Beta-amyloid Peptide Fragment Research Articles

Multi-Target Mechanisms of Phytochemicals in Alzheimer's Disease: Effects on Oxidative Stress, Neuroinflammation and Protein Aggregation.

Alzheimer’s disease (AD) is a neurodegenerative disease characterized by a tangle-shaped accumulation of beta-amyloid peptide fragments and Tau protein in brain neurons. The pathophysiological mechanism involves the presence of Aβ-amyloid peptide, Tau protein, oxidative stress, and an exacerbated neuro-inflammatory response. This review aims to offer an updated compendium of the most recent and promising advances in AD treatment through the administration of phytochemicals. The literature survey was carried out by electronic search in the following specialized databases PubMed/Medline, Embase, TRIP database, Google Scholar, Wiley, and Web of Science regarding published works that included molecular mechanisms and signaling pathways targeted by phytochemicals in various experimental models of Alzheimer’s disease in vitro and in vivo. The results of the studies showed that the use of phytochemicals against AD has gained relevance due to their antioxidant, anti-neuroinflammatory, anti-amyloid, and anti-hyperphosphorylation properties of Tau protein. Some bioactive compounds from plants have been shown to have the ability to prevent and stop the progression of Alzheimer’s.

Age‐dependent behavioral and neuropathological assessment of analysis of AppNL‐G‐F/NL‐G‐Fknock‐in (KI) mice

Mitochondria play a crucial role in Alzheimer's disease (AD) onset and progression. Traditional transgenic AD mouse models share the limitation that the proteolytic processing of overexpressed mutant APP results in overproduction of amyloid‐beta (Aß) and other APP peptide fragments resulting in artificial, non‐clinically relevant phenotypes. Here we performed an in‐depth and time‐resolved behavioral and metabolic characterization of a clinically relevant AD mouse model engineered to express normal physiological levels of APP harboring humanized Swedish (K670N/M671L), Beyreuther/Iberian (I716F), and Arctic (E693G) mutations, (AppNL‐G‐F/NL‐G‐F), termed APP knock‐in (APPKI) mice. At 7 months of age, APPKI mice exhibited only learning deficit confirmed by both the water maze test and fear conditioning test. At the same age, we also observed mitochondrial dysfunction consisting of decreased ATP production, higher membrane potential with subsequent greater production of reactive oxygen species (ROS). Furthermore, mitochondria were larger in volume, accompanied by lower molecular markers of mitochondrial dynamics, mitofusin‐2 (Mfn2), at 6‐8 m. In contrast, the oxygen consumption rate (OCR) from 6 m APPKI hippocampal homogenates was similar to that of WT mice. At 12 months of age, APPKI mice exhibited learning and memory deficit, together with significantly decreased mitochondrial oxygen consumption rate detected by high‐resolution respirometry. Histopathology revealed minor amyloidosis accompanied by microgliosis (Iba1) as early as 3 months, which became more apparent at 6 and 12 m. These data indicate a wide range of brain mitochondrial dysfunction and associated cognitive deficits in APPKImice. Moreover, the identified behavioral and bioenergetic alterations in this physiologically relevant AD transgenic mouse model provide a valuable tool to optimize the temporal component for therapeutic interventions to treat AD.

Adaptation to intermittent hypoxia prevents the decrease in cerebral vascular density in rats with experimental Alzheimer’s disease

