A Biomimetic Approach to Lignin Degradation - ACS Symposium Series

Chapter 12

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A Biomimetic Approach to Lignin Degradation Metalloporphyrins Catalyzed Oxidation of Lignin and Lignin Model Compounds 1

1

2

Claudia Crestini , Pietro Tagliatesta , and Raffaele Saladino 1

Dipartimento di Scienze e Tecnologie Chimiche, Università di Tor Vergata, Via della ricerca Scientifica, 00133, Roma, Italy Dipartimento di Agrobiologia e Agrochimica, Universitàdella Tuscia, Via San Camillo de Lellis, 01100, Viterbo, Italy 2

An overview of the state of the art for the use of synthetic metalloporphyrins in the catalytic oxidation of lignin and lignin model compounds is presented. The biomimetic oxidation of 5-5' condensed and diphenylmethane lignin model compounds with several water soluble anionic and cationic iron and manganese porphyrins in the presence of hydrogen peroxide is described. Manganese porphyrins were found more effective in degrading lignin substructures than iron porphyrins. Among them the cationic manganese meso-tetrakis(N-methyl-pyridinio) porphyrin pentaacetate [TPyMePMn(CH COO) ], never used before in lignin oxidation, proved to be the best catalyst. The catalytic activity of porphyrins in hydrogen peroxide oxidation of residual kraft lignin was also investigated. TPyMePMn(CH COO) was able to perform the most extensive degradation of the lignin structure, as demonstrated by the decrease of aliphatic hydroxyl groups and increase of carboxylic acids, as measured by quantitative P-NMR. No significant condensation reactions occurred during manganese porphyrin catalyzed oxidations of residual kraft lignin, while in the presence of iron porphyrins an increase of condensed moieties was detected. 3

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3

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© 2001 American Chemical Society In Oxidative Delignification Chemistry; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

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Introduction A main goal in the development of catalytic processes in chemistry is the mimicking of biological transformations. The objective is to perform reactions in a context isolatedfromthe biological one, and to use a catalyst with a minor cost than the enzyme itself. A modern approach to the development of new catalytic systems goes beyond these former intentions. The development of synthetic catalysts which can mimic the course of biological processes is proposed. Such systems should also provide a wider margin of catalyst stability, conversion yields, selectivity in the reaction, and lower substrate selectivity than the enzyme itself. The goal of such a process is the development of a system that is both environmentally friendly and economically suitable for the scale up to plant dimensions. In the paper production processes, environmental concerns have prompted the study of pulping and bleaching sequences avoiding the use of chlorinated compounds. Several totally chlorinefree(TCF) processes have been developed, for example by the use of oxygen, hydrogen peroxide, and ozone respectively.(l) However their major drawback consists in a lack of selectivity in the degradation of lignin, which leads to the partial degradation of the cellulose contained in pulps, and ultimately in a lower final yield. The reason for this lack of selectivity in the oxidation reactions, is due fundamentally to the formation of common radical intermediates such as hydroxyl radicals, that are able to attack both cellulose and lignin.(2)

HOOC

Figure L Fe(III) protoporpyrin IX

The selective removal of lignin in wood is accomplished, in nature, by the white-rot basidiomycetes fungi. Such fungi are able to produce three classes of ligninolitic enzymes: laccases, lignin peroxidases (LiP) and manganese

In Oxidative Delignification Chemistry; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.


214 dependent peroxidases (MnP).(3,4,5) The former are oxygenases with an active site containing four copper atoms. The oxidation of phenolic lignin subunits is performed, in the presence of oxygen, by the generation of phenoxy radicals. LiP and MnP are enzymes that can perform the heterolytic oxygen transfer from hydrogen peroxide to the active center. MnP is able to oxidize only phenolic lignin substructures, while LiP, due to its higher redox potential, can oxidize also non-phenolic substrates. The active site of these enzymes is constituted by a heme center, the protoporphyrin IX (iron~PPIX)(Figure 1). (6) Synthetic metalloporphyrins are biomimetic catalysts that can yield highly oxidized metallo-oxo species. They have been used as lignin peroxidase models, and their potentiality for lignin degradation has been a subject of several studies. (7-21)

