Molecular Structure Of Amyloid Fibrils Research Articles

Structural polymorphism and cytotoxicity of brain-derived β-amyloid extracts.

To date, more than 37 amyloidogenic proteins have been found to form toxic aggregates that are implicated in the progression of numerous debilitating protein misfolding diseases including Alzheimer's disease (AD). Extensive literature highlights the role of β-amyloid (Aβ) aggregates in causing excessive neuronal cell loss in the brains of AD patients. In fact, major advances in our understanding of Aβ aggregation process, including kinetics, toxicity, and structures of fibrillar aggregates have been revealed by examining in vitro preparations of synthetic Aβ peptides. However, ongoing research shows that brain-derived Aβ aggregates have specific characteristics that distinguish them from in vitro prepared species. Notably, the molecular structures of amyloid fibrils grown in the human brain were found to be markedly different than synthetic Aβ fibrils. In addition, recent findings report the existence of heterogeneous Aβ proteoforms in AD brain tissue in contrast to synthetically produced full-length aggregates. Despite their high relevance to AD progression, brain-derived Aβ species are less well-characterized compared with synthetic aggregates. The aim of this review is to provide an overview of the literature on brain-derived Aβ aggregates with particular focus on recent studies that report their structures as well as pathological roles in AD progression. The main motivation of this review is to highlight the importance of utilizing brain-derived amyloids for characterizing the structural and toxic effects of amyloid species. With this knowledge, brain-derived aggregates can be adopted to identify more relevant drug targets and validate potent aggregation inhibitors toward designing highly effective therapeutic strategies against AD.

Multiscale Modeling of Amyloid Fibrils Formed by Aggregating Peptides Derived from the Amyloidogenic Fragment of the A-Chain of Insulin.

Computational prediction of molecular structures of amyloid fibrils remains an exceedingly challenging task. In this work, we propose a multi-scale modeling procedure for the structure prediction of amyloid fibrils formed by the association of ACC1-13 aggregation-prone peptides derived from the N-terminal region of insulin’s A-chain. First, a large number of protofilament models composed of five copies of interacting ACC1-13 peptides were predicted by application of CABS-dock coarse-grained (CG) docking simulations. Next, the models were reconstructed to all-atom (AA) representations and refined during molecular dynamics (MD) simulations in explicit solvent. The top-scored protofilament models, selected using symmetry criteria, were used for the assembly of long fibril structures. Finally, the amyloid fibril models resulting from the AA MD simulations were compared with atomic force microscopy (AFM) imaging experimental data. The obtained results indicate that the proposed multi-scale modeling procedure is capable of predicting protofilaments with high accuracy and may be applied for structure prediction and analysis of other amyloid fibrils.

Structural model of amyloid fibrils for amyloidogenic peptide from Bgl2p–glucantransferase of S. cerevisiae cell wall and its modifying analog. New morphology of amyloid fibrils

We performed a comparative study of the process of amyloid formation by short homologous peptides with a substitution of aspartate for glutamate in position 2 – VDSWNVLVAG (AspNB) and VESWNVLVAG (GluNB) – with unblocked termini. Peptide AspNB (residues 166–175) corresponded to the predicted amyloidogenic region of the protein glucantransferase Bgl2 from the Saccharomyces cerevisiae cell wall. The process of amyloid formation was monitored by fluorescence spectroscopy (FS), electron microscopy (EM), tandem mass spectrometry (TMS), and X-ray diffraction (XD) methods. The experimental study at pH3.0 revealed formation of amyloid fibrils with similar morphology for both peptides. Moreover, we found that the morphology of fibrils made of untreated ammonia peptide is not mentioned in the literature. This morphology resembles snakes lying side by side in the form of a wave without intertwining. Irrespective of the way of the peptide preparation, the rate of fibril formation is higher for AspNB than for GluNB. However, preliminary treatment with ammonia highly affected fibril morphology especially for AspNB. Such treatment allowed us to obtain a lag period during the process of amyloid formation. It showed that the process was nucleation-dependent. With or without treatment, amyloid fibrils consisted of ring-like oligomers with the diameter of about 6nm packed either directly ring-to-ring or ring-on-ring with a slight shift. We also proposed the molecular structure of amyloid fibrils for two studied peptides.