Introduction. Patients with Alzheimer’s disease (AD) have reduced cerebral vascular density (VD), which impairs blood flow to neurons and may contribute to progression of AD. Earlier we showed that prior adaptation to intermittent hypobaric hypoxia (IHH) prevented memory loss and degeneration of cortical neurons in rats with experimental AD (EAD). The aim of this study was to test if IHH might prevent EAD-induced vascular rarefaction in rats. Materials and methods. EAD was induced with bilateral injection of neurotoxic beta-amyloid peptide fragment (A) (25–35) into n. basalis magnocellularis. IHH was simulated at a 4,000 m altitude, for 4 hours a day, for 14 days. Brain blood vessels were stained by transcardiac infusion of Indian ink; brain sections were stained with 0.3 % cresyl violet by Nissle method. Vascular density was assessed in the cortex and hippocampus using the Infinity Analysis Software. Results. In the EAD rats, VD was significantly decreased in the hippocampus (13.3±0.9 vs 17.8±1.0 in field of view, FOV, p<0.03) and in the cortex (17.3±1.5 vs 22.3±1.3 in FOV, p<0.03). AIH increased VD in the hippocampus to 27.0±3.5 in FOV (p=0.01) and in cortex to 26.0±1.1 in FOV (p<0.03). In EAD+AIH rats, VD did not differ significantly from the control rats neither in the hippocampus, nor in the cortex. AIH may stimulate angiogenesis through hypoxia inducible factor-1α-mediated expression of vascular endothelial growth factor and/or by increasing expression and activity of antioxidant enzymes. Conclusion. One of the mechanisms of AIH beneficial effect in AD-related neurodegeneration is preserving the capability for compensatory angiogenesis in brain.

Absolute Quantitation of Tryptophan-Containing Peptides and Amyloid β-Peptide Fragments by Coulometric Mass Spectrometry.

Isotope-labeled internal standards are routinely used for mass spectrometry (MS)-based absolute quantitation. However, syntheses of isotope-labeled peptides are time-consuming and costly. To tackle this issue, we recently developed a coulometric mass spectrometric (CMS) approach for absolute quantitation without the use of standards, based on the electrochemical oxidation of cysteine or tyrosine-containing peptides followed by mass spectrometric measurement of the oxidation yield. To further expand the utility of this method, herein we present the CMS method for absolute quantitation of peptides based on tryptophan electrochemical oxidation. Several tryptophan-containing peptides, such as WGG, WQPPRARI, WAGGDASGE, RTRPLWVRME, and KVPRNQDWL, were successfully quantified with a quantification error ranging from -4.5 to +4.3%. Furthermore, this quantitation approach is also applicable to protein, in which protein can be digested and a surrogate peptide can be selected for quantification to reflect the amount of the parent protein, as exemplified by CMS analysis of peptide GITWK from cytochrome c. The CMS result agreed well with the traditional isotope dilution method, with only a small difference of 3.5%. In addition, CMS was used to successfully quantify amyloid beta (Aβ) peptide fragments (up to 28 amino acid residues) based on tyrosine oxidation. The validity of the CMS method for peptide and protein absolute quantitation without using isotope-labeled peptide standards would greatly facilitate proteomics research.

Nerve Growth Factor Peptides Bind Copper(II) with High Affinity: A Thermodynamic Approach to Unveil Overlooked Neurotrophin Roles.

Nerve growth factor (NGF) is a protein essential to neurons survival, which interacts with its receptor as a non-covalent dimer. Peptides belonging to NGF N-terminal domain are able to mimic the activity of the whole protein. Such activity is affected by the presence of copper ions. The metal is released in the synaptic cleft where proteins, not yet identified, may bind and transfer to human copper transporter 1 (hCtr1), for copper uptake in neurons. The measurements of the stability constants of copper complexes formed by amyloid beta and hCtr1 peptide fragments suggest that beta-amyloid (Aβ) can perform this task. In this work, the stability constant values of copper complex species formed with the dimeric form of N-terminal domain, sequence 1–15 of the protein, were determined by means of potentiometric measurements. At physiological pH, NGF peptides bind one equivalent of copper ion with higher affinity of Aβ and lower than hCtr1 peptide fragments. Therefore, in the synaptic cleft, NGF may act as a potential copper chelating molecule, ionophore or chaperone for hCtr1 for metal uptake. Copper dyshomeostasis and mild acidic environment may modify the balance between metal, NGF, and Aβ, with consequences on the metal cellular uptake and therefore be among causes of the Alzheimer’s disease onset.