Discussion In the presence of hydrogen peroxide, the active center of LiP and MnP performs a one-electron oxidation of the lignin aromatic moieties.(6,22) The catalytic cycle consists in a two electron oxidation of Fe (III) protoporphyrin IX (high spin) to give a highly reactive oxo-iron (IV) protoporphyrin IX π-cation radical, the LiP I complex. The LiP compound I is then reduced to the initial state by two different one electron reductions by the substrates (Figure 2). LiP is a fragile enzyme. When exposed to an excess of hydrogen peroxide (more than 20 eq.), it is subject to inactivation by overoxidation, and gives the inactive form LiP III (Fig. 2).The protein scaffold around the active center provides stabilization of the metal complex, protection of the active site from possible overoxidation processes, and water solubility. When synthetic metalloporphyrins are used as biomimetic catalysts in the presence of hydrogen peroxide, several side reactions can occur. The peroxidic bond can undergo homolytic scission to yield Fe (IV)-OH and hydroxyl radical in a Fenton like fashion. This reactivity is more significant in the presence of iron complexes and hydrogen peroxide as oxygen donor. A second molecule of peroxide may react with the metal oxo complex in a catalase like fashion to yield the formation of H 0 and 0 , and ultimately the degradation of the active oxidant species. The metal oxo complex may react to yield μ-οχο dimers. (7) 2

2

In Nature the polypeptidic envelope of the enzyme protects the active site from side reactions, and activates it. In fact, the heterolytic cleavage of the peroxidic bond is subject to acid catalysis. The proxy mal His residue activates the complex to heterolytic cleavage by enhancing the electrophilic character of the oxygen atom, and reducing the strength of the metal-oxygen bond, the addition of a nitrogen base such as pyridine or imidazole to the oxidizing


215


solution can mimic this situation. (7) The manganese porphyrins are not strictly biomimetic systems of LiP, since the natural enzyme active site is an iron complex. However the use of manganese complexes could override many reactivity problems. More specifically, manganese porphyrins are usually more reactive than the iron porphyrins;(7) they form single adducts with nitrogen bases and in this way can be easily activated. Manganese shows a minor tendency than iron to undergo the homolytic cleavage of the peroxidic bond.

Resting Enzyme

Compound III

Compound I

H 0 2

H 0 2

2

Compound II

Figure 2. Catalytic cycle of lignin peroxidase

Oxidation of lignin model compounds Early studies on metalloporphyrins as ligninase biomimetic catalysts were based on the use of iron-PPIX. (8-11) Shimada et al. (11) reported that iron PP IX catalyzes the oxidation of β-1, β-Ο-4 and β-5 lignin model compounds thus mimicking ligninase. Also, the regiospecifity found in iron-PPIX oxidation of veratryl alcohol was the same observed in LiP; this showed that the regiospecificity of oxygenation is not necessarily governed by the protein moiety of ligninase.(Figure 3) However, simple métallo porphyrins suffer from the major disadvantage of being unstable in the presence of excess oxidants. Their lability is due either to self-destruction or to the formation of inactive μ-οχο complexes. (7,12) Sterically protected porphyrins were then studied. The presence of phenyl groups in the meso positions of the porphyrin ring highly increased its stability to



216 oxidants. A series of differently ring-substituted catalysts with increased redox potential and /or water solubility were then analyzed.(7,12) The presence of sulphonic groups highly increased water solubility, while the introduction of halogen atoms onto the phenyl ring or the β-positions enhanced the resistance to oxidants. Several studies have been reported aimed at evaluating the possible use of such catalysts as ligninase models. Veratryl alcohol is a typical non-phenolic lignin model and a secondary metabolite that acts as a mediator in the ligninase oxidation of lignin.

