicked mixture and diluted bioconversion broth.

Conclusions

The application of the direct downstream catalysis concept to bio-based succinate and 2,5-furandicarboxylate is a promising and potential processing alternative for carboxylates produced by fermentation, resulting in the production of diesters without the need for prior carboxylate purification. Ion exchange as a capturing step for raw dicarboxylates can be efficient, as demonstrated for 2,5-furandicarboxylate for which a capacity of 0.3 g FDCA²⁻ per g dry resin was achieved. The presence of competing anions, which reduce the sorption capacity, has to be minimized by optimization of fermentation conditions.

The main characteristics of the reaction system used for O-alkylation, described initially for succinate, were also applicable to 2,5-furandicarboxylate and appear to be general for other carboxylates. A reaction feature such as dehydration of certain alpha-hydroxyacids, such as malic acid, during ester formation was observed and could be of important relevance if extended to other substrates. In general, the reaction rate is relatively slow, especially for carboxylic acids with a low pK_a. Further evaluation of this integration concept for bio-based carboxylates could pave the way for the development of fully sustainable building blocks produced by biological transformations.

Acknowledgements

The authors would like to thank Prof. Marco Oldiges and Katharina Kinast from Forschungszentrum Jülich for providing succinate broth samples, Zita van der Krogt and Bird Engineering (currently part of Corbion) for providing FDCA samples and assistance with HPLC analysis and the TU Delft Biocatalysis group for NMR analysis. This study was partly carried out within the European Union's Sixth Research Framework Programme through the ERA-IB BioProChemBB consortium and partly within the BE-Basic R&D Program, which was granted a FES subsidy from the Dutch Ministry of Economic affairs, agriculture and innovation (EL&I).

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Footnote

† Current address: Novozymes A/S, Recovery Development, Krogshoejvej 36, 2880 Bagsvaerd, Denmark.

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