Carnitine palmitoyltransferase 1 (CPT1) alleviates oxidative stress and apoptosis of hippocampal neuron in response to beta-Amyloid peptide fragment Aβ25-35

ABSTRACT CPT1C, which is expressed in hippocampus, influences ceramide level, endogenous cannabinoid and oxidation process, as well as plays an important role in various brain functions such as learning. This study aimed to investigate the role of CPT1C in Alzheimer’s disease (AD) and its underlying mechanism. We established a model of Alzheimer’s disease in vitro by exposing primary hippocampal neurons to beta-Amyloid peptide fragment 25–35 (Aβ25-35). The cell viability, lactate dehydrogenase (LDH) level, expressions of reactive oxygen species (ROS), malondialdehyde (MDA) and superoxide dismutase (SOD) were detected using Cell Counting Kit-8 (CCK-8), LDH assay, ROS kits, malondialdehyde (MDA) kits and SOD kits, respectively. Moreover, the expression of oxidative stress-related proteins as well as the expressions of amyloid precursor protein (App), p-Tau andβ-site APP-cleaving enzyme1 (Bace-1) were measured using quantitative reverse transcription PCR (RT-qPCR) and western blot. Tunel and western blot were adopted to detect apoptosis as well as its related proteins. After the treatment of peroxisome proliferators-activated receptor alpha (PPARα), CPT1C expression was detected with the application of RT-qPCR and western blot. CPT1C expression was reduced in Aβ25-35-induced HT22 cells. Overexpression of CPT1C relieved cell viability and toxic injury as well as attenuated oxidative stress, apoptosis and expression levels of AD marker proteins. Moreover, higher doses of PPARα agonist activate the expression of CPT1C in Aβ25-35-induced HT22 cells. In conclusion, CPT1C alleviates Aβ25-35-induced oxidative stress, apoptosis and deposition of AD marker proteins in hippocampal neurons, suggesting that CPT1C has favorable effects on alleviating AD and participates in PPARα activation.

Improving separation optimization in capillary electrophoresis by using a general quality criterion

In this paper we extend the use of the quality criterion t’ to optimize separations in capillary electrophoresis (CE). The theoretical parameter t’ takes into account not only the relative separation between a given pair of compounds but also their separation from the neutral species migrating with the electroosmotic flow (EOF). Furthermore, it can be composed for complex mixtures as a global multicriterium optimization function T′, for a rapid, simple and reliable selection of optimized separation conditions by mathematical maximization. Here, we demonstrate the applicability of T′ using as a variable the electrophoretic mobility (me) for the optimization of pH in the separation of a mixture of amyloid beta (Aβ) peptide fragments. In addition, it is shown the versatility of T′ using other variables related to me, which do not require experimental measurements. This is the case with ionizable compounds as the Aβ peptide fragments, whose charge-to-mass ratios can be calculated if accurate pKa values are available in the literature. The excellent performance of T’ for Aβ peptide fragments is further validated optimizing the pH for the separation of mixtures of harmala alkaloids (HAlks) and quinolone antibiotics.

The Effect of The Neuroprotector Isatin on Complex Formation of Beta-Amyloid Peptide Fragments with Some Intracellular Proteins

The Effect of The Neuroprotector Isatin on Complex Formation of Beta-Amyloid Peptide Fragments with Some Intracellular Proteins

Commentary on Some Recent Theses Relevant to Combating Aging: October 2018.

Theses reviewed in this issue include "Computational Pathology for Quantifying Spatial Heterogeneity in Digital Images of Tissue Sections from Solid Tumors," "Molecular Analysis of Amyotrophic Lateral Sclerosis and Frontotemporal Degeneration Brain Tissue Identifies Disease Mechanisms Associated with Repetitive DNA Elements," "Neuroprotective Potential of the N-Terminal Beta Amyloid Peptide Fragment in the Neurodegeneration, Synaptic Dysfunction and Memory Deficits in Models of Alzheimer's Disease," "pH-triggered Self-Assembly of a PEGylated Peptide Amphiphilic Contrast Agent," "Quantitative Approaches for Profiling the T Cell Receptor Repertoire in Human Tissues," and "Regulation and Repair of Neural Stem Cells and the Neurogenic Niche."