R=CH OH, 1-Bu 2

R=COCH ,CHO,CO-CPh 3

Figure 3. Oxidation of lignin model compounds catalyzed by métallo porphyrins and some of the oxidation products (nonexhaustive list). When submitted to oxidation with iron mesO-tetraphenylporphyrin (FeTPPS) or manganese mesO-tetraphenylporphyrin (MnTPPS) ( H 0 or K H S O 5 ) it was oxidized to veratraldehyde and p-quinone (Figure 3). (13) The presence of different iron or manganese porphyrin did not affect the nature of the oxidation products. As oxygen donors were employed H 0 , MMP, r-BuOOH, mCPBA, K H S O 5 . (13-16) Hydrogen peroxide would be the oxidant of choice for 2

2

2


2

217 its low cost and low environmental impact. However H 0 show the tendency to undergo homolytic cleavage of the peroxidic bond more than other oxygen donors. The oxidation of methoxy arenes carrying electron-withdrawing groups such as -CHO, -COCH follows a dramatically different reaction pathway. (14) In fact in this case the different distribution of the electron density, due to the captodative effect of the substituents, yields products of aromatic ring cleavage, the muconate derivatives (Figure 3). Dimeric lignin model compounds have also been studied. The β-Ο-4 linkage is by far the most common substructure in lignin. (13-17) The oxidation of l-(4methoxy-3-methoxyphenyl)-2(2-methoxyphenoxy) propan-1, 3-diol a β-Ο-4 dimeric lignin model, when performed in iron meso-tetrakis(2,6-dichloro-3sulphonatophenyl) porphyrin chloride (TDCSPPFeCl), iBuOOH in H 0/CH CN, yielded products of side-chain oxidation and cleavage. In the presence of TPPS and KHS0 or H 0 , the p-quinone was also obtained (Figure 3). Mansuy and Dolphin studied the reactivity of diarylpropane units. (14,17) In the presence of TDCSPP, TPPS, me50-tetrakis(2,6-dichlorophenyl) porphyrin (TDCPP), and wesΌ-tetrakis(pentaf^uoro-β-tetrasulfonatophenyl) porphyrin ( T F 5 P S 4 P ) , the main products obtained were side chain oxidation products, and the corresponding p-quinone. The phenyl coumaran (β-5) substructure constitutes about 9-12% of the substructure in softwood lignin, while the 5-5' units are about 9.5-11%. The study of the degradation of such lignin subunits showed that both of them are actively oxidized by metalloporphyrins in the presence of /-BuOOH or H 0 . The products obtained are again side-chain oxidation products, muconate derivatives from aromatic ring cleavage reactions, and p-quinones. (17) A major drawback to the use of iron porphyrins in lignin oxidation is that their maximum activity is at pH about 3. At acid pH the homolytic cleavage of the peroxidic bond is favored. The reaction pattern observed could be due both to the presence of the high valence Me (IV) P * and to the formation of hydroxyl radicals. Mn porphyrins show their maximum activity at pH 6. (7) Furthermore, manganese is less prone than iron to catalyze the Fenton reaction. We designed a protocol to evidenciate the possible reactivity of several iron and manganese porphyrins toward condensed lignin subunits. (18) Residual lignin after kraft pulping is in fact heavily modified with respect to native lignin. Residual Kraft lignin contains significant amounts of 5-5' biphenyl and diphenylmethane substructures. (19) The oxidative behavior of porphyrins on isolated lignins and residual kraft lignins has not been much explored. (9,11,20,21) The oxidation pathway of 5-5' biphenyl and diphenylmethane lignin models, in the presence of cationic and anionic water-soluble porphyrins, using hydrogen peroxide as oxidant has been recently reported. (18) The oxidative efficiency of anionic manganese and iron TDCSPP, and TSPP was compared on the basis of 2

2


3

2

5

2

3

2

l

2

+


2

218 the oxidation extent of the tested models. The catalytic activity of the cationic manganese meso-tetrakis(N-methyl-pyridinio) porphyrin pentaacetate [TPyMePMn(CH COO) ] on lignin model compounds was determined at two different pH values: 3 and 6. 3

5

,

,

Oxidation of2,2',3,3 -tetramethoxy-5,5 'dimethylbiphenyl


tetramethoxy-5, 5 '-dimethyl diphenylmethane

1 and2,2',

3,3'-

5.

