Advances on Selective C-6 Oxidation of Chitosan by TEMPO Nicolas Bordenave, Ste ´phane Grelier, and Ve ´ ronique Coma* Universite ´ Bordeaux 1, INRA, CNRS, UMR 5103 US2B, 351 cours de la Libe ´ ration, F-33405 Talence, France Received April 8, 2008; Revised Manuscript Received June 13, 2008 The specific C-6 oxidation by TEMPO of chitosan and chitosan derivatives were studied to obtain tailored bioactive biopolymers. The modifications on chitosan presented many difficulties and showed the adverse effect of the amine moieties of chitosan on this reaction. Thus, protections of the amino groups by N-acetylation or N-phthaloylation were studied and followed by the C-6 specific oxidations of the resulting polymers. The desired 6-carboxychitosan could not be obtained after deprotection; the reactions with TEMPO led to degradation of the polymers. The specific oxidation of a potentially bioactive derivative of chitosan was then achieved by the oxidation of a quaternized chitosan: N,N,N-trimethylchitosan. N,N,N-Trimethyl-6-carboxychitosan was characterized by FTIR spectroscopy, 1 H, and 13 C NMR spectroscopy. Introduction Chitosan is a biopolymer composed of N-acetyl-D-glu- cosamine and D-glucosamine units linked by ,1-4 bonds, with 70-95% of D-glucosamine units (Figure 1) according to ref 1. It is obtained by deacetylation of chitin, one of the major components of crustacean exoskeleton. Chitosan exhibits film forming and antimicrobial properties due to its cationic character, depending on its deacetylation degree and the pH of the solution. Moreover, its chelating properties with various anions and substances, such as fats, metals, proteins, and so on, are remarkable. Common applications of chitosan are waste treatment, 2,3 controlled drug delivery, 4 wound dressing, 5,6 and medical and biocompatible materials. 7–9 Moreover, as a renewable resource, and thanks to its biodegradable character, chitosan can have numerous applications in agriculture or in the food industry. 10 The field of applications of this biopolymer is potentially very wide, but chitosan often requires modifications and improve- ments of its natural properties. Chemical modification is a practical way to improve the properties of a natural substance. Among the various potential chemical modifications of chitosan, 11,12 this work aims to develop a new derivative of chitosan that could be modified and adapted “on demand”. Indeed, the conversion of the primary alcohols borne by C-6 of chitosan would allow numerous easy modifications depending on the desired application: cross-linking, esterification thanks to fat alcohols grafting, grafting of phosphorescent functions, and so on. This conversion was well described on cellulose and starch. The use of common oxidative agents such as periodate is prohibited because they lead to the formation of 2,3-dialdehyde cellulose or starch. 13–16 Thus, a specific oxidant for primary alcohols was used on starch and cellulose: 2,2,6,6-tetrameth- ylpiperidine-1-oxy radical, TEMPO. 17–22 TEMPO was used with a co-oxidative system, and NaOCl/NaBr has been shown to be the system leading to the best yields. Figure 2 shows the mechanism of action of TEMPO proposed by Bragd et al. 23,24 This reaction applied to chitin or chitosan has not been extensively studied. Muzzarelli et al. 25 and Jiang et al. 26 have, respectively, carried out the selective C-6 oxidations of chitin and hyaluronan (copolymer of N-acetylglucosamine and D- glucoronic acid units). Kato et al. 27 performed this reaction on chitin and N-reacetylated chitosans. Finally, there was no characterization of reaction products in the only study, to our knowledge, which described selective C-6 oxidation of chito- san. 28 This paper deals with the selective C-6 oxidation of chitosan, considering several premodifications of the substrate to protect chitosan amino groups or modify chitosan reactivity. Among these premodifications, N-acetylation, 29,30 N-deacetylation, 31–33 N-phthaloylation, 34,35 and N,N,N-trimethylation 36–40 have been considered. Experimental Section Materials. Compounds. Chitosan 244 (deacetylation degree higher than 98%, Mw ) 400 kDa) and chitosan 652 (deacetylation degree * To whom correspondence should be addressed. E-mail: v.coma@ us2b.u-bordeaux1.fr. Figure 1. Structure for chitosan (deacetylation degree ) 67%). Figure 2. Oxidation of a primary alcohol to an aldehyde and a carboxylic acid by NaOCl/NaBr/TEMPO system. 23 Biomacromolecules 2008, 9, 2377–2382 2377 10.1021/bm800375v CCC: $40.75 2008 American Chemical Society Published on Web 08/14/2008
Transcript
Advances on Selective C-6 Oxidation of Chitosan by TEMPO
Nicolas Bordenave, Stephane Grelier, and Veronique Coma*
Universite Bordeaux 1, INRA, CNRS, UMR 5103 US2B, 351 cours de la Liberation,F-33405 Talence, France
Received April 8, 2008; Revised Manuscript Received June 13, 2008
The specific C-6 oxidation by TEMPO of chitosan and chitosan derivatives were studied to obtain tailored bioactivebiopolymers. The modifications on chitosan presented many difficulties and showed the adverse effect of theamine moieties of chitosan on this reaction. Thus, protections of the amino groups by N-acetylation orN-phthaloylation were studied and followed by the C-6 specific oxidations of the resulting polymers. The desired6-carboxychitosan could not be obtained after deprotection; the reactions with TEMPO led to degradation of thepolymers. The specific oxidation of a potentially bioactive derivative of chitosan was then achieved by the oxidationof a quaternized chitosan: N,N,N-trimethylchitosan. N,N,N-Trimethyl-6-carboxychitosan was characterized by FTIRspectroscopy, 1H, and 13C NMR spectroscopy.