An impedimetric micro-immunosensing assay to detect Alzheimer's disease biomarker: Aβ40

A miniaturized biosensing platform, based on monoclonal amyloid-beta antibodies (mAβab) that were immobilized on a disc-shaped platinum/iridium (Pt/Ir) microelectrode surface coupled with an impedimetric signal transducer, was developed for the label-free and sensitive detection of amyloid-beta peptide fragment 1–40 (Aβ40); a reliable biomarker for early diagnosis of Alzheimer's disease (AD). A Pt/Ir microelectrode was electropolymerized with poly (ortho-phenylenediamine), a conducting free amine-containing aromatic polymer; followed by crosslinking with glutaraldehyde for subsequent coupling of mAβab on the microelectrode surface. This modification strategy efficiently improved the impedimetric detection performance of Aβ40 in terms of charge transfer resistance (∼400-fold difference) and normalized impedance magnitude percentage change (∼40% increase) compared with a passive adsorption-based immobilization method. The sensitivity of the micro-immunosensing assay was found to be 1056 kΩ/(pg/mL)/cm2 and the limit of detection was found to be 4.81 pg/mL with a dynamic range of 1−104 pg/mL (R2 = 0.9932). The overall precision of the assay, as measured by relative standard deviation, ranged from 0.84 to 5.15%, demonstrating its reliability and accuracy; while in respect to assay durability and stability, the immobilized mAβab were able to maintain 80% of their binding activity to Aβ40 after incubation for 48 h at ambient temperature (25 °C). To validate the practical applicability, the assay was tested using brain tissue lysates prepared from AD-induced rats. Results indicate that the proposed impedimetric micro-immunosensing platform is highly versatile and adaptable for the quantitative detection of other disease-related biomarkers.

The effect of the neuroprotector isatin on complex formation of beta-amyloid peptide fragments with some intracellular proteins

Amyloid-β peptide (1-42) (Aβ1-42) is a key player in the development and progression of Alzheimer's disease (AD) and related pathologies, determined by formation of protein aggregates in the central nervous system. Aβ1-42 binding to crucial intracellular targets (and their subsequent inactivation) obviously represents one of the earliest events preceding extracellular pathogenic oligomerization/aggregation of Aβ1-42. It is reasonable to expect that dissociation of the Aβ1-42 complexes with intracellular proteins by means of inhibitors followed by subsequent degradation of Aβ1-42 would not only protect critically important proteins but also prevent intracellular accumulation of Aβ1-42. The aim of this study was to investigate the effect of the neuroprotector isatin (100 mM) on interaction of known Aβ-binding proteins, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and pyruvate kinase, with Aβ1-42 and its fragments (Aβ1-28, Aβ12-28, Aβ25-35). Aβ1-42 and its fragments (Aβ1-28, Aβ12-28, Aβ25-35) immobilized on the Biacore optical biosensor chip interacted with GAPDH and pyruvate kinase. The lowest and basically equal Kd values were determined for GAPDH and pyruvate kinase complexes with immobilized Aβ1-42 and Aβ25-35. The presence of 100 mM isatin caused a significant (more than fivefold) increase in the Kd values for GAPDH complexes with all Aβ peptides except Aβ1-28. In contrast to GAPDH isatin increased dissociation of pyruvate kinase complexes only with Aβ1-42 (causing a 30-fold increase in Kd) and to a lesser extent with Aβ12-28 and Aβ25-35 (a 10-fold increase in Kd). It should be noted that in the presence of isatin the Kd values for GAPDH and pyruvate kinase complexes with all Aβ studied were in a narrower concentration range (10-7 M - 10-6 M) than in the absence of this neuroprotector (10-8 M - 10-6 M). Data obtained suggest existence of principal possibility of (pharmacological) protection of crucial intracellular targets against both Aβ1-42, and its aggressive truncated peptides (Aβ25-35).

