Dynamical Behavior of Human α-Synuclein Studied by Quasielastic Neutron Scattering

<div><p>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 (<i>DNA</i>), 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.</p></div

Summary of parameters estimated from the fits to the EISF curves.

<p>The parameters calculated using <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0151447#pone.0151447.e002" target="_blank">Eq 2</a>, shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0151447#pone.0151447.g004" target="_blank">Fig 4</a> are shown. The parameters shown are (a) the fraction of frozen atoms (<i>p</i> in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0151447#pone.0151447.e002" target="_blank">Eq 2</a>), and (b) the radius of the confined sphere (<i>a</i> in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0151447#pone.0151447.e002" target="_blank">Eq 2</a>).</p

Examples of quasielastic neutron scattering spectra, S(Q,ω).

<p>The spectra of the samples, the solvent, and the difference spectra between the sample and the solvent, of αSyn in (a) the monomeric state and (b) the fibril state, at Q = 1.225 Å^-1 and at 280 K, are shown. Filled symbols in black and open symbols in blue show the spectra of the sample solutions and those of the solvent, respectively. The spectra of the empty cell were subtracted from these spectra. Filled symbols in red show the difference spectra between the sample and the solvent. (c) Fits using <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0151447#pone.0151447.e001" target="_blank">Eq 1</a> to the difference spectra, which are due to monomeric αSyn molecules in the sample solution. (d) Fits to the difference spectra, which are due to αSyn molecules in the fibril state. In the upper panels, open squares denote the data, thick solid lines in blue denote the total fits, solid lines in green and red denote the narrow and wide Lorentzian functions, corresponding to <i>L</i>_global(Q,ω) and <i>L</i>_local(Q,ω), respectively, thin solid lines in blue show the background, and dashed lines in black show the resolution functions. The lower panels show the residuals of the fits. (e) Reduced-χ² of the fits to the data. Filled squares and open squares are for the values to the data of αSyn in the monomeric state and the fibril state, respectively, at 280 K. Note that similar values of Reduced-χ² were obtained for the data measured at other temperatures.</p

The EISF curves of αSyn in (a) the monomeric state and (b) the fibril state.

<p>Solid lines are the fits with <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0151447#pone.0151447.e002" target="_blank">Eq 2</a>. The error bars are within in symbols where not shown.</p

Ligation-Dependent Picosecond Dynamics in Human Hemoglobin As Revealed by Quasielastic Neutron Scattering

Hemoglobin, the vital O₂ carrier in red blood cells, has long served as a classic example of an allosteric protein. Although high-resolution X-ray structural models are currently available for both the deoxy tense (T) and fully liganded relaxed (R) states of hemoglobin, much less is known about their dynamics, especially on the picosecond to subnanosecond time scales. Here, we investigate the picosecond dynamics of the deoxy and CO forms of human hemoglobin using quasielastic neutron scattering under near physiological conditions in order to extract the dynamics changes upon ligation. From the analysis of the global motions, we found that whereas the apparent diffusion coefficients of the deoxy form can be described by assuming translational and rotational diffusion of a rigid body, those of the CO form need to involve an additional contribution of internal large-scale motions. We also found that the local dynamics in the deoxy and CO forms are very similar in amplitude but are slightly lower in frequency in the former than in the latter. Our results reveal the presence of rapid large-scale motions in hemoglobin and further demonstrate that this internal mobility is governed allosterically by the ligation state of the heme group

Relaxation in a Prototype Ionic Liquid: Influence of Water on the Dynamics

The influence of water on the relaxation of a prototype ionic liquid (IL) C₈mimBF₄ is examined in the IL-rich regime combining quasi-elastic neutron scattering (QENS) and molecular dynamics (MD) simulations. The QENS and MD simulations results for relaxation of IL and the equimolar mixture with water probed by the dynamics of the C₈mim hydrogen atoms in the time range of 2 ps to 1 ns are in excellent agreement. The QENS data show that translational relaxation increases by a factor of 7 on the addition of water, while rotational relaxation involving multiple processes fitted by a KWW function with low β values is speeded up by a factor of 3 on the time scale of QENS measurements. The MD simulations show that the cation diffusion coefficient, inverse viscosity, and ionic conductivity increase on the addition of water, consistent with the very small change in ionicity. The difficulties in obtaining rotational and translational diffusion coefficients from fits to QENS experiments of pure ILs and IL–water mixtures are discussed