Design of Cellulose Dissolving Ionic Liquids Inspired by Nature
Design of Cellulose Dissolving Ionic Liquids Inspired by Nature
复制标题
DOI：
10.1002/cssc.201100427
复制
发表时间：
2012-02-13
期刊：
影响因子：
8.4
通讯作者：
Itoh, Toshiyuki
中科院分区：
文献类型：
--
作者：
Ohira, Kazutaka;Abe, Yoshikazu;Itoh, Toshiyuki
纤维素是生产生物复合材料和生物燃料醇的重要可再生资源。[1]然而，由于它由线性葡萄糖聚合物链组成，形成非常紧密的氢键超分子结构，纤维素抵抗酶降解。因此，人们越来越关注开发一种方法来改变纤维素结构，使其易于生物降解。[2]Rogers及其同事在2002年首次报道了使用离子液体（IL）技术的突破：将纤维素溶解在离子液体1-丁基-3-甲基咪唑氯化物（[C4 mim] Cl）中，从IL溶液中再生的纤维素结晶较少，易于被纤维素酶消化。[3]从那时起，已经进行了广泛的研究，以开发具有溶解纤维素和降低其结晶度的能力的IL。[2]大多数报道的离子液体是咪唑基或烷氧基烷基取代的铵盐，其具有氯离子、甲酸根、乙酸根、丙酸根或磷酸根作为抗衡阴离子。[2，4]当从离子液体溶液再生的纤维素由于结晶度降低而经受酶促反应时，实现了纤维素酶介导的水解的增加的反应速率。[5]然而，尚未建立溶解纤维素的离子液体的合理设计。[2d相反，固体纤维素的水解可以通过使用纤维素酶，例如内切葡聚糖酶（EG）和纤维二糖水解酶（CBH）来实现。[7]前者可以水解纤维素微纤丝内无定形区域中纤维素聚合物中的内部β-1，4-糖苷键，而后者可以作用于纤维素聚合物链的自由端。两种类型的纤维素酶都具有纤维素结合模块，其促进它们吸附到结晶纤维素上，使催化结构域物理上接近它们的作用位点。[7，8]受此启发，我们假设设计IL的关键点可能是利用纤维素酶蛋白的信息来增加IL与纤维素之间的亲和力。我们研究了几种纤维素酶的蛋白质序列，使底物结合裂缝，并确定纤维素酶的葡萄糖基结合位点经常形成在暴露的表面上的芳香族侧链。[7]构成封闭的纤维素结合通道的四个结合位点中的三个含有里氏木霉Cel 6 A（CBH II）的色氨酸残基侧链。[8]我们预期由氨基酸制成的离子液体可能对纤维素的某一部分具有亲和力，这可能导致纤维素在液体中溶解。Ohno和同事制备了以氨基酸部分作为阴离子的离子液体。[9]基于这一想法，我们最初通过使用微晶纤维素（Avicel）作为模型化合物来测试纤维素在1-丁基-3-甲基咪唑色氨酸（[C4 mim][Trp]）中的溶解。[10]然而，纤维素不溶于该IL中。然后，我们用铵、磷或吡啶阳离子制备色氨酸盐，并发现N，N-二乙基-N-（2-甲氧基乙基）-N-甲基色氨酸铵（[N221 ME][Trp]）在100 ℃下溶解纤维素（5wt%）（表1，条目1）。受该结果的鼓舞，我们制备了具有[N221 ME]阳离子的各种类型的基于氨基酸的IL，并评价了它们相对于模型纤维素的溶解特性（表1）。
Cellulose is an attractive and important renewable resource for the production of biocomposites and biofuel alcohols.[1] However, as it consists of linear glucose polymer chains that form a very tight hydrogen-bonded supramolecular structure, cellulose resists enzymatic degradation. Therefore, there has been a growing interest in the development of a means to modify the cellulose structure in such a way that it can be easily biodegraded.[2] Rogers and coworkers first reported a breakthrough by using ionic-liquid (IL) technology in 2002: Cellulose dissolved in an ionic liquid, 1-butyl-3-methylimidazolium chloride ([C4mim] Cl), and the cellulose regenerated from the IL solution was less crystalline and easily digested by a cellulase.[3] Since then, extensive investigations have been performed to develop an IL that possesses the capability to dissolve cellulose and reduce its crystallinity.[2] Most reported ILs are imidazoliumbased or alkyloxyalkyl-substituted ammonium salts with chloride, formate, acetate, propionate, or phosphate as counter anion.[2, 4] An increased reaction rate of cellulase-mediated hydrolysis was realized when cellulose regenerated from the ionic liquid solution was subjected to the enzymatic reaction because of reduced crystallinity.[5] However, no rational design of ILs dissolving cellulose has yet been established.[2d, 6] Conversely, hydrolysis of solid celluloses can be achieved by using cellulases, such as endoglucanases (EGs) and cellobiohydrolases (CBHs).[7] The former can hydrolyze internal β-1, 4-glycosidic bonds in a cellulose polymer in the amorphous regions within the cellulose micro-fibril, and the latter can act on the free ends of cellulose polymer chains. Both types of cellulases have cellulose-binding modules that facilitate their adsorption onto crystalline cellulose, bringing the catalytic domains physically close to their site of action.[7, 8] Inspired by this, we hypothesized that the key point of designing ILs might be to increase the affinity between ILs and cellulose by using information of the cellulase protein. We examined the protein sequences of several cellulases that make up the substrate-binding cleft and determined that glucosyl-binding sites of cellulases were frequently formed on the exposed surface of aromatic side-chains.[7] Three of the four binding sites making up the enclosed cellulose-binding tunnel contain the tryptophan residue side-chains of Trichoderma reesei Cel6 A (CBH II).[8] We expected that ILs made from amino acids might have an affinity toward a certain part of cellulose and that this may cause dissolution of the cellulose in the liquids.Ohno and coworkers prepared ILs with amino-acid moieties as anions.[9] Based on this idea, we initially tested the dissolution of cellulose in 1-butyl-3-methylimidazolium tryptophan ([C4mim][Trp]) by using microcrystalline cellulose (Avicel) as a model compound.[10] However, the cellulose did not dissolve in this IL. We then prepared tryptophan salts with ammonium, phosphonium, or pyrridinium cations, and found that N, N-diethyl-N-(2-methoxyethyl)-N-methylammonium tryptophan ([N221ME][Trp]) dissolved cellulose (5wt%) at 1008C (Table 1, Entry1). Encouraged by the result, we prepared various types of amino acid-based ILs with the [N221ME] cation, and evaluated their dissolution properties against the model cellulose (Table 1).