Molecular dynamics simulation of the nanofibrils formed by amyloid-based peptide amphiphiles

Atomistic molecular dynamics simulations have been performed on the peptide amphiphiles (PAs) with four amyloid beta peptide fragments as head groups. The stable structures were monitored by the root mean square deviation with respect to the energy minimised initial structures. Random coil and β-sheet structures with hydrogen bonds along and perpendicular to the long axis of the nanofibre were obtained due to the different nature of the head groups. Influences of pH and capping ends on the nanofibre structures were investigated through variation of the protonation states of the ionic amino acids in the peptides. The peptides with opposite charges on both sides were found to have the fewest β-sheet structures, and the charges on the outer terminal tended to destruct the β-sheets while those at the inner side did not. The isolated charge in the centre of peptides was found to be able to promote the formation of regular β-sheets, while multiple charged residues could not support ordered β-sheet structures. When charge neutralisation occurred between adjacent residues, regular β-sheet laminates might also occur for systems with charges at the outer terminal. With the increase of β-sheet structures formed, the original twisted structures found for random coil structures of the PAs could be diminished by the hydrogen bonds.

NMR Studies of the Mn2+ Interactions with Amyloid Peptide Aβ13-23 in Water Environment

In this paper, binding of Mn2+ ions to the fragment of beta-amyloid peptide (Aβ13-23) was studied. Manganese complexation induces important structural changes within the C-terminal segment of the peptide. Investigation of peptide–metal ion binding was made by MnCl2 salt titration and recording 2D 1H–1H NMR TOCSY spectra (TOtal Correlation SpectroscopY). Multidimensional NMR techniques were performed to understand the details of the conformational behavior of the peptide and to reveal the metal-binding sites. According to changes in NMR spectra, the manganese-binding center of the Aβ13-23 peptide is associated with the aspartate residue.

Ginsenoside Rg1 protects against neurodegeneration by inducing neurite outgrowth in cultured hippocampal neurons.

Ginsenoside Rg1 (Rg1) has anti-aging and anti-neurodegenerative effects. However, the mechanisms underlying these actions remain unclear. The aim of the present study was to determine whether Rg1 affects hippocampal survival and neurite outgrowth in vitro after exposure to amyloid-beta peptide fragment 25-35 (Aβ25-35), and to explore whether the extracellular signal-regulated kinase (ERK) and Akt signaling pathways are involved in these biological processes. We cultured hippocampal neurons from newborn rats for 24 hours, then added Rg1 to the medium for another 24 hours, with or without pharmacological inhibitors of the mitogen-activated protein kinase (MAPK) family or Akt signaling pathways for a further 24 hours. We then immunostained the neurons for growth associated protein-43, and measured neurite length. In a separate experiment, we exposed cultured hippocampal neurons to Aβ25-35 for 30 minutes, before adding Rg1 for 48 hours, with or without Akt or MAPK inhibitors, and assessed neuronal survival using Hoechst 33258 staining, and phosphorylation of ERK1/2 and Akt by western blot analysis. Rg1 induced neurite outgrowth, and this effect was blocked by API-2 (Akt inhibitor) and PD98059 (MAPK/ERK kinase inhibitor), but not by SP600125 or SB203580 (inhibitors of c-Jun N-terminal kinase and p38 MAPK, respectively). Consistent with this effect, Rg1 upregulated the phosphorylation of Akt and ERK1/2; these effects were reversed by API-2 and PD98059, respectively. In addition, Rg1 significantly reversed Aβ25-35-induced apoptosis; this effect was blocked by API-2 and PD98059, but not by SP600125 or SB203580. Finally, Rg1 significantly reversed the Aβ25-35-induced decrease in Akt and ERK1/2 phosphorylation, but API-2 prevented this reversal. Our results indicate that Rg1 enhances neurite outgrowth and protects against Aβ25-35-induced damage, and that its mechanism may involve the activation of Akt and ERK1/2 signaling.

