制药工程专业英语 第6单元

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1、PART2BIOCHEMICALPHARMACEUTICALSUnit6IsolationofCaffeinefromTeaInthisexperiment，Caffeinewillbeisolatedfromtealeaves.Themajorproblemoftheisolationisthatcaffeinedoesnotoccuraloneintealeaves，butisaccompaniedbyothernaturalsubstancesfromwhichitmustbeseparated.T

2、hemajorcomponentoftealeavesiscellulose，whichisthemajorstructuralmaterialofallplantcells.Celluloseisapolymerofglucose.Sincecelluloseisvirtuallyinsolubleinwater，itpresentsnoproblemsintheisolationprocedure.Caffeine，ontheotherhand，iswatersolubleandisoneofthem

3、ajorsubstancesextractedintothesolutioncalled"tea.”Caffeinecomprisesasmuchas5percentbyweightoftheleafmaterialinteaplants.Tanninsalsodissolveinthehotwaterusedtoextracttealeaves.Thetermtannindoesnotrefertoasinglehomogeneouscompound，oreventosubstanceswhichhav

4、esimilarchemicalstructure.Itreferstoaclassofcompoundswhichhavecertainpropertiesincommon.Tanninsarephenoliccompoundshavingmolecularweightsbetween500and3000.Theyarewidelyusedto"tan”leather.Theyprecipitatealkaloids'zandproteinsfromaqueoussolutions.Tanninsare

5、usuallydividedintotwoclasses:thosewhichcanbehydrolyzedandthosewhichcannot.Tanninsofthefirsttypewhicharefoundinteagenerallyyieldglucoseandgallicacidwhentheyarehydrolyzed.Thesetanninsareestersofgallicacidandglucose.Theyrepresentstructuresinwhichsomeofthehyd

6、roxylgroupsinglucosehavebeenesterifiedbydigalloylgroups.Thenon-hydrolyzabletanninsfoundinteaarecondensationpolymersofcatechin.Thesepolymersarenotuniforminstructure，butcatechinmoleculesareusuallylinkedtogetheratringpositions4and8.Whentanninsareextractedint

7、ohotwater，thehydrolyzableonesarepartiallyhydrolyzed，meaningthatfreegallicacidisalsofoundintea.Thetannins，byvirtueoftheirphenolicgroups，andgallicacidbyvirtueofitscarboxylgroups，arebothacidic.Ifcalciumcarbonate，abase，isaddedtoteawater，thecalciumsaltsofthese

8、acidsareformed.Caffeinecanbeextractedfromthebasicteasolutionwithchloroform，butthecalciumsaltsofgallicacidandthetanninsarenotchloroformsolubleandremainbehindintheaqueoussolution.Thebrowncolorofateasolutionisduetoflav