Self assembly of a model amphiphilic phenylalanine peptide/polyethylene glycol block copolymer in aqueous solution

There has been great interest recently in peptide amphiphiles and block copolymers containing biomimetic peptide sequences due to applications in bionanotechnology. We investigate the self-assembly of the peptide-PEG amphiphile FFFF-PEG5000 containing the hydrophobic sequence of four phenylalanine residues conjugated to PEG of molar mass 5000. This serves as a simple model peptide amphiphile. At very low concentration, association of hydrophobic aromatic phenylalanine residues occurs, as revealed by circular dichroism and UV/vis fluorescence experiments. A critical aggregation concentration associated with the formation of hydrophobic domains is determined through pyrene fluorescence assays. At higher concentration, defined beta-sheets develop as revealed by FTIR spectroscopy and X-ray diffraction. Transmission electron microscopy reveals self-assembled straight fibril structures. These are much shorter than those observed for amyloid peptides, the finite length may be set by the end cap energy due to the hydrophobicity of phenylalanine. The combination of these techniques points to different aggregation processes depending on concentration. Hydrophobic association into irregular aggregates occurs at low concentration, well-developed beta-sheets only developing at higher concentration. Drying of FFFF-PEG5000 solutions leads to crystallization of PEG, as confirmed by polarized optical microscopy (POM), FTIR and X-ray diffraction (XRD). PEG crystallization does not disrupt local beta-sheet structure (as indicated by FTIR and XRD). However on longer lengthscales the beta-sheet fibrillar structure is perturbed because spheruilites from PEG crystallization are observed by POM. (C) 2009 Elsevier B.V. All rights reserved.

Self-assembly in aqueous solution of a modified amyloid beta peptide fragment

The self-assembly in films dried from aqueous solutions of a modified amyloid beta peptide fragment is studied. We focus on sequence A beta(16-20), KLVFF, extended by two alanines at the N-terminus to give AAKLVFF. Self-assembly into twisted ribbon fibrils is observed, as confirmed by transmission electron microscopy (TEM). Dynamic light scattering reveals the semi-flexible nature of the AAKLVFF fibrils, while polarized optical microscopy shows that the peptide fibrils crystallize after an aqueous solution of AAKLVFF is matured over 5 days. The secondary structure of the fibrils is studied by FT-IR, circular dichroism and X-ray diffraction (XRD), which provide evidence for beta-sheet structure in the fibril. From high resolution TEM it is concluded that the average width of an AAKLVFF fibril is (63 +/- 18) nm, indicating that these fibrils comprise beta-sheets with multiple repeats of the unit cell, determined by XRD to have b and c dimensions 1.9 and 4.4 nm with an a axis 0.96 nm, corresponding to twice the peptide backbone spacing in the antiparallel beta-sheet. (C) 2008 Elsevier B.V. All rights reserved.

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.

Stepwise self-assembly of a tripeptide from molecular dimers to supramolecular beta-sheets in crystals and amyloid-like fibrils in the solid state

The terminally protected tripeptide Boc-Ala(1)-Leu(2)-Ala(3)-OMe 1 forms antiparallel hydrogen-bonded dimers of two different conformers in the asymmetric unit and the individual dimers then self-associate to form supramolecular beta-sheet structures in crystals and amyloid-like fibrils in the solid state.

The role of the disulfide bond in amyloid-like fibrillogenesis in a model peptide system

The role of the disulfide bond in amyloid-like fibrillogenesis in a model peptide system Apurba Kumar Das,(a) Michael G. B. Drew,(b) Debasish Haldar(a) and Arindam Banerjee*(a) (a)Department of Biological Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India. E-mail: bcab@mahendra.iacs.res.in; Fax: +91-33-2473-2805 b School of Chemistry, The University of Reading, Whiteknights, Reading, UK RG6 6AD Received 28th June 2005, Accepted 20th July 2005 First published as an Advance Article on the web 11th August 2005

alpha-Aminoisobutyric acid modified protected analogues of beta-amyloid residue 17-20: a change from sheet to helix

The strong intermolecular interactions mediated by short hydrophobic sequences (e.g., 17-20, -L-Leu-L-Val-L-Phe-L-Phe-) in the middle of A beta are known to play a crucial role in the neuropathology of Alzheimer's disease. FTIR, TEM and Congo red binding studies indicated that a series of L-Ala substituted terminally protected peptides related to the sequence 17-20 of the beta-amyloid peptide, adopted D-sheet conformations. However, the Aib-modified analogues disrupt the D-sheet structure and switch over to a 310 helix with increasing number of Aib residues. X-ray crystallography shed some light on the change from sheet to helix at atomic resolution. (c) 2006 Elsevier Ltd. All rights reserved.

