制药工程专业英语第6单元

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1、PART2BIOCHEMICALPHARMACEUTICALSUnit6IsolationofCaffeinefromTeaInthisexperiment，Caffeinewillbeisolatedfromtealeaves.Themajorproblemoftheisolationisthatcaffeinedoesnotoccuraloneintealeaves，butisaccompaniedbyothernaturalsubstancesfromwhichitmustbeseparated.Themajorcompone

2、ntoftealeavesiscellulose，whichisthemajorstructuralmaterialofallplantcells.Celluloseisapolymerofglucose.Sincecelluloseisvirtuallyinsolubleinwater，itpresentsnoproblemsintheisolationprocedure.Caffeine，ontheotherhand，iswatersolubleandisoneofthemajorsubstancesextractedintot

3、hesolutioncalled"tea.”Caffeinecomprisesasmuchas5percentbyweightoftheleafmaterialinteaplants.Tanninsalsodissolveinthehotwaterusedtoextracttealeaves.Thetermtannindoesnotrefertoasinglehomogeneouscompound，oreventosubstanceswhichhavesimilarchemicalstructure.Itreferstoaclass

4、ofcompoundswhichhavecertainpropertiesincommon.Tanninsarephenoliccompoundshavingmolecularweightsbetween500and3000.Theyarewidelyusedto"tan”leather.Theyprecipitatealkaloids'zandproteinsfromaqueoussolutions.Tanninsareusuallydividedintotwoclasses:thosewhichcanbehydrolyzedan

5、dthosewhichcannot.Tanninsofthefirsttypewhicharefoundinteagenerallyyieldglucoseandgallicacidwhentheyarehydrolyzed.Thesetanninsareestersofgallicacidandglucose.Theyrepresentstructuresinwhichsomeofthehydroxylgroupsinglucosehavebeenesterifiedbydigalloylgroups.Thenon-hydroly

6、zabletanninsfoundinteaarecondensationpolymersofcatechin.Thesepolymersarenotuniforminstructure，butcatechinmoleculesareusuallylinkedtogetheratringpositions4and8.Whentanninsareextractedintohotwater，thehydrolyzableonesarepartiallyhydrolyzed，meaningthatfreegallicacidisalsof

7、oundintea.Thetannins，byvirtueoftheirphenolicgroups，andgallicacidbyvirtueofitscarboxylgroups，arebothacidic.Ifcalciumcarbonate，abase，isaddedtoteawater，thecalciumsaltsoftheseacidsareformed.Caffeinecanbeextractedfromthebasicteasolutionwithchloroform，butthecalciumsaltsofgal

8、licacidandthetanninsarenotchloroformsolubleandremainbehindintheaqueoussolution.Thebrowncolorofateasolutionisduetoflav