5-5' Condensed model 1 was oxidized in the presence of hydrogen peroxide and several Mn or Fe porphyrins at pH 3 or 6. More specifically, TDCSPPMnCl and TSPPMnCl were used at pH 6, while, in accord to the maximum activity pH previously reported (16), TDCSPPFeCl was used at pH 3. TPyMePMn (CH COO) was used at pH 3 and 6 in order to test the pH of its optimal catalytic activity. The same reactivity pattern was observed under all the tested experimental conditions, the main oxidation products detected being in every case the p-quinones 2 and 3 (Scheme 1, Table I). 3

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Table I. Conversion of lignin model compound 1 and product yields. MnTPyMeP a

pH3

Residual 1 (%) 2 (%) 3 (%) 4(%)

MnTPyMeP

u

MnTSPP

Fe

Mn

e

TDCSPP

TDCSPP

d

pH6

27.1

41.2

72.7

49.9

55.3

51.2 2.04 1.14

53.9 2.22 1.19

21.6 3.14 1.57

43.9 2.55 1.90

Trace 3.14 2.17

a: oxidation in the presence of TPyMePMnAc at pH3; b: oxidation in the presence of TPyMePMnAc at pH6; c: oxidation in the presence of TSPPMnCl at pH6; d: oxidation in the presence of TDCSPPMnCl at pH6; e: oxidation in the presence of TDCSPPFeCl at pH3. Reprinted from Bioorg. Med. Chem. 1999, 7, 1897, with permission from Elsevier Science 5

5

In analogy with the general oxidative mechanisms previously reported for porphyrin-catalyzed reactions, the following pathway could be hypothesized. The formation of compound 2 can be rationalized on the basis of the electronic effect exerted by the methyl group. The intermediate radical cation 1 , that is generated during the first one electron oxidatio^ step, could be further oxidized by the highly electrophilic species (P) Me =0. The site of oxidation is determined by the distribution of the electronic density on the aromatic ring,


219


which is in turn modulated by the electronic effect of the substituent at the C-5 position (15). The electron donating effect of the methyl group could direct the second oxidation step at the C-6, and yield the corresponding p-quinone 2 (Scheme 2). This behavior had been previously reported on monomeric compounds carrying electron-releasing substituents. A side-chain oxidation reaction yielded the product of benzylic oxidation 4. The occurrence of side chain oxidation reactions had been previously described (14,16,17). The pquinone 3 could be formed, in analogy with 2, by the further oxidation of 4.

1,2,3,4 η - o 5,6,7,8 n=l HO'

4,8 Scheme 1. Products obtainedfrom and 5.Reprintedfrom

9

the oxidation of lignin model compounds 1

Bioorg. Med. Chem. 1999,

1, 1897, with permission from

Elsevier Science

In Table I are reported the yields of products 2, 3, 4 and the amount of 1 recovered after reaction under the different experimental conditions examined. A comparison between TDCSPPMnCl and TDCSPPFeCl showed that the manganese porphyrin was able to perform a more extensive oxidation of model compounds 1 and 5 than the iron porphyrin. This could be due to a higher stability under oxidative conditions rather than to an intrinsic higher reactivity toward the lignin moieties. (16) Under such experimental conditions the homolytic cleavage of the peroxidic bond in a Fenton like fashion is in competition with the heterolytic reaction (12,23,24). The anionic porphyrin TSPPMnCl proved to be the less active catalyst. In fact it lacks of any electron withdrawing substituent, which can enhance the redox potential. The corresponding TDCSPPMnCl, in which two chlorine atoms have been added to each phenyl ring, proved to be more reactive. TPyMePMn (CH3COO)5 showed a higher conversion rate at pH 3 than 6. (Table I). However the amount of products 2, 3, and 4 were comparable. Probably, when the reaction was performed at pH 3, the higher acidity of the medium underwent further radical coupling reactions yielding higher molecular


220


weight condensed products (25). Under both the experimental conditions TPyMePMn (CH3COO)s was found to be more effective in the degradation of 1 than the anionic catalysts. The oxidation of the diphenylmethane model compound 5 showed an analogous reaction pathway with p-quinone 6 being the main product recovered (about 10 % yields) (Schemes 1, 2).

l,2:n = 0 5,6:n = 1 Scheme 2. Proposed reaction pathway for the formation Reprinted from

Bioorg. Med. Chem. Elsevier

of p-quinones 2 and 6.

