Dual Behavior of Antiferromagnetic Uncompensated Spins in NiFe/IrMn Exchange Biased Bilayers

We present a comprehensive study of the exchange bias effect in a model system. Through numerical analysis of the exchange bias and coercive fields as a function of the antiferromagnetic layer thickness we deduce the absolute value of the averaged anisotropy constant of the antiferromagnet. We show that the anisotropy of IrMn exhibits a finite size effect as a function of thickness. The interfacial spin disorder involved in the data analysis is further supported by the observation of the dual behavior of the interfacial uncompensated spins. Utilizing soft x-ray resonant magnetic reflectometry we have observed that the antiferromagnetic uncompensated spins are dominantly frozen with nearly no rotating spins due to the chemical intermixing, which correlates to the inferred mechanism for the exchange bias.Comment: 4 pages, 3 figure

A label-free, quantitative assay of amyloid fibril growth based on intrinsic fluorescence.

Kinetic assay of seeded growth: The graph shows the variation in intrinsic fluorescence intensity of amyloid fibrils. Fluorescence increases during the seeded aggregation of α-synuclein seeds with α-synuclein monomeric protein (blue curve) but not when α-synuclein seeds are incubated with β-synuclein monomeric protein (black curve), thus showing that no seeded growth occurred in this case

Origin of the reduced exchange bias in epitaxial FeNi(111)/CoO(111) bilayer

We have employed Soft and Hard X-ray Resonant Magnetic Scattering and Polarised Neutron Diffraction to study the magnetic interface and the bulk antiferromagnetic domain state of the archetypal epitaxial Ni$_{81}$Fe$_{19}$(111)/CoO(111) exchange biased bilayer. The combination of these scattering tools provides unprecedented detailed insights into the still incomplete understanding of some key manifestations of the exchange bias effect. We show that the several orders of magnitude difference between the expected and measured value of exchange bias field is caused by an almost anisotropic in-plane orientation of antiferromagnetic domains. Irreversible changes of their configuration lead to a training effect. This is directly seen as a change in the magnetic half order Bragg peaks after magnetization reversal. A 30 nm size of antiferromagnetic domains is extracted from the width the (1/2 1/2 1/2) antiferromagnetic magnetic peak measured both by neutron and x-ray scattering. A reduced blocking temperature as compared to the measured antiferromagnetic ordering temperature clearly corresponds to the blocking of antiferromagnetic domains. Moreover, an excellent correlation between the size of the antiferromagnetic domains, exchange bias field and frozen-in spin ratio is found, providing a comprehensive understanding of the origin of exchange bias in epitaxial systems.Comment: 8 pages, 5 figures, submitte

Direct observation of heterogeneous amyloid fibril growth kinetics via two-color super-resolution microscopy.

The self-assembly of normally soluble proteins into fibrillar amyloid structures is associated with a range of neurodegenerative disorders, such as Parkinson's and Alzheimer's diseases. In the present study, we show that specific events in the kinetics of the complex, multistep aggregation process of one such protein, α-synuclein, whose aggregation is a characteristic hallmark of Parkinson's disease, can be followed at the molecular level using optical super-resolution microscopy. We have explored in particular the elongation of preformed α-synuclein fibrils; using two-color single-molecule localization microscopy we are able to provide conclusive evidence that the elongation proceeds from both ends of the fibril seeds. Furthermore, the technique reveals a large heterogeneity in the growth rates of individual fibrils; some fibrils exhibit no detectable growth, whereas others extend to more than ten times their original length within hours. These large variations in the growth kinetics can be attributed to fibril structural polymorphism. Our technique offers new capabilities in the study of amyloid growth dynamics at the molecular level and is readily translated to the study of the self-assembly of other nanostructures

Long range antiferromagnetic order of formally nonmagnetic Eu3 Van Vleck ions observed in multiferroic Eu1 xYxMnO3

We report on resonant magnetic x ray scattering and absorption spectroscopy studies of exchange coupled antiferromagnetic ordering of Eu3 magnetic moments in multiferroic Eu1 amp; 8722;xYxMnO3 in the absence of an external magnetic field. The observed resonant spectrum is characteristic of a magnetically ordered 7F1 state that mirrors the Mn magnetic ordering, due to exchange coupling between the Eu 4f and Mn 3d spins. Here, we observe long range magnetic order generated by exchange coupling of magnetic moments of formally nonmagnetic Van Vleck ions, which is a step further towards the realization of exotic phases induced by exchange coupling in systems entirely composed of non magnetic ion

Nanoscopic insights into seeding mechanisms and toxicity of α-synuclein species in neurons.

