中孔洞分子篩內限制空間的化學反應

Abstract

固定化分子催化劑不但可以有效率的將催化劑從反應溶液中分離出來，固定化的催化系統也可以用來改善反應的活性與選擇性。反應選擇性的增加主要是來自於載體擁有規則孔洞排列與限制的空間，反應基質在限制空間中較容易受到活性中心的誘導基團所影響，此時反應物可以擁有較適當的位向進入催化中心進行催化反應。 在本篇論文中，我們合成一雙核銅金屬錯合物並將它負載於分子篩MCM-41和Y沸石中，並利用這模型來模擬自然界的酵素。在這其中，我們以3,5-di-tert-butylcatechol (DTBC)為反應基質，並進行催化反應形成對應的quinone (DTBQ)，用以模擬酵素 catechol oxidases的功能。在這其中，Y沸石的孔洞太小，HPC 錯合物僅能吸附在的孔洞外。EXAFS光譜指出： HPC負載於Al-MCM-41孔洞中時，其Cu•••Cu距離為3.51 Å，這個距離與DTBC分子中的兩個氫氧基團的O•••O距離是相當符合的，這也使得兩氫氧基可以很容易地同時配位到兩個銅金屬中心，進一步DTBC轉移兩個電子到雙銅金屬中心而生成DTBQ。 Al-MCM-41擁有奈米級孔洞，它提供了限制的空間與表面的電荷，防止雙核銅金屬在催化的過程中因為氫氧橋基的移除所造成的分離。 進一步的我們將真實的酵素cyt c 附載於限制空間的中孔洞材料中，這不同孔洞大小、超高水熱穩定性的中孔洞鋁金屬矽氧分子篩可經由沸石奈米晶種作為先驅物自組裝形成。在這其中，MAS-9 樣品顯示出最大的吸附能力，因為它比NaY、MCM-41與MCM-48 樣品擁有較大的孔洞體積和孔洞大小，cyt c 較容易藉由擴散進到大體積的超大孔洞中。藉由光譜的研究顯示出固定化的酵素存在著high spin 與low spin 組態，這顯示出heme Fe(Ⅲ) 上的Met-80配位基已被水所取代，因此heme 上的活性中心溝槽被充分的打開，此時氧化剤與反應基質可以更容易的接近活性中心並增加了反應的催化活性。MCM-41 與 MCM-48樣品其孔洞大小 與cyt c 的分子直徑互相吻合，其顯示出最好的水熱穩定性。MAS-9 過大的孔洞使得 cyt c 容易在孔洞中改變構形，NaY 沸石孔洞比cyt c 分子來得小，cyt c主要吸附於孔洞的外表面，如此酵素直接的暴露在大環境中造成快速的失活。此外，藉由EPR自旋捕捉實驗，我們更可以確認以中孔洞材料固定化的cyt c主要是經由均勻斷裂過氧化氫的O-O 鍵並產生一陽離子蛋白質自由基中間體，其在低溫固態EPR光譜中顯現於g = 2.00峰。藉由一系列的光譜研究分析，吾人提出一個cyt c催化氧化的機制。 不同孔洞大小 (30-300Å)有機官能基化的中孔洞材料也進一步的被合成用來固定化cyt c 。在這當中，我們分別合成含有硫醇、酯基與磷酯基修飾的中孔洞材料，其顯示出以硫醇來固定化cyt c 有較差的活性。低活性主要來自於在固定化的過程中，硫原子會配位到血基質中的Fe(Ⅲ)因此造成酵素被毒化的現象。其中，硫醇基團強的親核性更進一步的破壞活性中心的血基質，進一步的造成鐵離子由環上掉落出來。為了改善硫醇官能基的毒化現象，我們改用酯類官能基修飾的中孔洞材料藉由共價鍵結來固定化酵素，在這當中顯示出固定化後反應活性明顯增加，其主要來自於強共價鍵結可以防止酵素在反應的過程中失去原有的摺疊。 酯類官能基化的中孔洞材料可以進一步的經由水解得到酸基，由於酸性官能基在中性條件下提供充分的負電荷，帶正電荷的酵素可以經由靜電引力被固定化於中孔洞材料中。在這當中顯示出經由酸官能基固定化的酵素比起利用含鋁的中孔洞材料有較高的活性(3-4倍高)，其主要來自於有機官能基化的孔洞材料可以避免酵素分子直接吸附在載體表面造成蛋白質失去原有的摺疊。