1999, 7, 1897, with permission from Science

Bleaching of kraft pulps. Paszczynski (9) studied the treatment of kraft pulps in the presence of iBuOOH with a variety of natural and synthetic porphyrins. The biomimetic system was able to effectively bleach the pulps. After treatments the pulps were found essentially lignin free, although a little loss occurred in the cellulose content. Extensive delignification was obtained using the water-soluble catalysts FeTDCSPP, iron meso-tetrakis(2,6-dichloro-3-sulfonatophenyl)-Poctachloroporphyrin (FeTDCSPCl P), and iron meso-tetrakis(4sulfonatophenyl)-P-octachloroporphyrin FeTSPCigP (21). The use of the corresponding manganese porphyrins did not significantly alter the lignin degradation extent (K number). However, a decrease in the viscosity of pulps, due to cellulose degradation, was evident in all the experiments. Possible 8


221 technological applications require a better control of the oxidative reactions and milder reaction conditions. From this viewpoint the elucidation of the oxidation mechanism on lignin is of pivotal importance.


Oxidation of lignin Kurek (20) reported a study on the structural modification induced on milled wood lignin and extractive free wood by FeTF PS P in the presence of hydrogen peroxide. It was established that demethylation, aromatic ring cleavage and side chain oxidation reactions occurred. 5

4

In order to elucidate the reactivity pattern of iron and manganese porphyrins on kraft pulps we designed a study on the oxidation of softwood residual kraft lignin with hydrogen peroxide and different metalloporphyrins. More specifically, the different activity of Mn and Fe porphyrins was tested on FeTDCSPP and MnTDCSPP. The effect of the chlorine atoms was studied by the comparison of MnTSPP and MnTDCSPP. Moreover, the catalytic effect of MnTMePyP, a water-soluble cationic porphyrin, never used to date in the oxidation of lignin or lignin models was studied. The oxidation of residual kraft lignin with TDCSPPMnCl and TSPPMnCl was carried out at pH 6, while the oxidation with TDCSPPFeCl was performed at pH 3. Such values were chosen according to the maximum activity of iron and manganese porphyrins previously found at different pH values (16). The oxidations with TPyMePMn (CH3COO)5 were performed both at pH 3 and 6. The structural modifications induced on the polymer were quantitatively determined by P-NMR spectroscopy (26-31). In Table II the quantitative data obtained from the spectra of lignin before and after the different treatments are reported. The comparison of the efficiency of TDCSPPFeCl and TDCSPPMnCl in oxidations performed at pH 3 and 6 showed, under both the experimental conditions, a decrease of aliphatic O H groups. This shows the occurrence of side-chain oxidation reactions. The increase of C O O H units further confirms this behavior. In the case of TDCSPPMnCl such increase was found too high to be explained only on the basis of side chain oxidation reactions. Thus the presence of aromatic ring cleavage reactions cannot be ruled out. The effect of H2O2 porphyrin mediated oxidation on lignins, was also evident on the modification of the phenolic groups. In the presence of TDCSPPMnCl only a decrease of guaiacyl groups was observed, while in the presence of TDCSPPFeCl an increase in the N M R chemical shift region of condensed structures was evident. Such structures have been previously assigned to β-5, 4-0-5 and 5-5' condensed units (32). This seems to indicate a different reaction pathway for iron and manganese porphyrins, with the latter being able to carry out an oxidative process with a low amount of coupling reactions. The


222 coupling reactions that occur during the oxidations catalyzed by iron porphyrins could be due to the formation of hydroxyl radicals generated by the homolytic cleavage of hydrogen peroxide. Furthermore, the overall oxidation was found more extended on lignins treated with TDCSPPMnCl than with TDCSPPFeCl. This behavior is in accord with the previously examined extent of oxidation of lignin model 1. A comparison of the reaction course in the presence of the cationic porphyrin TPyMePMn ( C H C O O ) at different pH values showed that the oxidation of lignin was improved at pH 6 rather than pH 3 (Table II). In fact the treatments performed at pH 6 resulted in a net increase of the carboxylic units and decrease of aliphatic hydroxyl groups. Since the C O O H increase was found higher than the aliphatic O H decrease, the carboxylic units cannot be formed only by side-chain oxidation reactions, but also aromatic ring cleavage processes could occur.