New strategies for visualizing self-assembly processes at the nanoscale give deep insights into the molecular origins of disease. An example is the self-assembly of misfolded proteins into amyloid fibrils, which is related to a range of neurodegenerative disorders, such as Parkinson's and Alzheimer's diseases. Here, we probe the links between the mechanism of α-synuclein (AS) aggregation and its associated toxicity by using optical nanoscopy directly in a neuronal cell culture model of Parkinson's disease. Using superresolution microscopy, we show that protein fibrils are taken up by neuronal cells and act as prion-like seeds for elongation reactions that both consume endogenous AS and suppress its de novo aggregation. When AS is internalized in its monomeric form, however, it nucleates and triggers the aggregation of endogenous AS, leading to apoptosis, although there are no detectable cross-reactions between externally added and endogenous protein species. Monomer-induced apoptosis can be reduced by pretreatment with seed fibrils, suggesting that partial consumption of the externally added or excess soluble AS can be significantly neuroprotective.We thank Dr Q. Jeng and Dr A. Stephens for technical assistance and Dr J. Skepper for TEM imaging. This work was funded by grants from the U.K. Medical Research Council (MR/K015850/1 and MR/K02292X/1), Alzheimer’s Research UK (ARUK-EG2012A-1), U.K. Engineering and Physical Sciences Research Council (EPSRC) (EP/H018301/1) and the Wellcome Trust (089703/Z/09/Z). D.P. wishes to acknowledge support from the Swiss National Science Foundation and the Wellcome Trust through personal fellowships. A.K.B thanks Magdalene College, Cambridge and the Leverhulme Trust for support.This is the author accepted manuscript. The final version is available from the National Academy of Sciences via http://dx.doi.org/10.1073/pnas.1516546113

Direct observations of amyloid β self-assembly in live cells provide insights into differences in the kinetics of Aβ(1-40) and Aβ(1-42) aggregation.

Insight into how amyloid β (Aβ) aggregation occurs in vivo is vital for understanding the molecular pathways that underlie Alzheimer's disease and requires new techniques that provide detailed kinetic and mechanistic information. Using noninvasive fluorescence lifetime recordings, we imaged the formation of Aβ(1-40) and Aβ(1-42) aggregates in live cells. For both peptides, the cellular uptake via endocytosis is rapid and spontaneous. They are then retained in lysosomes, where their accumulation leads to aggregation. The kinetics of Aβ(1-42) aggregation are considerably faster than those of Aβ(1-40) and, unlike those of the latter peptide, show no detectable lag phase. We used superresolution fluorescence imaging to examine the resulting aggregates and could observe compact amyloid structures, likely because of spatial confinement within cellular compartments. Taken together, these findings provide clues as to how Aβ aggregation may occur within neurons

Export of functional Streptomyces coelicolor alditol oxidase to the periplasm or cell surface of Escherichia coli and its application in whole-cell biocatalysis

Streptomyces coelicolor A3(2) alditol oxidase (AldO) is a soluble monomeric flavoprotein in which the flavin cofactor is covalently linked to the polypeptide chain. AldO displays high reactivity towards different polyols such as xylitol and sorbitol. These characteristics make AldO industrially relevant, but full biotechnological exploitation of this enzyme is at present restricted by laborious and costly purification steps. To eliminate the need for enzyme purification, this study describes a whole-cell AldO biocatalyst system. To this end, we have directed AldO to the periplasm or cell surface of Escherichia coli. For periplasmic export, AldO was fused to endogenous E. coli signal sequences known to direct their passenger proteins into the SecB, signal recognition particle (SRP), or Twin-arginine translocation (Tat) pathway. In addition, AldO was fused to an ice nucleation protein (INP)-based anchoring motif for surface display. The results show that Tat-exported AldO and INP-surface-displayed AldO are active. The Tat-based system was successfully employed in converting xylitol by whole cells, whereas the use of the INP-based system was most likely restricted by lipopolysaccharide LPS in wild-type cells. It is anticipated that these whole-cell systems will be a valuable tool for further biological and industrial exploitation of AldO and other cofactor-containing enzymes.

Oxygen Hole Character and Lateral Homogeneity in PrNiO2 delta Thin Films

Using x ray absorption spectroscopy with lateral resolution from the submillimeter to submicrometer range, we investigate the homogeneity, the chemical composition, and the nickel 3d oxygen 2p charge transfer in topotactically reduced epitaxial PrNiO2 amp; 948; thin films. To this end, we use x ray absorption spectroscopy in a standard experimental setup and in a soft x ray microscope to probe the element and spatially resolved electronic structure modifications through changes of the nickel 2p and oxygen 1s absorption spectrum upon soft chemistry reduction. We find that the reduction process is laterally homogeneous across a partially reduced PrNiO2 amp; 948; thin film sample for length scales down to 50 n