Introduction
Chitosan is a biopolymer composed of N-acetyl-D-glu-cosamine and D-glucosamine units linked by �,1-4 bonds, with70-95% of D-glucosamine units (Figure 1) according to ref 1.It is obtained by deacetylation of chitin, one of the majorcomponents of crustacean exoskeleton.
Chitosan exhibits film forming and antimicrobial propertiesdue to its cationic character, depending on its deacetylationdegree and the pH of the solution. Moreover, its chelatingproperties with various anions and substances, such as fats,metals, proteins, and so on, are remarkable.
Common applications of chitosan are waste treatment,2,3
controlled drug delivery,4 wound dressing,5,6 and medical andbiocompatible materials.7–9 Moreover, as a renewable resource,and thanks to its biodegradable character, chitosan can havenumerous applications in agriculture or in the food industry.10
The field of applications of this biopolymer is potentially verywide, but chitosan often requires modifications and improve-ments of its natural properties.
Chemical modification is a practical way to improve theproperties of a natural substance. Among the various potentialchemical modifications of chitosan,11,12 this work aims todevelop a new derivative of chitosan that could be modifiedand adapted “on demand”.
Indeed, the conversion of the primary alcohols borne by C-6of chitosan would allow numerous easy modifications dependingon the desired application: cross-linking, esterification thanksto fat alcohols grafting, grafting of phosphorescent functions,and so on.
This conversion was well described on cellulose and starch.The use of common oxidative agents such as periodate isprohibited because they lead to the formation of 2,3-dialdehydecellulose or starch.13–16 Thus, a specific oxidant for primaryalcohols was used on starch and cellulose: 2,2,6,6-tetrameth-ylpiperidine-1-oxy radical, TEMPO.17–22 TEMPO was used witha co-oxidative system, and NaOCl/NaBr has been shown to bethe system leading to the best yields. Figure 2 shows themechanism of action of TEMPO proposed by Bragd et al.23,24
This reaction applied to chitin or chitosan has not beenextensively studied. Muzzarelli et al.25 and Jiang et al.26 have,respectively, carried out the selective C-6 oxidations of chitinand hyaluronan (copolymer of N-acetylglucosamine and D-glucoronic acid units). Kato et al.27 performed this reaction onchitin and N-reacetylated chitosans. Finally, there was nocharacterization of reaction products in the only study, to ourknowledge, which described selective C-6 oxidation of chito-san.28
This paper deals with the selective C-6 oxidation of chitosan,considering several premodifications of the substrate to protectchitosan amino groups or modify chitosan reactivity. Amongthese premodifications, N-acetylation,29,30 N-deacetylation,31–33
N-phthaloylation,34,35 and N,N,N-trimethylation36–40 have beenconsidered.
* To whom correspondence should be addressed. E-mail: [email protected].
Figure 1. Structure for chitosan (deacetylation degree ) 67%).
