利用X光與中子散射研究生物分子及粒子聚集現象

Using X-ray and Neutron Scattering to Investigate the Aggregation of Biomolecules and Particles

林智敏(Jhih-Min Lin)

指導教授 ： 林滄浪

國立清華大學/原子科學院/工程與系統科學系/博士(2007年)

摘要

β型類澱粉胜肽(β-amyloid peptide, Aβ)是由39到43個氨基酸所組成，其生成方式為類澱粉蛋白前驅蛋白(amyloid precursor protein)經由蛋白酵素切割(proteolytic cleavage)產生。β型類澱粉胜肽在單體(monomer)或寡聚物(oligomer)型態是可溶性的，在高濃度之下，將自我聚集(self-assemble)形成纖維狀型態(fibril)。這些纖維狀的聚集物被認為與阿茲海默症(Alzheimer’s disease)發生有關，而且β型類澱粉胜肽單體和細胞膜的交互作用也可能是發生阿茲海默症重要的因素。目前已經知道不同的兩性分子例如界面活性劑依不同特性會促進或是抑制纖維狀聚集的生長。在本項研究中，我們利用散射法及其他的技術研究β型類澱粉胜肽和界面活性劑以及脂質分子膜的交互作用。我們利用小角度中子散射(SANS)及小角度X光散射法(SAXS)研究由β型類澱粉胜肽和十二烷基硫酸鈉(sodium dodecyl sulfate, SDS)所形成的結構及結構衍生物，分別研究在高於及低於SDS臨界微胞濃度(critical micelle concentration (CMC)，SDS的臨界微胞濃度為8 mM)時的交互作用。利用低於臨界微胞濃度的氘化之SDS添加進0.115 mM之β型類澱粉胜肽水溶液以改變其散射對比，由SANS量測所得之數據顯示部分β型類澱粉胜肽會和SDS形成複合的短棒狀的結構，其半徑為13 Å，長度為49 Å，由2個β型類澱粉胜肽和60個SDS單體組成，且這些短棒複合粒子也會構成碎形的較大群聚體，碎形維度為1.6，相關長度為430 Å，但部分β型類澱粉胜肽仍會聚集形成纖維。對於0.115 mM β型類澱粉胜肽添加20 mM SDS，time-resolved SAXS實驗數據可得知β型類澱粉胜肽會吸附在SDS微胞表面，並形成胜肽和SDS所組成的層殼結構。β型類澱粉胜肽並不會形成纖維結構。同時藉由圓二色光譜可得知在添加6 mM以及20 mM SDS的樣品，β型類澱粉胜肽二級結構均為a螺旋(a-helix)。對於β型類澱粉胜肽嵌入脂質分子膜，我們利用掠角入射小角度散射(GISAXS)研究在不同溼度環境下，β型類澱粉胜肽嵌入矽基板上脂質分子膜的效應，其胜肽與脂質分子重量比為1比10。我們藉由分析在繞射峰附近橫向上的漫散射(diffuse scattering)可以發現β型類澱粉胜肽會形成聚集，以柱狀聚集估算其半徑約為9 Å。當相對溼度超過94%，這些β型類澱粉胜肽聚集會呈有序的排列，間距約為300 Å。我們也進行了β型類澱粉胜肽纖維及單體對棕櫚醯磷脂醯膽鹼(DPPC)液胞(vesicle)作用之研究，我們發現β型類澱粉胜肽纖維的存在會限制脂質雙層膜由膠態(gel phase)到波紋態(ripple phase)之變化。而在添加β型類澱粉胜肽單體，我們由散射圖形可看出發現β型類澱粉胜肽單體進到脂質層，並造成會使脂質多層液胞層與層的排列失去有序的排列，呈現類似單層液胞結構的散射圖形，或是鬆開脂質多層結構。

關鍵字

小角度散射 ； 掠角小角度散射 ； β型類澱粉胜肽 ； 脂質分子膜 ； 棕櫚醯磷脂醯膽鹼 ； 液胞

並列摘要

The β-amyloid peptide (Aβ) is a 39-43 amino acid peptide, fragment-derived from proteolytic cleavage of the large amyloid precursor protein (APP). The monomers or oligomers of Aβ are soluble, but at higher concentrations Aβ will self-assemble into fibrils. The fibrillar aggregates of Aβ are associated to Alzheimer’s disease. The interaction between Aβ monomer and membrane is a key factor to the Alzheimer’s disease. Amphiphilic molecules such as surfactants were known to either promote or reduce the fibril aggregation of Aβ. In this study, we used scattering methods and other techniques to investigate the interaction of Aβ with surfactants and lipid membranes. The structure and structural evolution of the complexes formed by β-Amyloid (Aβ) 1-40 peptides with sodium dodecyl sulfate (SDS) surfactants of a critical micelle concentration (CMC) of 8 mM were investigated by small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) at below the CMC and above the CMC of SDS. With the scattering contrast varied by the deuteration of the 6 mM SDS added in the aqueous solutions of 0.115 mM Aβ peptides, the measured SANS data indicate that the typical fibril aggregation of the Aβ peptides is suppressed by the SDS monomers via the formation of short rod-like Aβ peptide/SDS complexes. The rod-like complexes, characterized by a rod radius of 13 Å, a rod length of 49 Å, and aggregation numbers of two Aβ peptides and sixty SDS monomers, further form fractal-like clusters of a fractal dimension of 1.6 and a correlation length of 430 Å. For the solution of 0.115 mM Aβ peptides added with 20 mM SDS (with SDS micelles), the time-dependent SAXS data measured indicate that the Aβ peptides adsorbed to the surfaces of the SDS micelles, and formed peptide/SDS complexes of a core-shell structure based on the preexisting SDS micelles. Using an ellipsoid-like model of a core-shell structure, we have extracted respectively the semi-major and semi-minor axes of 22 Å and 16 Å of the ellipsoidal core and a shell thickness of ~4 Å for the peptide/SDS micelle complexes. With circular dichroism (CD), we have also shown that either 6 mM SDS monomers or 20 mM SDS with micelles can maintain the secondary structures of the Aβ peptides largely in an a-helix structure. As for the interaction of the Aβ with lipid membrane, the effect of the insertion of Aβ in the 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) multibilayers supported on silicon wafer was studied by using grazing-incident small-angle X-ray scattering at different bilayer hydration levels (changing the relatively humidity) at the weight ratio of peptide to lipid weight ratio of 1 to 10. It was found that the amyloid peptides form clusters in the bilayer and possess in-plane correlation. From analyzing the diffuse scattering around the Bragg peak in the lateral direction, the amyloid peptides are found to form clusters in the bilayer with a radius of about 9 nm. As the relative humidity exceeds about 94 %, the Aβ clusters seem to develop ordered structure with a spacing of about 300 Å. We have also investigated the amyloid peptide fibrils/monomers interacting with DPPC vesicles. In the gel to ripple transition of the lipid bilayers, we found that in the presence of amyloid fibrils the riple phase of lipid bilayers was suppressed. In the presence of amyloid monomers, the lipid multilamellar liposomes were turned into structures similar to unilamellar vesicles or unbinding multilamellar membranes.

並列關鍵字

small angle scattering ； grazing incidence small angle scattering ； β-amyloid peptide ； lipid membrane ； DPPC ； vesicle

參考文獻

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