3

5

Table II. Distribution of aliphatic, phenolic and carboxylic hydroxyl groups in residual Kraft lignins before and after porphyrin catalyzed oxidations. lignin/treatment

Aliphatic OH

Condensed Guaiacyl OH phenolic OH

COOH

(mmol/g) TPyMePMnAcs pH 3 TPyMePMnAc pH 6 TSPPMnCl pH 6 TDCSPPMnCl p H 6 TDCSPPFeCl pH 3 Kraft lignin H 0 pH 3 H 0 pH 6 5

3

b

2

2

2

2

C

1.78 1.64 1.79 1.76 1.75 1.94 1.78 1.80

0.97 0.91 0.93 0.90 1.01 0.92 0.94 0.89

1.04 1.09 1.07 0.98 1.14 1.27 1.02 1.08

0.35 0.65 0.30 0.48 0.41 0.27 0.36 0.34

3Î

Data obtained from quantitative P-NMR spectra of samples phosphytylated with 2chloro-4, 4,5,5-tetramethyl-l, and 3,2-dioxaphosphoiane. a: starting reference sample; b: control experiment in the absence of porphyrins at pH 3; c: control experiment in the absence of porphyrins at pH 6. Reprinted from Bioorg. Med, Chem. 1999, 7, 1897, with permission from Elsevier Science

The comparison of anionic and cationic water-soluble Mn porphyrins TSPPMnCl and TPyMePMn ( C H C O O ) respectively, showed an increased efficiency in the cationic TPyMePMn (CH3COO)s toward the oxidation of lignin (Table II). The efficiency was evaluated on the basis of the aliphatic O H decrease and C O O H increase. To our knowledge this is the first example of the use of a cationic porphyrin for lignin oxidation. It is worth noting that in all the 3

5


223 oxidations carried out at pH 6 the amount of condensed phenolic substructures did not vary significantly. When the oxidations were performed at pH 3, processes occurring with condensation reactions were found active. In particular the iron porphyrin TDCSPPFeCl was more prone to undergo condensation than TPyMePMn (CH COO) . 3

5


Concluding remarks Anionic and cationic water-soluble manganese porphyrins are more effective in degrading residual kraft lignin and lignin substructures than iron porphyrins. This effect could be due to a higher stability of Mn porphyrins under oxidizing conditions. Among these the cationic TPyMePMn (CH3COO)5, never used before in lignin oxidation, showed to be the best catalyst. Both Fe and Mn porphyrins are able to oxidize such recalcitrant condensed lignin substructures as those represented in models 1 and 5, and yield p-quinones that in turn can be easily cleaved. However during hydrogen peroxide oxidation of residual kraft lignin in the presence of TDCSPPFeCl further condensation reactions are likely to occur. The final products are modified lignins with increased amounts of condensed substructures. On the other hand, residual kraft lignin treated with Mn porphyrins at pH 6 did not show any significative increase in the condensed units. These findings suggest that the overall lignin oxidation by H2O2 in the presence of Fe and Mn porphyrins follows different reaction pathways. Formation of hydroxyl radicals by the homolytic cleavage of the peroxidic bond of H 2 O 2 can occur, especially in the case of iron porphyrins. This reaction pattern is in competition with the heterolytic reaction. The suppression of condensation reactions that occurs during the oxidations of residual kraft lignin catalyzed by manganese porphyrins prompts at a further evaluation of their applicability in lignin degradation processes.

Points to ponder and future directions Despite the progress achieved in the synthesis of métallo porphyrins resistant to oxidation, a major drawback is still present to their applicability. Métallo porphyrins are, to date, expensive catalysts. Their potential use in lignin oxidation is bound to the possibility of a further increase of their stability toward hydrogen peroxide. Furthermore, the need to recover and recycle the catalysts after their use is also pressing. A possible approach to the development of such new catalysts has been attempted taking into consideration that these two aims could be reached by immobilization of the catalyst onto a suitable support. Several approaches of this kind are possible ranging from organic synthetic


