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α-synuclein (αSyn) is a protein consisting of 140 amino acid residues and is abundant in the presynaptic nerve terminals in the brain. Although its precise function is unknown, the filamentous aggregates (amyloid fibrils) of αSyn have been shown to be involved in the pathogenesis of Parkinson's disease, which is a progressive neurodegenerative disorder. To understand the pathogenesis mechanism of this disease, the mechanism of the amyloid fibril formation of αSyn must be elucidated. Purified αSyn from bacterial expression is monomeric but intrinsically disordered in solution and forms amyloid fibrils under various conditions. As a first step toward elucidating the mechanism of the fibril formation of αSyn, we investigated dynamical behavior of the purified αSyn in the monomeric state and the fibril state using quasielastic neutron scattering (QENS). We prepared the solution sample of 9.5 mg/ml purified αSyn, and that of 46 mg/ml αSyn in the fibril state, both at pD 7.4 in D2O. The QENS experiments on these samples were performed using the near-backscattering spectrometer, BL02 (DNA), at the Materials and Life Science Facility at the Japan Accelerator Research Complex, Japan. Analysis of the QENS spectra obtained shows that diffusive global motions are observed in the monomeric state but largely suppressed in the fibril state. However, the amplitude of the side chain motion is shown to be larger in the fibril state than in the monomeric state. This implies that significant solvent space exists within the fibrils, which is attributed to the αSyn molecules within the fibrils having a distribution of conformations. The larger amplitude of the side chain motion in the fibril state than in the monomeric state implies that the fibril state is entropically favorable.

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ABSTRACT:2-Dimensional X-ray and neutron diffraction patterns have been successfully measured for deuterated and hydrogenated polyoxymethylene (POM) samples obtained by -ray induced solid-state polymerization reaction. More than 700 reflections were collected from the X-ray diffraction data at À150C by utilizing a high-energy synchrotron X-ray beam at SPring-8, Japan, from which the crystal structure of POM has been refined thoroughly including the extraction of hydrogen atomic positions as clearly seen in the difference Fourier synthesis map. As the first trial the nonuniform (9/5) helical model was analyzed with the reliability factor (R factor) 6.9%. The structural analysis was made also using the X-ray reflections of about 400 observed at room temperature (R 8.8%), and the thermal parameters of constituent atoms were compared between the low and high temperatures to discuss the librational thermal motion of the chains. The 2-dimensional neutron diffraction data, collected for the deuterated and hydrogenated POM samples using an imaging plate system specifically built-up for neutron scattering experiment, have allowed us to pick up the D and H atomic positions clearly in the Fourier synthesis maps. Another possible model, (29/16) helix, which was proposed by several researchers, has been also investigated on the basis of the X-ray diffraction data at À150 C. The direct method succeeded in extracting this (29/16) model straightforwardly. The R factor was 8.6%, essentially the same as that of (9/5) helical model. This means that the comparison of the diffraction intensity between the data collected from the full-rotation X-ray diffraction pattern and the intensity calculated for both the (9/5) and (29/16) models cannot be used for the unique determination of the superiority of the model, (9/5) or (29/16) helix. However, we have found the existence of 001 and 002 reflections which give the longer repeating period 55.7 Å . Besides there observed a series of meridional 00l reflections forbidden for (9/5) helical model. These additional evidences support the nonuniform (29/16) helical model as the most plausible chain conformation of POM crystal.

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