【作者】 杨荣玲；

【导师】 宗敏华；

【作者基本信息】 华南理工大学 ， 生物化工， 2010， 博士

【摘要】 近年来,随着糖生物学的飞速发展,糖类化合物的多样性和复杂性被阐明,它们众多的生物学功能也逐渐为人们所认识。在药物化学领域,这类化合物愈来愈受到人们的重视。糖苷及糖苷酯类化合物均是重要的候选糖类药物,化学法合成糖苷及糖苷酯存在选择性差、需要保护／脱保护步骤和环境不友好等诸多问题。无疑,反应条件温和、高效、高选择性和绿色的酶法合成是一条极具前景的替代途径。基于上述情况,本论文首先进行了植物来源糖苷酶的筛选,并以其为催化剂,合成了各种烷基及芳基糖苷；研究了在含离子液体介质中糖苷酶的催化特性；随后以熊果苷为模型糖苷化合物,揭示了在不同反应体系中固定化Penicillium expansum旨肪酶(PEL)催化其酰化反应的特性,并在此基础上探讨了固定化PEL在催化熊果苷酰化反应中对酰基供体的识别规律。研究表明,桃仁粗酶具有最高的水解活力,为75.2 U/g；其次是黑布林籽、苹果籽、西梅籽和苦杏仁粗酶,为20.3-34.7 U/g；而西瓜籽、郁李仁和柚子籽粗酶的比活较低,仅为7.1-12.4 U/g。其中黑布林籽、西梅籽、郁李仁、西瓜籽和柚子籽为新的β-葡萄糖苷酶来源。无溶剂体系中,所筛选的几种糖苷酶均能催化逆水解反应合成各种醇糖苷,收率介于14-78%之间。随着烷基醇链长的增加,糖基化产物收率逐渐下降,这可能由于碳链较长的糖基受体空间位阻较大。经NMR鉴定,所合成的糖苷均为β-葡萄糖苷。在含离子液体介质中,离子液体的组成和含量对糖苷酶的催化特性影响显著。研究结果表明,离子液体对糖苷酶催化糖基化反应的影响规律不遵循Hofmeister序列,其对酶反应的影响不能简单地归结于离子效应。以不同芳醇为糖基受体时,离子液体对酶反应的影响规律类似,但影响幅度却存在较大差异。在所研究范围内,在反应体系中添加适量的离子液体均能有效提高糖苷酶催化芳基糖苷的合成收率。在含10%(v/v)BMIm·I的介质中,西梅籽β-糖苷酶催化各种芳基糖苷(熊果苷除外,收率6%)合成的收率介于15-28%之间。尤其值得一提的是,在该反应介质中,酶催化对硝基苯甲醇糖基化反应的平衡收率达15%,是不添加该离子液体介质中的1.56倍。底物识别研究表明,糖基受体的结构对酶促糖基化反应速率和产物收率影响显著。这是由酶活性中心结构、大小、形状以及底物分子空间位阻和羟基亲核性共同决定的。探讨了有机溶剂中利用廉价的固定化脂肪酶PEL催化熊果苷区域选择性丁酰化反应特性。发现该反应的最适反应介质、酶量、底物摩尔比(丁酸乙烯酯／熊果苷)、反应温度分别为脱水四氢呋喃、50 U/mL、7.5和35℃。在上述条件下,反应初速度、熊果苷最大转化率和6’-区域选择性分别为75.1 mM/h、>99%及>99%。酰基供体结构对固定化脂肪酶PEL催化熊果苷酰化反应速度影响显著,然其对反应的区域选择性影响甚微。产物的NMR分析表明,酶促熊果苷酰化反应均发生在6’-羟基上,其区域选择性高达99%以上。在所所研究的酰基供体[10种直链脂肪酸乙烯酯(C2-C18)和6种带取代基或官能团的羧酸乙烯酯]中,固定化PEL催化熊果苷10-十一烯酸酰化活性最高。当酰基供体α-位含有取代基(如甲基或乙基)或含有与羰基共轭的不饱和键(如双键)时,由于空间位阻或共振效应,酶促酰化反应速度显著降低。在固定化PEL催化各种熊果苷芳香酸酯合成中,以苯丙酰化反应最快,反应4 h,转化率达99%。当酰基供体中含有与羰基共轭的不饱键时,酶反应速度降低。当酰基供体的苯环上含有取代基时,由于空间位阻或电子效应,酶促反应速度和转化率急剧下降。所建立的糖苷酰化反应体系具有较好的普适性,基于该体系,成功合成了26个结构新颖的熊果苷酯类衍生物(脂肪酸酯与芳香酸酯)。尤其值得一提的是,在本研究中熊果苷肉桂酸酯的分离产率可达到88%,大大高于前人报道的对应值(28%)。并且,该酶在重复使用至11批次时,其残余活性达63%。以熊果苷香草酰化为模型反应,研究了混合溶剂中固定化PEL催化熊果苷区域选择性酰化反应的特性。在四氢呋喃-异丙醚(20%,v/v)中,该酶可高效、高6’-区域选择性(>99%)地催化熊果苷香草酰化。采用3因素5水平的响应面法探讨了混合体系中酶量、底物摩尔比和反应温度对熊果苷香草酰化反应的影响。其最优酶量、底物摩尔比(香草酸乙烯酯／熊果苷)和反应温度分别为93 U/mL、11和50℃。在上述条件下,反应初速度、熊果苷最大转化率及6’-区域选择性分别为8.2 mM/h、93%及>99%,其中熊果苷最大转化率远高于在纯四氢呋喃体系中的对应值(30%)。当以对羟基肉桂酸乙烯酯、对甲氧基肉桂酸乙烯酯或3,4-二甲氧基肉桂酸乙烯酯为酰基供体时,熊果苷转化率分别由四氢呋喃中的36%、80%和70%提高到97%、99%和99%。此外,与纯四氢呋喃体系相比,混合溶剂中酶的操作稳定性有所提高,重复使用至11批次,其残余活性仍高达71%。研究结果表明利用混合溶剂体系不仅能有效促进酶催化熊果苷芳香酰化反应的发生,而且还能增强酶的操作稳定性。本研究不仅丰富了酶学基础理论知识,而且开辟了一条可用于糖苷及其酯衍生物高效、高选择性合成的新途径。同时,本研究合成了一系列糖苷及其酯类衍生物,为研究糖类化合物的构效关系、筛选糖类新药奠定了物质基础。更多还原

【Abstract】 With the rapid development of glycobiology over the last decade, the diversity and complexity of glycoconjugate compounds have been well elucidated, and their biological functions have been gradually understood. So these compounds are attracting increasing attention in medicinal chemistry. Glycosides and their esters are an important group of candidates of glycoconjugate drugs. The conventional chemical approaches are characterized by low selectivity, the requirement of protection/deprotection, and being environmentally unfriendly. Doubtlessly, green enzymatic routes with mild reaction conditions, high efficiency and high selectivity are a promising alternative. Hence, in the dissertation, a variety of glycosidases of plant origin were screened to catalyze the synthesis of alkyl and aryl glycosides. Besides, the catalytic performance of glycosidase in ionic liquids-containing systems was examined. Then, with