Figure 2. Oxidation of a primary alcohol to an aldehyde and acarboxylic acid by NaOCl/NaBr/TEMPO system.23
Biomacromolecules 2008, 9, 2377–2382 2377
10.1021/bm800375v CCC: $40.75 2008 American Chemical SocietyPublished on Web 08/14/2008
higher than 85%, Mw ) 165 kDa) were furnished by France Chitine(Marseille, France). Acetic acid (assay >99.5%), acetic anhydride (assay>99%), N-methyl-2-pyrrolidinone (assay >99%), phthalic anhydride(assay >99%), dimethylsulfate (assay >99%), TEMPO (assay >96%),sodium hypochlorite (solution, assay >10-13%), sodium borohydride(powder, assay >98.5%), and phosphorus pentoxide (technical) werefurnished by Sigma-Aldrich (Saint-Quentin-Fallavier, France). Absoluteethanol was furnished by Fluka (L’Isle d’Abeau Chesnes, France).Methanol, acetone, ethanol, and diethyl ether (HPLC grade) werefurnished by SDS (Val de Reuil, France). Sodium hydroxide (granules,assay >99%), hydrochloric acid (solution, assay >37%), and formal-dehyde (solution, assay >36%) were furnished by VWR (Fontenaysous Bois, France). Sodium bromide (powder, assay >98%) wasfurnished by Prolabo (Fontenay sous Bois, France). Sodium sulfate(powder, assay >99%) was furnished by Riedel-de-Haen (Seelze,Germany). Deuterium oxide was furnished by EurisoTop (Gif-sur-Yvette, France).
Methods. N-Acetylation of Chitosan. The method was described byKato et al.27
Briefly, 3 g of oven-dried chitosan were suspended in 27 mL ofwater over a period of 1 h. To dissolve chitosan, 30 mL of 20% aqueousacetic acid solution were added to the suspension. A total of 120 mLof methanol were then added and the resulting solution was filtered toeliminate the insoluble impurities. Acetic anhydride (1.2 mol per molof glucosamine units in the chitosan solution) was poured into 120mL of methanol, added to the chitosan solution, and stirred at 500 rpm.As the solution was becoming a gel, stirring was intensified (1200 rpm)for 18 h. The resulting N-acetylchitosan was then washed and filtratedfive times using water/acetone (1/7 by weight) and one time with waterand then dried under vacuum with P2O5.
N-Phthaloylation of Chitosan. The method was described by Kuritaet al.35
A total of 6.77 g of phthalic anhydride (0.046 mol) and 3 g ofchitosan (0.015 mol of glucosamine units) were added to a DMF/watersolution (95/5 v/v), heated up to 120 °C, and stirred for 8 h at 500 rpmunder a nitrogen atmosphere. The solution was cooled at roomtemperature and poured onto crushed ice. The precipitated product wasrecovered by filtration, washed five times with methanol, and driedunder vacuum with P2O5.
N,N,N-Trimethylation of Chitosan. The method was described byBelalia.38,39
First step: preparation of N-methylchitosan. A total of 4 g of drychitosan 652 was dissolved in 400 mL of a 1% w/w acetic acid aqueoussolution. The resulting solution was then filtered to eliminate impuritiesand formaldehyde was added (3 mol formaldehyde per mol of chitosanfree amine groups). The solution was stirred for 30 min at 500 rpm atroom temperature before adding NaBH4 (1.5 mol of NaBH4 per molof formaldehyde) and stirred again for 1 h. The pH was adjusted to 10with 1 M NaOH. The precipitate was filtered and abundantly washeduntil neutrality. The unreacted products were removed by Soxhletextraction with ethanol/diethyl ether (80/20 v/v), for 48 h. The productwas partially dried at room temperature and relative humidity for 48 h(a complete drying led to an insoluble product).
Second step: preparation of N,N,N-trimethylchitosan. N-Methylchi-tosan was dispersed in 120 mL of N-methyl-2-pyrrolidinone withNa2SO4 (0.1 mol ·L-1) at 60 °C, 1200 rpm for 1 h. A total of 22 mLof 15% w/w aqueous solution of NaOH containing (CH3O)2SO2 (1.5mol of per mol of free amine groups in the starting material) wereadded. The mixture was stirred for 6 h at 500 rpm and 60 °C. Finally,N,N,N-trimethylchitosan was precipitated and washed three times withacetone and dried under vacuum at room temperature with P2O5.
SelectiVe Oxidation of Chitosan and its DeriVatiVes. The followingmethods were applied to chitosan or its derivatives, prepared accordingto the previous descriptions. Their different monomeric units werenamed by the generic term “glucosidic unit” and written GlU. Thedifferent products designed to be oxidized were named by the genericterm “substrate”.