Amyloid Beta-Protein Fibrils from Human Alzheimer's Brain Tissue and from Mouse Models of Alzheimer's Differ in Structures

Electron microscopy (EM) and two dimensional infrared spectroscopy have shown that the molecular structure of amyloid fibrils prepared in vitro from synthetic 40-residue amyloid β-protein (Aβ40), are not uniquely determined by the amino acid sequence of Aβ40. Instead, fibril structure can vary depending on growth conditions. Therefore, the suitability of fibrils from mouse models of Alzheimer's disease (AD) as models of fibrils that form in human AD is uncertain. In this study, fibrils extracted from brain tissue of three different human AD patients were used to seed the growth of synthetic Aβ40 fibrils. Because amyloid fibril structures self-propagate via seeded growth, the structure of seeded Aβ40 fibrils is likely to reflect the structure of fibrils formed in AD brain. Mass-per-length (MPL) measurements derived from dark-field transmission EM images indicate that the most prevalent number of filaments in human-fibril-seeded-fibrils is 3, with a secondary peak at 2 filaments per fibril. The same result was obtained from all three cases of human AD, but it differs from MPL measurements on Aβ40 fibrils seeded with extracted fibrils from the brain tissue of mouse models of AD (three strains of aged transgenic mice, 3xTg, J20 and 5xFAD), in which there are 3, 4 and 5 filaments per fibril. Moreover, the most prevalent number can vary among mice of the same strain. We conclude that amyloid fibrils in mouse models of AD and fibrils from human AD are fundamentally different in structure.

Self-assembled amyloid fibrils with controllable conformational heterogeneity.

Amyloid fibrils are a hallmark of neurodegenerative diseases and exhibit a conformational diversity that governs their pathological functions. Despite recent findings concerning the pathological role of their conformational diversity, the way in which the heterogeneous conformations of amyloid fibrils can be formed has remained elusive. Here, we show that microwave-assisted chemistry affects the self-assembly process of amyloid fibril formation, which results in their conformational heterogeneity. In particular, microwave-assisted chemistry allows for delicate control of the thermodynamics of the self-assembly process, which enabled us to tune the molecular structure of β-lactoglobulin amyloid fibrils. The heterogeneous conformations of amyloid fibrils, which can be tuned with microwave-assisted chemistry, are attributed to the microwave-driven thermal energy affecting the electrostatic interaction during the self-assembly process. Our study demonstrates how microwave-assisted chemistry can be used to gain insight into the origin of conformational heterogeneity of amyloid fibrils as well as the design principles showing how the molecular structures of amyloid fibrils can be controlled.

Physical and structural basis for polymorphism in amyloid fibrils.

As our understanding of the molecular structures of amyloid fibrils has matured over the past 15 years, it has become clear that, while amyloid fibrils do have well-defined molecular structures, their molecular structures are not uniquely determined by the amino acid sequences of their constituent peptides and proteins. Self-propagating molecular-level polymorphism is a common phenomenon. This article reviews current information about amyloid fibril structures, variations in molecular structures that underlie amyloid polymorphism, and physical considerations that explain the development and persistence of amyloid polymorphism. Much of this information has been obtained through solid state nuclear magnetic resonance measurements. The biological significance of amyloid polymorphism is also discussed briefly. Although this article focuses primarily on studies of fibrils formed by amyloid-β peptides, the same principles apply to many amyloid-forming peptides and proteins.

Unraveling the Link between Molecular Conformation and Morphology and Mechanics of Amyloid Fibrils

Nearly all proteins and peptides have the ability to self-assemble into amyloid fibrils when they are denatured. These highly ordered nanofibrils exhibit superior mechanical properties, and are therefore attractive candidates for applications in materials science and food industry. The flipside of the remarkable stability is their accumulation in tissues in the context of conformational diseases.It is thought that the high stability and rigidity of amyloid fibrils is caused by β-sheets, which are stabilized with hydrogen bridges. However, spectroscopic measurements show that amyloids contain not only β-sheets, but also have a pronounced α-helical and random coil content, and morphological measurements show that amyloids are highly polymorphic. The link between molecular conformation and the mesoscopic fibril structure and mechanical rigidity is still not understood. Our strategy to elucidate this link is to measure both the mechanical properties and the molecular structure of amyloid fibrils prepared from the model protein β-lactoglobulin (β-lg).The average β-sheet intensity of amyloid fibrils prepared under different conditions is studied by Fourier transform infrared (FT-IR) and sum-frequency generation (SFG) spectroscopy and varies between about 40 and 90%. Tip-enhanced Raman spectroscopy (TERS) is used to determine with nanometer resolution the structure and amino acid content of single fibrils, which shows that the surface of the fibrils is heterogeneous in terms of its molecular structure. Bending dynamics of single, fluctuating fibrils enable us to correlate the secondary structure of fibrils to their morphology and mechanical properties. Together, our results show that although the fibrils are composed of the same peptides, they are highly polymorphic and their shape and mechanical rigidity is strongly correlated with the underlying molecular structure.