the acylation of arbutin as a model reaction, the catalytic performance of immobilized Penicillium expansum lipase (PEL) in various reaction media was examined. At last, the acyl donor recognition of immobilized PEL in the acylation of arbutin was explored.It was found that peach seed meal displayed the highest hydrolytic activity (75.2 U/g) among the seeds tested. The glycosidases from the seeds of black plum, prune, apple and almond exhibited moderate hydrolytic activities (20.3-34.7 U/g), while the activity of glycosidases from the seeds of watermelon, bunge cherry and pomelo was low (7.1-12.4 U/g). To the best of our knowledge, the seeds from black plum, prune, bunge cherry and pomelo as the sources ofβ-glucosidase have not been reported. In solvent-free systems, these glycosidases could catalyze the synthesis of alkyl glycosides via reverse hydrolysis with 14-78% yields. With increasing chain length of alcohols, the enzymatic glucosylation became unfavorable possibly due to the steric hindrance. The products were exclusivelyβ-glycosides as characterized by 13C NMR and 1H NMR.In ionic liquid (IL)-containing media, the effects of the nature and content of ILs on the catalytic performance of glycosidases were remarkable. It was revealed that the effects of ILs on the enzymactic reaction did not follow the Hofmeister series, and it could not be ascribed simply to the ion effect. With different aryl alcohols as glycosyl acceptors, the effects of various ILs on the enzymatic reaction were similar, but they were different with each other in the degree. Among ILs tested, the yields of aryl glucosides were efficiently improved by adding these ILs of moderate amount. In the BMIm·I (10%, v/v)-containing mixtures, prune seed could efficiently catalyze the synthesis of alyl glycosides via reverse hydrolysis with 15-28% yields (except for arbutin with a yield of 6%). It was worth noting that the yield of p-nitrobenzylβ-D-glucopyranoside in this medium was 15%, which is 1.56 times that in IL-free systems. The investigation of enzyme substrate recognition revealed that the structure of glycosyl acceptors exerted significant effects on the enzymatic reaction. It might be attributed to the structure, size and shape of the enzyme active center as well as steric hindrance of the substrates or nucleophilicity of.hydroxyl groups.In organic solvents, the characteristics of the regioselective butanoylation of arbutin catalyzed by inexpensive immobilized PEL were elucidated. The optimum reaction medium, enzyme dosage, molar ratio of vinyl butyrate to arbutin and reaction temperature were anhydrous THF,50 U/mL,7.5 and 35℃, respectively. Under the optimal conditions, the initial rate, the maximum substrate conversion and the 6’-regioselectivity were 75.1 mM/h, >99% and>99%, respectively. The structures of acyl donors showed a great influence on the reaction rate, while little effect on the regioselectivity was observered. The acylation reaction occurred exclusively on 6’-hydroxyl of arbutin (regioselectivity>99%) as characterized by 13C NMR and 1H NMR. Among the acyl donors tested [10 straight-chain fatty acid vinyl esters (C2-C18) and 6 