Method 1. The oxidation was performed according to a modifiedmethod described by Muzzarelli et al.24
TEMPO (156 mmol per mol of GlU), NaBr (0.789 mol per mol ofGlU), and a solution containing 4% NaOCl (2.868 mol NaOCl per molof GlU) were added to a substrate suspension in water (2% w/w) atroom temperature. Immediately after the addition of NaOCl, pH wasadjusted and maintained at 10.8, thanks to the continuous addition ofNaOH.
The reaction was considered as finished when the NaOH consump-tion stopped. Ethanol was then poured into the solution to quench thereaction (2.5 mL per g of initial substrate). The solution was neutralizedwith diluted HCl.
The product was precipitated in acetone, recovered by centrifugation,and washed three times with water/acetone (1/10 by weight) andlyophilized.
Method 2. The oxidation was performed according to a modifiedmethod developed by Kato et al.27 The differences with Method 1 were(1) the reaction was conducted at 5 °C in an ice bath, with a pHmaintained at 10.8; (2) TEMPO, NaBr, and NaOCl (11% solution) wereadded as 200 mmol, 0.4, and 2.7 mol per mol of GlU, respectively;and (3) NaOCl was added progressively for 30 min.
Method 3. The oxidation was performed according to a modifiedmethod described by Yoo et al.28 The differences with Method 1 were(1) the reaction was conducted at 30 °C, with a maintained pH at 10.8;(2) TEMPO, NaBr, and NaOCl (11% solution) added as 596 mmol,0.541, and 2.2 mol per mol of GlU, respectively; and (3) NaOCl wasadded progressively for 30 min.
Deacetylation of Oxidized N-Acetylchitosan. The substrate wassuspended in 40% NaOH aqueous solution, at 80 °C, for 1 h. Thesolution was cooled in an ice bath and poured into acetone forprecipitation.
NMR Analyses. 1H and 13C NMR spectra of the products were carriedout on a Bruker AVANCE 300 MHz apparatus with D2O as solvent.
FTIR Analyses. InfraRed spectra of the products were carried out inKBr tablets (1% w/w of product in KBr), with a resolution of 4 cm-1,and 100 scans per sample, on a ThermoNicolet AVATAR 370apparatus.
N,N,N-Trimethyl-6-carboxychitosan. FT-IR (KBr tablets), wavenumbers (cm-1): 3440 (O-H stretching), 2920 (C-H asymmetricstretching of methyl groups), 2874 (C-H symmetric stretching ofmethyl groups), 1730 (CdO stretching of acetone), 1700 (CdOstretching of carboxylic acid groups), 1650 (CdO stretching of amidegroups), 1553 (N-H bending of amino groups), 1460 (C-H asymmetricbending of methyl groups), 1180 (C-N stretching of primary aminogroups), 895 (N-H bending out of plane of primary amino groups).
2378 Biomacromolecules, Vol. 9, No. 9, 2008 Bordenave et al.
Results and Discussion
Direct Oxidation of Chitosan. Chitosans 652 and 244 weresubmitted to TEMPO oxidation with the three proposed methods(Figure 3).
Reactions were repeated several times at 5 and 30 °C withreactants from various furnishers on both chitosans, but theirprogress was unrepeatable. The reaction was or was not initiated,without possible correlation with the experimental conditions.This could be due to the heterogeneous reaction, at solid/liquidinterface (chitosan vs other reactants in solution). Thus, theinitiation of the reaction might differ depending on the chitosanhydration, for instance.
Whenever reaction was initiated, pH steadily decreased andwas corrected by the addition of NaOH. The reaction wasconsidered as finished when the sodium hydroxide consumptionstopped. The total NaOH consumption was generally about of80% of the theoretical consumption. It was noted that theinitiated reactions progress was not repeatable. They could lastfrom a few hours to several days, without correlation with theexperimental temperature. In these cases, the product wasrecovered after neutralization of the solution, and the bestamount of recovered product was 20 mg, from 1 g of chitosanat the beginning. This suggested that chitosan was degradedand NMR spectra of recovered product could not allow anyidentification.
This could be due to an effective chitosan C6-oxidationassociated with a drastic decrease of the polymer molecularweight. As a result, Yoo et al.28 recovered the product bycentrifugation and ultrafiltration, while centrifugation andprecipitation were only used in our experiments. However, theobjective was to synthesize 6-carboxychitosan comparable tothe starting material. This step was then considered unsuccessful.