Structural complexity of a composite amyloid fibril.

The molecular structure of amyloid fibrils and the mechanism of their formation are of substantial medical and biological importance, but present an ongoing experimental and computational challenge. An early high-resolution view of amyloid-like structure was obtained on amyloid-like crystals of a small fragment of the yeast prion protein Sup35p: the peptide GNNQQNY. As GNNQQNY also forms amyloid-like fibrils under similar conditions, it has been theorized that the crystal's structural features are shared by the fibrils. Here we apply magic-angle-spinning (MAS) NMR to examine the structure and dynamics of these fibrils. Previously multiple NMR signals were observed for such samples, seemingly consistent with the presence of polymorphic fibrils. Here we demonstrate that peptides with these three distinct conformations instead assemble together into composite protofilaments. Electron microscopy (EM) of the ribbon-like fibrils indicates that these protofilaments combine in differing ways to form striations of variable widths, presenting another level of structural complexity. Structural and dynamic NMR data reveal the presence of highly restricted side-chain conformations involved in interfaces between differently structured peptides, likely comprising interdigitated steric zippers. We outline molecular interfaces that are consistent with the observed EM and NMR data. The rigid and uniform structure of the GNNQQNY crystals is found to contrast distinctly with the more complex structural and dynamic nature of these "composite" amyloid fibrils. These results provide insight into the fibril-crystal distinction and also indicate a necessary caution with respect to the extrapolation of crystal structures to the study of fibril structure and formation.

Molecular Structure of Amyloid Fibrils Controls the Relationship between Fibrillar Size and Toxicity

BackgroundAccording to the prevailing view, soluble oligomers or small fibrillar fragments are considered to be the most toxic species in prion diseases. To test this hypothesis, two conformationally different amyloid states were produced from the same highly pure recombinant full-length prion protein (rPrP). The cytotoxic potential of intact fibrils and fibrillar fragments generated by sonication from these two states was tested using cultured cells.Methodology/Principal FindingsFor one amyloid state, fibril fragmentation was found to enhance its cytotoxic potential, whereas for another amyloid state formed within the same amino acid sequence, the fragmented fibrils were found to be substantially less toxic than the intact fibrils. Consistent with the previous studies, the toxic effects were more pronounced for cell cultures expressing normal isoform of the prion protein (PrPC) at high levels confirming that cytotoxicity was in part PrPC-dependent. Silencing of PrPC expression by small hairpin RNAs designed to silence expression of human PrPC (shRNA-PrPC) deminished the deleterious effects of the two amyloid states to a different extent, suggesting that the role of PrPC-mediated and PrPC-independent mechanisms depends on the structure of the aggregates.Conclusions/SignificanceThis work provides a direct illustration that the relationship between an amyloid's physical dimension and its toxic potential is not unidirectional but is controlled by the molecular structure of prion protein (PrP) molecules within aggregated states. Depending on the structure, a decrease in size of amyloid fibrils can either enhance or abolish their cytotoxic effect. Regardless of the molecular structure or size of PrP aggregates, silencing of PrPC expression can be exploited to reduce their deleterious effects.

Molecular Structure of Amyloid Fibrils Formed by Residues 127 to 147 of the Human Prion Protein

Amyloid fibrils are filamentous and insoluble forms of peptides or proteins. Proline has long been considered to be incompatible with the cross-beta structural motif of amyloid fibrils. On the basis of solid-state NMR spectroscopy data, we present a structural model of an in-register parallel beta sheet for the amyloid fibrils formed from a human prion protein fragment, huPrP(127-47). We have developed a simple solid-state NMR spectroscopy technique to identify solvent-protected backbone amide protons in a H/D exchange experiment without disaggregating the amyloid fibrils, from which we find that proline residue P(137) does not disrupt the beta-sheet structure from G(127) to G(142). We suggest that the resultant kink at P(137) generates a twist between adjacent peptide strands to maintain hydrogen bonding in the beta-sheet regions flanking the P(137) residue. Although proline can be well integrated into the cross-beta structure of amyloid fibrils, the kink formed at the position of the proline residue will considerably weaken the hydrogen bonding between the neighboring strands, especially when the mutation site is near the central region of a beta sheet.