Effects of dipole modifiers on channel-forming activity of amyloid and amyloid-like peptides in lipid bilayers

We have studied the steady-state transmembrane current induced by amyloid and amyloid-like peptides in lipid bilayers in the presence of dipole modifiers. It has been shown that the addition of dipole modifier, phloretin, to the membrane bathing solutions leads to an increase in the multichannel activity of amyloid beta-peptide fragment 25-35, [Gly35]-amyloid beta-peptide fragment 25--35, prion protein fragment 106-126 and amyloid-like peptides myr-BASP1 (1--13), myr-BASP1(1--19) and GAP-43(1--40). We have found that the effect of phloretin is not the result of dipole potential changes due to adsorption of this modifier on the membrane. Using the various fragments of amyloid beta-peptide, presenilin, prion protein and neuronal proteins BASP1 and GAP-43 allowes to conclude that the steady-state peptide-induced transmembrane current in the case of addition of phloretin is due to the electrostatic interaction between the positively charged channel-forming agents and negatively charged dipole modifier. The results obtained by electron microscopy have demonstrated that this interaction increases degree of peptide oligomerization.

Global properties and propensity to dimerization of the amyloid-beta (12-28) peptide fragment through the modeling of its monomer and dimer diffusion coefficients and electrophoretic mobilities.

Neuronal activity loss may be due to toxicity caused mainly by amyloid-beta (1-40) and (1-42) peptides forming soluble oligomers. Here the amyloid-beta (12-28) peptide fragment (monomer) and its dimer are characterized at low pH through the modeling of their diffusion coefficients and effective electrophoretic mobilities. Translational diffusion coefficient experimental values of monomer and dimer analogs of this peptide fragment and monomer and dimer mixtures at thermodynamic equilibrium are used as reported in the literature for different monomer initial concentrations. The resulting electrokinetic and hydrodynamic global properties are employed to evaluate the amyloid-beta (12-28) peptide fragment propensity to dimerization through a thermodynamic theoretical framework. Therefore equilibrium constants are considered at pH 2.9 to elucidate one of the amyloidogenic mechanisms involving the central hydrophobic region LVFFA of the peptide spanning residues 17-21 associated with phenylalanine at positions 19 and 20 in the amino acid sequence of amyloid-beta peptides. An analysis demonstrating that peptide aggregation is a concentration-dependent process is provided, where both pair and intraparticle charge regulation phenomena become relevant. It is shown that the modeling of the effective electrophoretic mobility of the amyloid-beta (12-28) peptide fragment is crucial to understand the effect of hydrophobic region LVFFA in the amyloidogenic process.

Sphingosine Kinase-1 Protects Differentiated N2a Cells Against Beta-Amyloid25–35-Induced Neurotoxicity Via the Mitochondrial Pathway

Although the etiology of Alzheimer's disease (AD) is not fully understood, multiple lines of evidence suggests the importance of amyloid-β (Aβ) in the initiation/progression of the disease. Aβ has been shown to induce neuronal apoptosis via the sphingomyelin/ceramide pathway. This study was designed to elucidate whether the sphingosine kinase-1 (SPK1), a critical regulator of the ceramide/sphingosine 1-phosphate rheostat, plays a pivotal role in the regulation of death and survival of differentiated neuro-2a cells in response to beta-amyloid peptide fragment 25-35 (Aβ25-35). These results show that the expression of SPK1 was markedly decreased in Aβ25-35-induced neurotoxicity, as evidenced by the decreased cell viability and the increased apoptotic rate. Overexpression of SPK1 significantly attenuated Aβ25-35-induced neurotoxicity, whereas silencing the expression of SPK1 exacerbated it. Moreover, overexpression of SPK1 can significantly attenuate Aβ25-35-induced upregulation of Bax and rehabilitate the level of Bcl-2; concomitantly, it can ameliorate mitochondrial ultrastructure. These studies demonstrate that overexpression of SPK1 may moderate Aβ25-35-induced neurotoxicity by regulating the Bcl-2/Bax ratio and improving mitochondrial ultrastructure. Based on these findings, SPK1 is a potential therapeutic target for AD.