224 polymers to biopolymers or to inorganic matrices. To date several procedures for métallo porphyrins immobilization onto inert matrices have been developed. (8,13) However the reactivity of the immobilized systems is usually lower than the soluble porphyrin. Meunier reported the oxidation of veratryl alcohol and a β-Ο-4 model dimer with FeTPPS and MnTPPS immobilized on the exchange resin Amberlite IRA 900 EGA using H 0 or KHSO as oxygen donor. The system immobilized-MnTPPS KHS0 showed to be the most active. (14) Much work is still to be done on the degradation of lignin by immobilized metalloporphyrins. From this view-point the use of hydrophobic membranes, that allow the equimolar transfer of H 0 and substrate to the catalyst, and avoid porphyrin bleaching is a promising technique. Smectite clays provide negatively charged layers large enough for the accommodation one porphyrin and one substrate molecule only. Thus, immobilization onto clays would prevent from the formation of μ-οχο dimers. The possibility to use clays as support is also a promising approach. 2

2

s


5

2

2

Acknowledgments This work is dedicated to the memory of prof. Tristano Boschi. Italian M.U.R.S.T. and the University of Tor Vergata are acknowledged for financial support.

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

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225 10. Hattori, T. ; Shimada, M. ; Umezawa, T. ; Higuchi, T. ; Leisola, M.S.A. ; Fiechter, A. Agric. Biol. Chem. 1988, 52, 879. 11. Shimada, M.; Habe, T.; Higuchi, T.; Okamoto, T.; Panijpan, B. Holzforshung, 1977, 41, 277. 12. Meunier, B. In Metalloporphyrins catalyzed oxidations. Montanari, F.; Casella, L. editors, Kluwer academic publishers: Dordrecht, 1994; pp. 11-19 13. Labat, G.; Meunier, B. J. Org. Chem. 1989, 54, 5008. 14. Artaud, I.; Ben-Aziza, K.; Mansui, D. J. Org. Chem. 1993, 58, 3373. 15. Cui, F.; Dolphin, D. Can. J. Chem. 1992, 70, 2314. 16. Cui, F.; Wijesekera, T.; Dolphin, D.; Farrel, R.; Skerker, P. J. Biotechnol. 1993, 30, 15. 17. Cui, F.; Dolphin, D. Bioorg. Med. Chem. 1994, 2, 735. 18. Crestini, C.; Saladino, R.; Tagliatesta, P.; Boschi, T. Bioorg. Med. Chem. 1999, 7, 1897. 19. Eriksson, T. ; Gierer, J. J. Wood Chem. Technol. 1985, 5, 53. 20. Kurek, B.; Artaud, I.; Pollet, B.; Lapierre, C.; Monties, B. J. Agric. Food Chem. 1996, 1953. 21. Skerker, P.S.; Farrel, R.L.; Dolphin, D.; Cui, F.; Wijesekera, T. In Biotechnology in pulp and paper maufacture. Kirk, T.K., Chang, H.M. Eds.; Butterworth-Heinemann: Stoneham, MA, 1990, pp203-210. 22. Xie, L. ; Dolphin, D. Handbook on metal-Ligand Interactions in Biological fluids, Marcel Dekker inc.; New York. 23. Panicucci, R.; Bruice, T.C. J. Am. Chem. Soc. 1990, 112, 6063. 24. Traylor, T.G.; Xu, F. J. Am. Chem. Soc. 1990, 112, 178. 25. Gierer, J.; Reitberger, T.; Yang, E. Holzforshung 1992, 46, 495. 26. Crestini, C.; Argyropoulos, D.S. J. Agric. and Food Chem. 1997, 49, 1212. 27. Argyropoulos, D.S. Res. Chem. Intermed. 1995, 21, 373. 28. Granata, Α.; Argyropoulos, D.S. J. Agric. and Food Chem. 1995, 33, 375. 29. Jiang, Z.H.; Argyropoulos, D.S.; Granata, A. Magn. Res. Chem., 1995, 43, 1538. 30. Argyropoulos, D.S. J. Wood Chem. Technol. 1994, 14, 45. 31. Crestini, C. ; Giovannozzi Sermanni G. ; Argyropoulos, D.S. Bioorg. Med. Chem. 1998, 6, 967. 32. Smit, R. ; Suckling, I.D. ; Ede, R.M. Proceedings of the 9 Int. Symp. on Wood and Pulping Chemistry. Montreal, 1997, L4-1 L4-6. th


A Biomimetic Approach to Lignin Degradation - ACS Symposium Series

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