Nanomechanical Properties of Human Prion Protein Amyloid as Probed by Force Spectroscopy

Amyloids are associated with a number of protein misfolding disorders, including prion diseases. In this study, we used single-molecule force spectroscopy to characterize the nanomechanical properties and molecular structure of amyloid fibrils formed by human prion protein PrP90-231. Force-extension curves obtained by specific attachment of a gold-covered atomic force microscope tip to engineered Cys residues could be described by the worm-like chain model for entropic elasticity of a polymer chain, with the size of the N-terminal segment that could be stretched entropically depending on the tip attachment site. The data presented here provide direct information about the forces required to extract an individual monomer from the core of the PrP90-231 amyloid, and indicate that the β-sheet core of this amyloid starts at residue ∼164–169. The latter finding has important implications for the ongoing debate regarding the structure of PrP amyloid.

Characterization of β-Sheet Structure in Ure2p1-89 Yeast Prion Fibrils by Solid-State Nuclear Magnetic Resonance

Residues 1-89 constitute the Asn- and Gln-rich segment of the Ure2p protein and produce the [URE3] prion of Saccharomyces cerevisiae by forming the core of intracellular Ure2p amyloid. We report the results of solid-state nuclear magnetic resonance (NMR) measurements that probe the molecular structure of amyloid fibrils formed by Ure2p1-89 in vitro. Data include measurements of intermolecular magnetic dipole-dipole couplings in samples that are 13C-labeled at specific sites and two-dimensional 15N-13C and 13C-13C NMR spectra of samples that are uniformly 15N- and 13C-labeled. Intermolecular dipole-dipole couplings indicate that the beta-sheets in Ure2p1-89 fibrils have an in-register parallel structure. An in-register parallel beta-sheet structure permits polar zipper interactions among side chains of Gln and Asn residues and explains the tolerance of [URE3] to scrambling of the sequence in residues 1-89. Two-dimensional NMR spectra of uniformly labeled Ure2p1-89 fibrils, even when fully hydrated, show NMR linewidths that exceed those in solid-state NMR spectra of fibrils formed by residues 218-289 of the HET-s prion protein of Podospora anserina [as originally reported in Siemer, A. B., Ritter, C., Ernst, M., Riek, R., and Meier, B. H. (2005) Angew. Chem., Int. Ed. 44, 2441-2444 and confirmed by measurements reported here] by factors of three or more, indicating a lower degree of structural order at the molecular level in Ure2p1-89 fibrils. The very high degree of structural order in HET-s fibrils indicated by solid-state NMR data is therefore not a universal characteristic of prion proteins, and is likely to be a consequence of the evolved biological function of HET-s in heterokaryon incompatibility. Analysis of cross peak intensities in two-dimensional NMR spectra of uniformly labeled Ure2p1-89 fibrils suggests that certain portions of the amino acid sequence may not participate in a rigid beta-sheet structure, possibly including portions of the Asn-rich segment between residues 44 and 76.

Molecular Alignment within β-Sheets in A β14-23 Fibrils: Solid-State NMR Experiments and Theoretical Predictions

We report investigations of the molecular structure of amyloid fibrils formed by residues 14–23 of the β-amyloid peptide associated with Alzheimer’s disease (A β 14-23), using solid-state nuclear magnetic resonance (NMR) techniques in conjunction with electron microscopy and atomic force microscopy. The NMR measurements, which include two-dimensional proton-mediated 13C- 13C exchange and two-dimensional relayed proton-mediated 13C- 13C exchange spectra, show that A β 14-23 fibrils contain antiparallel β-sheets with a registry of backbone hydrogen bonds that aligns residue 17+ k of each peptide molecule with residue 22− k of neighboring molecules in the same β-sheet. We compare these results, as well as previously reported experimental results for fibrils formed by other β-amyloid fragments, with theoretical predictions of molecular alignment based on databases of residue-specific alignments in antiparallel β-sheets in known protein structures. While the theoretical predictions are not in exact agreement with the experimental results, they facilitate the design of experiments by suggesting a small number of plausible alignments that are readily distinguished by solid-state NMR.