Morphofunctional Changes in Goldfish Mauthner Neurons after Application of β-Amyloid

The effects of applying aggregated beta-amyloid peptide fragment 25-35 on the three-dimensional structure and volume of Mauthner neurons (MN) and on motor asymmetry were assessed in goldfish using reconstructions based on serial histological sections. These experiments showed that the motor asymmetry of the fish was stable in the intact state and in controls and correlated tightly with structural asymmetry of neurons. beta-Amyloid produced large changes or inversion in motor asymmetry, which did not coincide with or even contradicted the structural asymmetry of MN. This occurred as a result of marked dystrophy or, conversely, hypertrophy of individual neurons and their individual dendrites, with changes in their proportions. It is suggested that the harmful action of beta-amyloid on MN structure and the discordant ("incorrect") behavior of the fish may result from mechanical deformation evoked by its tape-like fibrils. Overall, the results lead to the conclusion that MN provide a suitable system for studying the structural aspects of amyloidosis.

Influence of the Solvent on the Self-Assembly of a Modified Amyloid Beta Peptide Fragment. II. NMR and Computer Simulation Investigation

The conformation of a model peptide AAKLVFF based on a fragment of the amyloid beta peptide Abeta16-20, KLVFF, is investigated in methanol and water via solution NMR experiments and molecular dynamics computer simulations. In previous work, we have shown that AAKLVFF forms peptide nanotubes in methanol and twisted fibrils in water. Chemical shift measurements were used to investigate the solubility of the peptide as a function of concentration in methanol and water. This enabled the determination of critical aggregation concentrations. The solubility was lower in water. In dilute solution, diffusion coefficients revealed the presence of intermediate aggregates in concentrated solution, coexisting with NMR-silent larger aggregates, presumed to be beta-sheets. In water, diffusion coefficients did not change appreciably with concentration, indicating the presence mainly of monomers, coexisting with larger aggregates in more concentrated solution. Concentration-dependent chemical shift measurements indicated a folded conformation for the monomers/intermediate aggregates in dilute methanol, with unfolding at higher concentration. In water, an antiparallel arrangement of strands was indicated by certain ROESY peak correlations. The temperature-dependent solubility of AAKLVFF in methanol was well described by a van't Hoff analysis, providing a solubilization enthalpy and entropy. This pointed to the importance of solvophobic interactions in the self-assembly process. Molecular dynamics simulations constrained by NOE values from NMR suggested disordered reverse turn structures for the monomer, with an antiparallel twisted conformation for dimers. To model the beta-sheet structures formed at higher concentration, possible model arrangements of strands into beta-sheets with parallel and antiparallel configurations and different stacking sequences were used as the basis for MD simulations; two particular arrangements of antiparallel beta-sheets were found to be stable, one being linear and twisted and the other twisted in two directions. These structures were used to simulate circular dichroism spectra. The roles of aromatic stacking interactions and charge transfer effects were also examined. Simulated spectra were found to be similar to those observed experimentally (in water or methanol) which show a maximum at 215 or 218 nm due to pi-pi* interactions, when allowance is made for a 15-18 nm red-shift that may be due to light scattering effects.

Influence of the Solvent on the Self-Assembly of a Modified Amyloid Beta Peptide Fragment. I. Morphological Investigation

The solvent-induced transition between self-assembled structures formed by the peptide AAKLVFF is studied via electron microscopy, light scattering, and spectroscopic techniques. The peptide is based on a core fragment of the amyloid beta-peptide, KLVFF, extended by two alanine residues. AAKLVFF exhibits distinct structures of twisted fibrils in water or nanotubes in methanol. For intermediate water/methanol compositions, these structures are disrupted and replaced by wide filamentous tapes that appear to be lateral aggregates of thin protofilaments. The orientation of the beta-strands in the twisted tapes or nanotubes can be deduced from X-ray diffraction on aligned stalks, as well as FT-IR experiments in transmission compared to attenuated total reflection. Strands are aligned perpendicular to the axis of the twisted fibrils or the nanotubes. The results are interpreted in light of recent results on the effect of competitive hydrogen bonding upon self-assembly in soft materials in water/methanol mixtures.