other carboxylic acid vinyl esters bearing different substituents or functional groups], immobilized PEL showed the highest catalytic activity toward the regioselective acylation of arbutin with vinyl 10-undecenoate. Drastic decrease of the initial rate was observed in the acylation of arbutin with acyl donors bearing a substitute (i.e. methyl or ethyl) at the a-position or an unsaturated bond (i.e. C-C double bond) conjugated with the carbonyl group, due to steric hindrance or resonance effect. Among the synthesis of various aromatic acid esters of arbutin, the lipase was the most specific towards vinyl 3-phenylpropionate, with which a substrate conversion of 99% was recorded in 4 h. The reaction rate decreased significantly when the acyl donors had an unsaturated bond (i.e. C-C double bond) conjugated with the carbonyl group. Moreover, the substituents present in the phenyl moiety of the acyl donor exerted a negative impact on the reaction which could be attributed to the unfavorable resonance effect or steric hindrance. The enzymatic acylation route with immobilized PEL proved to be quite applicable and was successfully used for the synthesis of 26 novel ester derivatives of arbutin. It was noteworthy that the isolated yield of arbutin cinnamate was 88%, which is much higher than that (28%) reported previously. In addition, the lipase retained the residual activity of 63% when reused for 11 runs.In co-solvent mixtures, the acylation of arbutin with vanillic acid vinyl ester catalyzed by immobilized PEL was examined. In anhydrous THF-diisopropyl ether (20%, v/v), the lipase showed a high activitiy and an excellent regioselectivity (>99%). The effects of several key variables on the reaction were investigated in detail by response surface methodology of five-level-three-factors. The optimal enzyme dosage, the molar ratio of vinyl vanillic acid to arbutin and the reaction temperature were 93 U/mL,11 and 50℃, respectively, under which the initial reaction rate, the maximum arbutin conversion and the 6’-regioselectivity were 8.2 mM/h,93% and>99%, respectively. The maximum arbutin conversion of 93% was much higher in co-solvent system than that (30%) in THF. And the conversion in the p-hydroxylcinnamoylation,p-methoxylcinnamoylation and 3,4-dimethoxycinnamoylation were 97%,99% and 99%, respectively, which were much higher than those (36%,80% and 70%, respectively) in THF. Besides, the operational stability of the immobilized PEL in co-solvent mixtures was improved, as compared to that in THF. The lipase remained the residual activity of 71% when reused for 11 batches. It was revealed that using co-solvent system not only enhanced enzymatic acylation of arbutin, but also improved the operational stability of the enzyme.This study not only enriches the knowledge of fundamental enzymology, but also provides a novel, selective and efficient route to glucosides and their ester derivatives. Meanwhile, a group of novel glucosides and their ester derivatives have been successfully synthesized, which can be used for the investigation of structure-function of the glycoconjugate compounds and the screening of new glycoconjugate drugs.更多还原