A simple preparation of N-acetylated chitosan highly soluble

www.elsevier.nl/locate/carres

CarbohydrateResearch324(2000)268–274

AsimplepreparationofhalfN-acetylatedchitosanhighlysolubleinwaterandaqueousorganicsolvents

NaojiKubotaa,*,NobuhideTatsumotoa,TakayukiSanoa,KaoriToyab

b

GeneralEducationChemistryandPhysics,OitaMedicalUni6ersity,Hasama-machi,Oita879-5593,JapanDepartmentofAppliedChemistry,FacultyofEngineering,OitaUni6ersity,Dannoharu,Oita870-1192,Japan

Received8June1999;accepted3September1999

a

Abstract

Asimpleandimprovedmethodofpreparinghighlysolublechitosan(halfN-acetylatedchitosan)wasdevelopedusingaseriesofchitosansamplesoflowmolecularweights,andthesolubilityofthehalfN-acetylatedchitosaninwaterandorganicsolventswasinvestigatedindetail.Toreducethemolecularweight,chitosanwastreatedwithNaBO3undertheconditionthatchitosanwashomogeneouslydissolvedinaqueousaceticacid.Weight-averagemolecularweightsoftheobtainedchitosansamplesweredeterminedusingasize-exclusionchromatographysystemequippedwithalow-anglelaserlight-scatteringphotometer.EachchitosansamplewasthenN-acetylatedwithaceticanhydrideundertheconditionthatchitosanwashomogeneouslydissolvedinaqueousaceticacidagain.ThewatersolubilityofthehalfN-acetylatedchitosanthuspreparedincreasedwithdecreasingmolecularweight.From1HNMRspectroscopy,itwassuggestedthatthesequenceofN-acetylglucosamineandglucosamineresidueswasrandom.ThesolubilityofthehalfN-acetylatedchitosanoflowmolecularweightwasratherhigheveninaqueousdimethylacetamideanddimethylsulfoxide.©2000ElsevierScienceLtd.Allrightsreserved.

Keywords:Chitin;Chitosan;HalfN-acetylatedchitosan;Solubility;Molecularweight;DegreeofN-acetylation

1.Introduction

Chitin,poly-b-(1 4)-N-acetyl-D-glucosa-mine,isthesecondmostabundantnaturalpolysaccharideandexistslargelyintheshellsofcrustaceaandinsects.Chitosan,poly-b-(1 4)-D-glucosamine,canbereadilyob-tainedfromchitinbydeacetylationwithalkali.Theuseofchitinandchitosaninvari-ousfunctionalmaterials,includingbiomedicalmaterials,haverecentlybeendeveloped[1,2].Wehavealsoreportedonthemembraneap-plicationsofchitosananditsderivatives

[3–5].

*Correspondingauthor.Tel.:+81-97-586-5605;fax:+81-97-586-5619.

E-mailaddress:nkubota@oita-med.ac.jp(N.Kubota)

However,theapplicationsofchitinandchi-tosaninbiology,inwhichmanyenzymeas-saysareperformedatneutralpH,isquiterestricted,becausetheyareessentiallyinsolu-bleinneutralwater.Ifwater-solublechitinandchitosancouldbepreparedinasimplemanner,theirbiologicalandphysiologicalap-plicationswoulddevelopdramatically.

Chitinandchitosanbecomewatersolublethroughachemicalmodi cationinwhichthedegreeofsubstitutioniscontrolled.Forin-stance,itisknownthatwater-solublechitinwithadeacetylationdegreeofabout50%canbeobtainedfromchitinbyhydrolysiswithalkali(Fig.1)[6,7].Itwassuggestedthatthewatersolubilityresultedfromtherandomdis-tributionofN-acetylgroups.Ontheotherhand,water-solublechitosanwithabouta

PII:S0008-6215(99)00263-3

0008-6215/00/$-seefrontmatter©2000ElsevierScienceLtd.Allrightsreserved.

N.Kubotaetal./CarbohydrateResearch324(2000)268–274269

Fig.1.Syntheticroutestowater-solublechitinandchitosan.

50%degreeofN-acetylationwaspreparedfromchitosanbyN-acetylationwithaceticanhy-dride(Fig.1)[8].However,averylongreactiontime,i.e.,over2days,wasrequiredintheformer,andacomplexsolventsystem,i.e.,aceticacid–water–methanol–pyridine,wasusedinthelatter.Furthermore,Aibareportedthatreductionofthemolecularweightdidnotin uencethewatersolubilityofthepartiallyN-acetylatedchitosan[9].However,therela-tionshipbetweenmolecularweightandsuchwatersolubilitywasnotclear,sinceheusedonlytwodifferentmolecularweights.Inaddi-tion,therearefewreportsonthesolubilityofpartiallyN-acetylatedchitosanincommonor-ganicsolvents.

Inthispaper,therefore,wereportasimpleandimprovedmethodtopreparehighlywater-solublechitosan(halfN-acetylatedchitosan)usingaseriesofchitosansamplesoflowmolecularweights.Chitosanispreviouslyde-gradedbytreatmentwithNaBO3[10]andthenN-acetylatedwithaceticanhydrideinaqueous

aceticacid.TherelationbetweenmolecularweightandwatersolubilityoftheobtainedN-acetylatedchitosanisquantitativelyinvesti-gatedandthesolubilityofthehalfN-acetylatedchitosaninsomeorganicsolventsisexamined.2.Experimental

Materials.—Commerciallyavailablechi-tosanfromTokyoKaseiKogyo,Tokyo,Japan,waspuri edthricebyreprecipitationusingaqAcOH(5%)andaqKOHinadvance.Itwas nallywashedwithdeionizedwaterandace-tone,andthendriedinvacuo.Standardpullu-lansforsize-exclusionchromatography(SEC)werepurchasedfromShowaDenko,Tokyo,Japan.Aceticanhydride,acetone,N,N-dimethylacetamide(DMA),andMe2SOfromWakoPureChemicalIndustries,Osaka,Japan,weredistilledjustbeforeuse.Allotherchemicalswereofreagentgradeandusedasreceived.

Table1

Degradationconditionsandweight-averagemolecularweightofchitosanSamplecode

DegradationconditionNaBO3(%w/v)

600k380k340k230k150k110k78k63k33k19k12k8.8k

a

wM

Temperature(°C)

Time(h)

6.0×1053.8×1053.4×1052.3×1051.5×1051.1×1057.8×1046.3×1043.3×1041.9×1041.2×1048.8×103

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