Folding Landscapes of the Alzheimer Amyloid-β(12-28) Peptide

The energy landscape for folding of the 12-28 fragment of the Alzheimer amyloid β (Aβ) peptide is characterized using replica-exchange molecular dynamics simulations with an all-atom peptide model and explicit solvent. At physiological temperatures, the peptide exists mostly as a collapsed random coil, populating a small fraction (less than 10%) of hairpins with a β-turn at position V18F19, with another 10% of hairpin-like conformations possessing a bend rather than a turn in the central VFFA positions. A small fraction of the populated states, ∼14%, adopt polyproline II (PPII) conformations. Folding of the structured hairpin states proceeds through the assembly of two locally stable segments, VFFAE and EDVGS. The interactions stabilizing these locally folded structural motifs are in conflict with those stabilizing the global fold of A12-28, a signature of underlying residual frustration in this peptide. At increased temperature, the population of both β-strand and PPII conformations diminishes in favor of β-turn and random-coil states. On the basis of the conformational preferences of Aβ 12-28 monomers, two models for the molecular structure of amyloid fibrils formed by this peptide are proposed.

Polymorphic Fibril Formation by Residues 10–40 of the Alzheimer’s β-Amyloid Peptide

We report investigations of the morphology and molecular structure of amyloid fibrils comprised of residues 10–40 of the Alzheimer’s β-amyloid peptide (A β 10–40), prepared under various solution conditions and degrees of agitation. Omission of residues 1–9 from the full-length Alzheimer’s β-amyloid peptide (A β 1–40) did not prevent the peptide from forming amyloid fibrils or eliminate fibril polymorphism. These results are consistent with residues 1–9 being disordered in A β 1–40 fibrils, and show that fibril polymorphism is not a consequence of disorder in residues 1–9. Fibril morphology was analyzed by atomic force and electron microscopy, and secondary structure and inter-side-chain proximity were probed using solid-state NMR. A β 1–40 fibrils were found to be structurally compatible with A β 10–40: A β 1–40 fibril fragments were used to seed the growth of A β 10–40 fibrils, with propagation of fibril morphology and molecular structure. In addition, comparison of lyophilized and hydrated fibril samples revealed no effect of hydration on molecular structure, indicating that A β 10–40 fibrils are unlikely to contain bulk water.

High density lipoproteins bind Aβ and apolipoprotein C-II amyloid fibrils

Disease-associated amyloid deposits contain both fibrillar and nonfibrillar components. The majority of these amyloid components originate or coexist in the bloodstream. To understand the nature of the interaction between the nonfibrillar and fibrillar components, we have developed a centrifugation method to isolate fibril binding proteins from human serum. Amyloid fibrils composed of either Abeta peptide or apolipoprotein C-II (apoC-II) cosedimented with specific serum proteins. Gel electrophoresis, mass spectrometry peptide fingerprinting, and Western analysis identified the major binding species as proteins found in HDL particles, including apoA-I, apoA-II, apoE, clusterin, and serum amyloid A. Sedimentation analysis showed that purified human HDL and recombinant apoA-I lipid particles bound directly to Abeta and apoC-II amyloid fibrils. These studies reveal a novel function of HDL that may contribute to the well-established protective effect of this lipoprotein class in heart disease.

Solid-state NMR as a probe of amyloid structure.

Solid state nuclear magnetic resonance (NMR) has developed into one of the most informative and direct experimental approaches to the characterization of the molecular structures of amyloid fibrils, including those associated with Alzheimer's disease. In this article, essential aspects of solid state NMR methods are described briefly and results obtained to date regarding the supramolecular organization of amyloid fibrils and the conformations of peptides within amyloid fibrils are reviewed.

