M. Manno | Searcharxiv

arXiv2009

Bulk Spin Polarization of Co$_{(1-x)}$Fe$_x$S$_2$

We report on a new method to determine the degree of bulk spin polarization in single crystal Co$_{(1-x)}$Fe$_x$S$_2$ by modeling magnetic Compton scattering with {\it ab initio} calculations. Spin-dependent Compton profiles were measured for CoS$_2$ and Co$_{0.9}$Fe$_{0.1}$S$_2$. The {\it ab initio} calculations were then refined by rigidly shifting the bands to provide the best fit between the calculated and experimental directional profiles for each sample. The bulk spin polarizations, $P$, corresponding to the spin-polarized density of states at the Fermi level, were then extracted from the {\it refined} calculations. The values were found to be $P=-72 \pm 6 %$ and $P=18 \pm 7%$ for CoS$_2$ and Co$_{0.9}$Fe$_{0.1}$S$_2$ respectively. Furthermore, determinations of $P$ weighted by the Fermi velocity ($v_F$ or $v_F^2$) were obtained, permitting a rigorous comparison with other experimental data and highlighting the experimental dependence of $P$ on $v_F$.

cond-mat.str-el↗

arXiv2005

Early events in insulin fibrillization studied by time-lapse atomic force microscopy

The importance of understanding the mechanism of protein aggregation into insoluble amyloid fibrils relies not only on its medical consequences, but also on its more basic properties of self--organization. The discovery that a large number of uncorrelated proteins can form, under proper conditions, structurally similar fibrils has suggested that the underlying mechanism is a general feature of polypeptide chains. In the present work, we address the early events preceeding amyloid fibril formation in solutions of zinc--free human insulin incubated at low pH and high temperature. Aside from being a easy--to--handle model for protein fibrillation, subcutaneous aggregation of insulin after injection is a nuisance which affects patients with diabetes. Here, we show by time--lapse atomic force microscopy (AFM) that a steady-state distribution of protein oligomers with an exponential tail is reached within few minutes after heating. This metastable phase lasts for few hours until aggregation into fibrils suddenly occurs. A theoretical explanation of the oligomer pre--fibrillar distribution is given in terms of a simple coagulation--evaporation kinetic model, in which concentration plays the role of a critical parameter. Due to high resolution and sensitivity of AFM technique, the observation of a long-lasting latency time should be considered an actual feature of the aggregation process, and not simply ascribed to instrumental inefficency. These experimental facts, along with the kinetic model used, claim for a critical role of thermal concentration fluctuations in the process of fibril nucleation.

q-bio.BM↗