The better separability is a good advantage for immobilization of molecular catalysts. The immobilized systems have an improved activity and regioselectivity compared to the homogeneous system. The enhancement of selectivity might be explained by the steric constraints which the support with the uniform channels. The confinement of the substrate in the mesoporous channels could then lead to a larger influence of the directing group on the orientation of the substrate relative to the reactive catalytic center when compared to the homogeneous reaction. Dicupric complex (HPC) can be immobilized in MCM-41 and Y zeolite. We use this system to study the catalytic activities of HPC in the oxidation of 3,5-di-tert-butylcatechol (DTBC) to the corresponding quinone to mimic the functionality of catechol oxidases. HPC complexes can adsorb only on the outside surface of the Y zeolite due to its smaller pore size. The EXAFS spectrum gives 3.51 Å for the Cu--Cu distance in HPC encapsulated in the nanochannels of Al-MCM-41, which is comparable to the O--O distance of the two hydroxyl groups of DTBC, and this made a simultaneous coordination of the diol group to the dicupric center possible. The nanochannels of Al-MCM-41 provide stability, due to confined space and surface charge, which could prevent excessive separation of the dinuclear cupric centers after removal of the hydroxo bridge in the catalytic process. The study demonstrates that HPC encapsulated in the nanochannels of Al-MCM-41 mesoporous materials could be a viable system for a broad range of catalytic oxidation to mimic natural occurring enzymes. Enzyme cyt c can also be immobilized in the nanochannels of mesoporous silica by electrostatic attraction. The amount of cyt c adsorption could be increased by the introduction of aluminum into the framework of pure silica materials. Among these, MAS-9 showed the highest loading capacity due to its large pore size. However, cyt c immobilized in MAS-9 could undergo facile unfolding during hydrothermal treatments. MCM-41-S and MCM-48-S have the pore sizes that match well the size of cyt c. Hence the adsorbed cyt c in these two medium pore size have the highest hydrothermal stability and overall catalytic activity. On the other hand, the pore size of NaY zeolite is so small that cyt c is mostly adsorbed only on the outer surface and loses its enzymatic activity rapidly. The improved stability and high catalytic activity of cyt c immobilized in mesoporous silica are attributed to the electrostatic attraction between the pore surface and cyt c and the confinement provided by nanochannels. We further observed that cyt c immobilized in mesoporous exists in both high and low spin states. The high spin state arises from the replacement of Met-80 ligands of heme Fe (III) by water or silanol group on silica surface, which could open up the heme groove for easy access of oxidants and substrates to iron center and facilitate the catalytic activity. By the study of the EPR spin trapping experiments, we showed that cyt c catalyzes a homolytic cleavage of the O-O bond of hydroperoxide and generates a protein cation radical. Possible mechanisms for MPS-cyt c catalytic oxidation of hydroperoxides and PAHs are proposed based on the spectroscopic characterizations. Functionalised mesoporous silica which has its pore size in the range of 30-300 Å has been prepared and used for immobilization of the enzyme cyt c. Thio, carboxyl ester and phosphatyl ester functional groups were attached in the nanochannels of mesoporous silica surface by organo silane co-condensation with silanol groups. Coming from the the coordination of sulfur to Heme Fe(Ⅲ) center, the immobilization of cyt c in the thio-functionalised surface showed low activity . The strong nucleophiles of thio groups will cause the destruction of active site, and Fe(Ⅲ) center will leach from the Heme group. The immobilization of cyt c in the ester-functionalized mesoporous silica showed the increasing activity of pyrene oxidation. The high activity primarily came from the formation of good covalent bond. The strong covalent bond will prevent cyt c from unfolding in the catalytic process. The hydrolysis of ester bonds produced the corresponding carboxylic and phosphatic acid. The acidic groups provide the negative charge that could immobilize cyt c by the electrostatic attraction. We further observed that the immobilization of cyt c in the acidic functionalized mesoporous have 3-4 fold increase in activity than aluminosilicate as a support. The high activity comes from the support with organo silane in the surface that could prevent cyt c from unfolding by exposing the protein skeleton in the surface of silica.