Molecular structure of amyloid fibrils: insights from solid-state NMR

Solid-state nuclear magnetic resonance (NMR) measurements have made major contributions to our understanding of the molecular structures of amyloid fibrils, including fibrils formed by the beta-amyloid peptide associated with Alzheimer's disease, by proteins associated with fungal prions, and by a variety of other polypeptides. Because solid-state NMR techniques can be used to determine interatomic distances (both intramolecular and intermolecular), place constraints on backbone and side-chain torsion angles, and identify tertiary and quaternary contacts, full molecular models for amyloid fibrils can be developed from solid-state NMR data, especially when supplemented by lower-resolution structural constraints from electron microscopy and other sources. In addition, solid-state NMR data can be used as experimental tests of various proposals and hypotheses regarding the mechanisms of amyloid formation, the nature of intermediate structures, and the common structural features within amyloid fibrils. This review introduces the basic experimental and conceptual principles behind solid-state NMR methods that are applicable to amyloid fibrils, reviews the information about amyloid structures that has been obtained to date with these methods, and discusses how solid-state NMR data provide insights into the molecular interactions that stabilize amyloid structures, the generic propensity of polypeptide chains to form amyloid fibrils, and a number of related issues that are of current interest in the amyloid field.

Amyloid fibrils

Amyloidoses constitute a group of diverse diseases in which extracellular insoluble material is deposited either systemically or in specific organs, which leads to the destruction of surrounding cells. The amyloid diseases encompass some of today's most devastating maladies, both in terms of number of people affected and in the extent of human suffering, including Alzheimer's disease, spongiform encephalopathies and type II diabetes mellitus. The term amyloid was originally introduced to describe the tissue deposits seen in histological sections as it was thought that they were composed of starch-like material. Today it is known that protein constitutes the main component of amyloid deposits, although other constituents are found as well. In order to form these deposits, the proteins involved must convert from their native (folded or unfolded) states and aggregate into insoluble fibrils with cross β-sheet structure. The fibrils themselves and/or oligomers formed on the pathway to fibril formation are thought to be cytotoxic, and thereby directly responsible for disease initiation and progression. For each of the amyloid diseases, one specific protein or peptide forms the fibrils, for example the amyloid β-peptide (Aβ) in Alzheimer's disease, the prion protein (PrP) in spongiform encephalopathies and the islet amyloid polypeptide (IAPP) in type II diabetes. Our knowledge about amyloid diseases and the constituent proteins at a molecular level has expanded significantly during the last few years and the minireviews in this issue of FEBS Journal summarize the progress made from four different angles. The contribution by Per Westermark describes the pathology of the amyloidoses, defines the ≈ 25 known amyloidogenic proteins in a systematic manner and gives an outlook on future lines of research. Particularly interesting, and potentially horrifying, is the possibility of lateral spread of amyloidoses, as a result of the inherent self-propagating nature of fibril growth and the potential for cross-seeding between different amyloidogenic proteins. In the second review, Sumner Makin & Louise Serpell discuss current knowledge on the molecular structures of amyloid fibrils. Recent progress of these authors and others has resulted in the determination of high-resolution structures of amyloid fibrils. The details of these structures are described, highlighting, among other things, the impressive complementarity between the strands that build up the cross β-sheet polymers, and the ways in which this knowledge impacts our understanding of disease progression are discussed. In the third review, Thomas Jahn & Sheena Radford describe what has been learnt from biophysical studies of protein folding and unfolding as regards the driving forces of amyloid fibril formation. In the light of this they discuss how different perturbations of a biological system could lead to the switch from a kinetically favoured native state towards the globally most stable structure, the amyloid fibril. In the final contribution in the series, Ehud Gazit summarizes recent work on mechanistic aspects of fibril formation, in particular how the amino acid sequence determines the propensity to fibrillation and how short peptides can be used to study this aspect. Gazit also discusses how a detailed understanding of fibril formation and structure can be used to design inhibitors., The amyloidoses are fatal diseases and the number of affected people is expected to increase significantly in the coming decades, but no curative treatment is yet available for any of these conditions. The apparent close connection among amyloid disease, fibril formation and protein folding and misfolding, makes it likely that continued intense interest in these topics will eventually generate mechanism-based inhibitors that can be used to combat these devastating diseases.

FTIR reveals structural differences between native β‐sheet proteins and amyloid fibrils

The presence of beta-sheets in the core of amyloid fibrils raised questions as to whether or not beta-sheet-containing proteins, such as transthyretin, are predisposed to form such fibrils. However, we show here that the molecular structure of amyloid fibrils differs more generally from the beta-sheets in native proteins. This difference is evident from the amide I region of the infrared spectrum and relates to the distribution of the phi/psi dihedral angles within the Ramachandran plot, the average number of strands per sheet, and possibly, the beta-sheet twist. These data imply that amyloid fibril formation from native beta-sheet proteins can involve a substantial structural reorganization.