制藥工程專業(yè)英語(yǔ)-第6單元-逐句詳解

PART2BIOCHEMICALPHARMACEUTICALS生化制藥Unit6IsolationofCaffeinefromTea從茶葉中分離咖啡因Inthisexperiment,Caffeinewillbeisolatedfromtealeaves.在這個(gè)實(shí)驗(yàn)中，咖啡因會(huì)從茶葉中分離出來(lái)。Themajorproblemoftheisolationisthatcaffeinedoesnotoccuraloneintealeaves,butisaccompaniedbyothernaturalsubstancesfromwhichitmustbeseparated.分離過(guò)程中的主要問(wèn)題就是，茶葉中不僅僅只有咖啡因，還伴隨著需要被分離除去的其他天然物質(zhì)。Themajorcomponentoftealeavesiscellulose，whichisthemajorstructuralmaterialofallplantcells.茶葉中的主要成分是所有植物細(xì)胞中的主要結(jié)構(gòu)材料——纖維素。Celluloseisapolymerofglucose.纖維素是葡萄糖的聚合物。Sincecelluloseisvirtuallyinsolubleinwater,itpresentsnoproblemsintheisolationprocedure.然而纖維素實(shí)際上是不溶于水的，所以在隔離的程序上不用考慮去除纖維素。Caffeine,ontheotherhand,iswatersolubleandisoneofthemajorsubstancesextractedintothesolutioncalled"tea”.另一方面，咖啡因是水溶性的，也是萃取到溶液（也就是茶）里的主要物質(zhì)之一。Caffeinecomprisesasmuchas5percentbyweightoftheleafmaterialinteaplants.咖啡因包含了茶樹(shù)葉片材料總重的5%之多。Tanninsalsodissolveinthehotwaterusedtoextracttealeaves.丹寧（鞣酸）也溶解于用于萃取茶葉的熱水中。Thetermtannindoesnotrefertoasinglehomogeneouscompound,oreventosubstanceswhichhavesimilarchemicalstructure.丹寧一詞不是指單個(gè)均一的化合物，也不是具有相似化學(xué)結(jié)構(gòu)的物質(zhì)。Itreferstoaclassofcompoundswhichhavecertainpropertiesincommon.它是指某些具有共同屬性的化合物的種類。Tanninsarephenoliccompoundshavingmolecularweightsbetween500and3000.丹寧是一類分子質(zhì)量在500到3000的含酚的化合物。Theyarewidelyusedto"tan”leather.它們被廣泛用于鞣革。Theyprecipitatealkaloidsandproteinsfromaqueoussolutions.它們從水溶液中沉淀出生物堿和蛋白質(zhì)Tanninsareusuallydividedintotwoclasses:thosewhichcanbehydrolyzedandthosewhichcannot.丹寧通常被分為兩類：可水解和不可水解。Tanninsofthefirsttypewhicharefoundinteagenerallyyieldglucoseandgallicacidwhentheyarehydrolyzed.在茶中，第一類丹寧水解后通常產(chǎn)生葡萄糖和五倍子酸。Thesetanninsareestersofgallicacidandglucose.這些丹寧是五倍子酸和葡萄糖的酯類。Theyrepresentstructuresinwhichsomeofthehydroxylgroupsinglucosehavebeenesterifiedbydigalloylgroups.它們代表著葡萄糖中的一些羥基被酯化為鞣?；慕Y(jié)構(gòu)。Thenon-hydrolyzabletanninsfoundinteaarecondensationpolymersofcatechin.茶中的不可水解的丹寧是兒茶酚的縮合聚合物。Thesepolymersarenotuniforminstructure,butcatechinmoleculesareusuallylinkedtogetheratringpositions4and8.這些聚合物在結(jié)構(gòu)上是不一致的，但兒茶酚分子通常在環(huán)的4號(hào)和8號(hào)位是相連的。Whentanninsareextractedintohotwater,thehydrolyzableonesarepartiallyhydrolyzed,meaningthatfreegallicacidisalsofoundintea.當(dāng)?shù)幈惠腿〉綗崴?，可水解的部分水解掉了，意味著茶中還有多余的五倍子酸。Thetannins,byvirtueoftheirphenolicgroups,andgallicacidbyvirtueofitscarboxylgroups,arebothacidic.由于丹寧中含有酚，五倍子酸中含有羧基，所以它們都是酸性的。Ifcalciumcarbonate,abase,isaddedtoteawater,thecalciumsaltsoftheseacidsareformed.如果茶水中加入碳酸鈣，這些酸的鈣鹽就會(huì)形成。Caffeinecanbeextractedfromthebasicteasolutionwithchloroform,butthecalciumsaltsofgallicacidandthetanninsarenotchloroformsolubleandremainbehindintheaqueoussolution.咖啡因可以被茶的氯仿溶液萃取，但五倍子酸和丹寧的鈣鹽是不溶于氯仿的，所以它們會(huì)留在水溶液中。Thebrowncolorofateasolutionisduetoflavonoidpigmentsandchlorophylls,aswellastheirrespectiveoxidationproducts.茶水呈現(xiàn)出褐色是由于黃酮類色素和葉綠素，以及它們各自的氧化物。Althoughchlorophyllsaresomewhatchloroformsoluble,mostoftheothersubstancesinteaarenot.雖然葉綠素是微溶于氯仿的，但茶葉中的其他大多數(shù)物質(zhì)不是如此。Thus,thechloroformextractionofthebasicteasolutionremovesnearlypurecaffeine.因此，堿性茶水的氯仿提取去除了幾乎無(wú)雜質(zhì)的咖啡因。Thechloroformiseasilyremovedbydistillation(by61'C)toleavethecrudecaffeine.氯仿在61℃蒸餾中易被去除，從而分離出咖啡因的粗產(chǎn)物。Thecaffeinemaybepurifiedbyrecrystallizationorbysublimation.咖啡因可以通過(guò)結(jié)晶或升華來(lái)純化。Inasecondpartofthisexperiment,Caffeinewillbeconvertedtoaderivative.本次實(shí)驗(yàn)的第二個(gè)部分，咖啡因會(huì)被轉(zhuǎn)化為衍生物。Aderivativeofacompoundisasecondcompound,ofknownmeltingpoint,formedfromtheoriginalcompoundbyasimplechemical