Because the reaction was extensively described on celluloseand starch, it was supposed that the presence of amino groupsin the oxidation substrate could dramatically disturb the reaction,leading to chitosan degradation. Yoo et al.28 claimed to managea direct oxidation of chitosan primary alcohol. However, nosubstrate characterization was reported (degree of deacetylationand molecular weight) and the progress of the product synthesiswas only determined with respect to the NaOH consumption.Nevertheless, it seems that NaOH consumption was not a proofof effective selective oxidation of chitosan. It could beconsidered that, to our knowledge, no evidence of the synthesisof 6-carboxychitosan has ever been performed, and, accordingto our results, the feasibility of this reaction was not proven.
Due to the possible adverse effects of the chitosan aminogroup on the specific C6 oxidation, probably due to nitrogennucleophilic character and reactivity, same reactions wereconducted onto modified chitosans with protected amino groups.Furthermore, amino group deprotection was investigated torestore the antimicrobial properties of chitosan.
Chitosan C6 Specific Oxidation after Amino GroupProtection. Three ways of protections were considered: N-acetylation, N-phthaloylation and N,N,N-trimethylation.
Oxidation after N-Acetylation. Kato et al.27 and Muzzarelliet al.24 achieved the oxidation of chitin or N-acetylated chitosan.
Chitosan N-acetylation was then considered as a protection ofthe amino group, deprotection process being similar to chitosanproduction process from chitin: deacetylation in a warm andconcentrated basic media (Figure 4).
Both chitosans 244 and 652 were reacetylated according tothe method developed by Kato et al.27 and further oxidized withcontrasted results.
Concerning the reacetylated chitosan 652, 90% of thetheoretical NaOH consumption was achieved during the oxida-tion step, and both reactions (acetylation and oxidation) reacheda global yield of 88% by weight. The 13C NMR spectrum ofthe synthesized product is presented in Figure 5.
13C NMR spectrum of reacetylated-chitosan 652 oxidizedwith TEMPO is similar to the spectra obtained by Kato et al.27
with comparable noise. C6 signal of chitosan, commonlyobserved at 63 ppm, disappeared,41,42 whereas a peak corre-sponding to a CdO carbon appeared at 175 ppm. This is anevidence for the formation of N-acetyl-6-carboxychitosan. Thissuggests that a decrease of nitrogen reactivity was necessary toavoid any perturbation of the oxidation process.
Concerning reacetylated-chitosan 244, the oxidation reactionhas never been initiated, despite numerous attempts with variousexperimental conditions. Due to no chemical difference betweenboth chitosans (244 and 652), except their molecular weight,both polymers might have adopted different crystalline structuresduring drying, leading to different reactivities, with reactive sitesless accessible in the case of chitosan 244.
As a result, due to the low reactivity of chitosan 244, onlychitosan 652 was used in the continuation of the study.
Deacetylation of N-acetyl-6-carboxychitosan was investigated.Because the deacetylation of chitin is known to lead to severedegradations,32 the deacetylation process was reduced to 1 h in40% aqueous NaOH solution at 80 °C. Unfortunately, N-acetyl-
Figure 3. Oxidation of chitosan primary alcohol catalyzed by TEMPO.
Figure 4. N-Acetylation of chitosan, followed by formation of N-acetyl-6-carboxychitosan and deacetylation of N-acetyl-6-carboxychitosan.
Figure 5. 13C NMR spectrum of chitosan 652 reacetylated andoxidized with TEMPO.
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6-carboxychitosan was almost fully degraded during thisprocess, producing a brownish solution from which no productcould be recovered.
According to Roberts,43 degradation of chitin during deacety-lation process could be due to terminal aldehydes on the chitinchains. Potential side reaction with aldehydes can lead to randomdepolymerization of chitosan. This can be limited by the additionof reducing agent or antioxidants, such as NaBH4 or thiophe-nol.44 Thus, a reduction of potential aldehydes by NaBH4 wascarried out in the reactive media. NaBH4 does not reducecarboxylic acids and allows the reduction of the terminalaldehydes to alditols as a protection.45 Unfortunately, thismethod could not prevent degradations.
Due to too rough conditions of the amino group deprotectionby deacetylation, another protection has been considered.
Oxidation of Chitosan after N-Phthaloylation. Phthaloylationis a common protection of amines and its deprotection is carriedout under mild basic conditions with hydrazine. After thephthaloylation step, oxidation with TEMPO was carried outunder the described conditions and under milder conditions toavoid any unwanted deprotection (pH ) 8).
However, despite these precautions, phthaloyl groups havebeen removed from chitosan during the oxidation stage and theoxidation was not successful: the recovered product exhibiteda classic chitosan 1H NMR spectrum (spectrum not shown).