Self-assembly of beta-turn forming synthetic tripeptides into supramolecular beta-sheets and amyloid-like fibrils in the solid state

We have described here the self-assembling properties of the synthetic tripeptides Boc-Ala(1)-Aib(2) -Val (3)-OMe 1, BocAla(l)-Aib(2)-Ile(3)-OMe 2 and Boc-Ala(l)-Gly(2)-Val(3)-OMe 3 (Aib=alpha-arnino isobutyric acid, beta-Ala=beta-alanine) which have distorted beta-turn conformations in their respective crystals. These turn-forming tripeptides self-assemble to form supramolecular beta-sheet structures through intermolecular hydrogen bonding and other noncovalent interactions. The scanning electron micrographs of these peptides revealed that these compounds form amyloid-like fibrils, the causative factor for many neurodegenerative diseases including Alzheimer's disease, Huntington's disease and Prion-related encephalopathies. (C) 2004 Elsevier Ltd. All rights reserved.

Water-soluble tripeptide A beta(9-11) forms amyloid-like fibrils and exhibits neurotoxicity

A water-soluble, hydrophilic tripeptide GYE, having sequence identity with the N-terminal segment of amyloid peptides A,beta(9-11), upon self-association exhibits amyloid-like fibrils and significant neurotoxicity towards the Neuro2A cell line. However, the tripeptides GFE and GWE, in which the centrally located tyrosine residue has been replaced by phenylalanine or tryptophan, fail to show amyloidogenic behavior and exhibit little or no neurotoxicity.

A short water-soluble self-assembling peptide forms amyloid-like fibrils

A water-soluble tripeptide Val-Ile-Ala (VIA) 1, bearing sequence identity with the C-terminal portion of the Alzheimer A beta-peptide (A beta(40-42)), self-assembles, in crystalline form, to produce an intermolecularly hydrogen bonded supramolecular beta-sheet structure which self-associates to form straight, unbranched nanofibrils exhibiting amyloid-like behavior; in contrast, the synthetic tripeptide Ala-Val-Ile (AVI) 2 self-assembles to produce a beta-sheet structure that forms branched nanofibrils which do not show any characteristic features of amyloid-like fibrils.

Long-term monounsaturated fatty acid diets reduce platelet aggregation in healthy young subjects

The aim of the present study was to compare the response of a range of atherogenic and thrombogenic risk markers to two dietary levels of saturated fatty acid (SFA) substitution with monounsaturated fatty acids (MUFA) in students living in a university hall of residence. Although the benefits of such diets have been reported for plasma lipoproteins in high-risk groups, more needs to be known about effects of more modest SFA-MUFA substitutions over the long term and in young healthy adults. In a parallel design over 16 weeks, fifty-one healthy young subjects were randomised to one of two diets: (1) a moderate-MUFA diet in which 16 g dietary SFA/100 g total fatty acids were substituted with MUFA (n 25); (2) a high-MUFA diet in which 33 g dietary SFA/100 g total fatty acids were substituted with MUFA (n 26). All subjects followed an 8-week run-in diet (reference diet), with a fatty acid composition close to the UK average values. There were no differences in plasma lipid responses between the two diets over 16 weeks of the study with similar reductions in total cholesterol (P<0.001) and LDL-cholesterol (P<0.01) in both groups; a small but significant reduction in HDL-cholesterol was also observed in both groups (P<0.01). Platelet responses to ADP (P<0.01) and arachidonic acid (P<0.05) differed with time on the two diets; at 16 weeks, platelet aggregatory response to ADP was significantly lower on the high-MUFA than the moderate-MUFA (P<0.01) diet; ADP responses were also significantly lower within this group at 8 (P< 0.05) and 16 (P< 0.01) weeks compared with baseline. There were no differences in fasting factor VII activity (factors VIII and VIIag), fibrinogen concentration or tissue-type plasminogen activator activity between the diets. There were no differences in postprandial factor VIII responses to a standard meal (area under the curve) between the diets after 16 weeks, but postprandial factor VIII response was lower than on the high-MUFA diet compared with baseline (P<0.01). In conclusion, a high-MUFA diet sustains potentially beneficial effects on platelet aggregation and postprandial activation of factor VII. Moderate or high substitution of MUFA for SFA achieves similar reductions in fasting blood lipids in young healthy subjects.