N-Phthaloylation and N-acetylation appeared to be ineffectivemethods to protect amine groups and to achieve chitosan C6-oxidation. Other protection could be envisaged such as the useof t-butylcarbamate (Boc),46 but permanent modifications ofchitosan that would not need additional protection-removal stepswere preferred and studied.
Oxidation of Chitosan after N,N,N-Trimethylation. The useof chitosan with a quaternized amino group was then investi-gated. Jia et al.37 and Belalia38,39 studied the synthesis of N,N,N-trimethylchitosan (TMC). In particular, Belalia et al.38,39 haveshown that this derivative exhibited better antimicrobial activitythan unmodified chitosan and a good solubility in water over alarger pH range. The use of chitosan with nitrogen in a stablequaternized form appeared to be an alternative solution to avoiddegradations during the oxidation stage with TEMPO, becauseN,N,N-trimethylation considerably decreased the nucleophiliccharacter of nitrogen in the biopolymer and then reduced itsreactivity. Moreover, quaternization avoids deprotection processto recover the antimicrobial properties, due to stable cationiccharges in the biopolymer structure.
According to Belalia et al.,38,39 the iodine ion was used asthe counterion of TMC. Unfortunately, this latter would interferein the following oxidative process. The iodomethane alkylatingagent was thus replaced by dimethylsulfate, with SO4
2- as thenew counterion, which is inert for the following oxidation(Figure 6). Chitosan was trimethylated and characterized by 1HNMR (Figure 7) and FT-IR spectroscopy (Figure 8).
The spectra, proposed in Figure 7, is similar to those obtainedby Belalia et al.38,39 and Jia et al.37 and shows that trimethylationof chitosan amino groups led to minor side-reactions, such asmethylation of some hydroxyl groups or dimethylation of some
amino groups (signal at 3.4 and 2.9 ppm, respectively). Productwas then oxidized with the same protocols as specified above(Figure 9), and the reaction consumed 90% of the theoreticalvolume of the NaOH solution. Degradations of the substrateled to a yield of about 30%. Product was then characterizedwith 1H NMR (Figure 10), 13C NMR (Figure 11), and by FT-IR spectroscopy (Figure 12).
1H and 13C NMR spectra show that the structure of theglucosidic rings of chitosan was preserved. Indeed, on 1H NMRspectrum (Figure 10), the protons borne by carbons C-1 to C-5and those borne by the three methyl groups of the quaternaryammonium are still present.
13C NMR spectrum (Figure 11) is not correctly defined (noise)and the peaks are multiple. This could be due to the polydis-persity of the new polymer. However, the signals can beattributed to corresponding carbons27,41,42 (Figure 5). Moreover,the signal around 176 ppm reveals the presence of a carbonylgroup. Due to the noise on this spectrum, this peak could notbe certainly attributed to a carboxylic acid function.
However, the FT-IR spectrum (Figure 12) exhibits a band at1700 cm-1 attributed to CdO stretching of carboxylic acids
Figure 6. N,N,N-Trimethylation of chitosan.
Figure 7. 1H NMR spectra of quaternized (trimethylated) chitosan652.
Figure 8. FT-IR spectra of quaternized (trimethylated) chitosan 652.
Figure 9. Selective oxidation of C-6 on N,N,N-trimethylchitosan.
2380 Biomacromolecules, Vol. 9, No. 9, 2008 Bordenave et al.
groups. Thus, a carboxylic acid function seems to be formedon C-6. In addition, because the product was dried under vacuumwith P2O5 and without heating to avoid any degradation, theband at 1730 cm-1 is attributed to CdO stretching of acetone,without any interference with the band at 1700 cm-1. As a result,TMC was oxidized toward N,N,N-trimethyl-6-carboxychitosan.
Conclusion
This work showed that the direct C-6 selective oxidation ofchitosan is limited by the reactivity of chitosan amino groups.Several protection of this group has been then investigatedleading to other problems of reactivity or deprotection. Thus, apermanent modification of chitosan amino groups has been usedby their N,N,N-trimethylation, leading to a chitosan derivativewith improved solubility and bioactivity. This protection allowedthe C-6 specific oxidation of chitosan. Nevertheless, despite a
low yield of this synthesis, it opens a wide field of potential“on demand” modifications, for various applications of chitosan.The experimental conditions should now be optimized for lessdegradations.
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Figure 12. FT-IR spectra of chitosan 652 quaternized and oxidized.
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