Sulfation of genistein alters its antioxidant properties and its effect on platelet aggregation and monocyte and endothelial function

Soy isoflavones have been extensively studied because of their possible benefits to human health. Genistein, the major isoflavone aglycone, has received most attention; however, it undergoes extensive metabolism (e.g. conjugation with sulfuric acid) in the gut and liver, which may affect its biological proper-ties. This study investigated the antioxidant activity and free radical-scavenging properties of genistein, genistein-4'-sulfate and genistein-4'-7-disulfate as well as their effect on platelet aggregation and monocyte and endothelial function. Electron spin resonance spectroscopy (ESR) and spin trapping data and other standard antioxidant assays indicated that genistein is a relatively weak antioxidant compared to quercetin and that its sulfated metabolites are even less effective. Furthermore, genistein-4'-sulfate was less potent than genistem, and genistein-4'-7-disulfate even less potent, at inhibiting collagen-induced platelet aggregation, nitric oxide (NO) production by macrophages, and secretion by primary human endothelial cells of monocyte chemoattractant protein 1 (MCP-1), intercellular adhesion molecule 1 (ICAM-1) and vascular cell adhesion molecule 1 (VCAM-1). The current data suggest that sulfation of genistein, with the associated loss of hydroxyl groups, decreases its antioxidant activity and its effect on platelet aggregation, inflammation, cell adhesion and chemotaxis. (C) 2004 Elsevier B.V All rights reserved.

Effect of genistein and daidzein on platelet aggregation and monocyte and endothelial function

There has been much recent interest in the cardiovascular benefits of dietary isoflavones. The aim of the present in vitro studies was to investigate potential anti-thrombogenic and anti-atherogenic effects of the isoflavones genistein and daidzein in platelets, macrophages and endothelial cells. Pre-treatment with either isoflavone inhibited collagen-induced platelet aggregation in a dose-dependent manner. In a macrophage cell line (RAW 264-7) activated with interferon gamma plus lipopolysaccharide, both isoflavones were found to inhibit NO production and tumour necrosis factor alpha (TNF-alpha) secretion dose-dependently, but they did not affect mRNA levels for inducible nitric oxide synthase and cyclo-oxygenase-2. Both isoflavones also dose-dependently decreased monocyte chemoattractant protein-1 secretion induced by TNF-alpha in human umbilical vein endothelial cells. Compared with daidzein, genistein exerted greater inhibitory effects for all parameters studied. The present data contributes to our knowledge on the molecular mechanisms by which isoflavones may protect against coronary artery disease. Further studies are required to determine whether the effects of isoflavones observed in the current in vitro studies are relevant to the aetiology of coronary artery disease in vivo.

Alignment of a model amyloid peptide fragment in bulk and at a solid surface

The alignment of model amyloid peptide YYKLVFFC is investigated in bulk and at a solid surface using a range of spectroscopic methods employing polarized radiation. The peptide is based on a core sequence of the amyloid beta (A beta) peptide, KLVFF. The attached tyrosine and cysteine units are exploited to yield information on alignment and possible formation of disulfide or dityrosine links. Polarized Raman spectroscopy on aligned stalks provides information on tyrosine orientation, which complements data from linear dichroism (LD) on aqueous solutions subjected to shear in a Couette cell. LD provides a detailed picture of alignment of peptide strands and aromatic residues and was also used to probe the kinetics of self-assembly. This suggests initial association of phenylalanine residues, followed by subsequent registry of strands and orientation of tyrosine residues. X-ray diffraction (XRD) data from aligned stalks is used to extract orientational order parameters from the 0.48 nm reflection in the cross-beta pattern, from which an orientational distribution function is obtained. X-ray diffraction on solutions subject to capillary flow confirmed orientation in situ at the level of the cross-beta pattern. The information on fibril and tyrosine orientation from polarized Raman spectroscopy is compared with results from NEXAFS experiments on samples prepared as films on silicon. This indicates fibrils are aligned parallel to the surface, with phenyl ring normals perpendicular to the surface. Possible disulfide bridging leading to peptide dimer formation was excluded by Raman spectroscopy, whereas dityrosine formation was probed by fluorescence experiments and was found not to occur except under alkaline conditions. Congo red binding was found not to influence the cross-beta XRD pattern.