A triply templated artificial beta-sheet.

This paper describes the design, synthesis, and structural evaluation of a compound (4) comprising three molecular templates and a peptide strand that mimics a three-stranded protein beta-sheet. Two of the templates mimic the hydrogen-bonding functionality of peptide beta-strands and serve as the top and bottom strands by embracing the peptide strand, which is located in the middle of the sheet. The remaining template holds the three strands next to each other. The synthesis of artificial beta-sheet 4 begins with the bottom template and involves the sequential addition of the middle and top strands. (1)H NMR chemical shift and NOE studies establish that this compound folds to adopt a hydrogen-bonded beta-sheetlike structure in CDCl(3) solution. Chemical shift studies indicate that triply stranded artificial beta-sheet 4 is more tightly folded than its smaller doubly stranded homologue, artificial beta-sheet 1.     

Controlling the morphology of cross beta-sheet assemblies by rational design

Low molecular weight peptidomimetics with simple amphiphilic sequences can help to elucidate the structures of cross beta-sheet assemblies, such as amyloid fibrils. The peptidomimetics described herein comprise a dibenzofuran template, two peptide strands made up of alternating hydrophilic and hydrophobic residues, and carboxyl termini, each of which can be varied to probe the structural requirements for beta-sheet self-assembly processes. The dibenzofuran template positions the strands approximately 10 A apart, allowing corresponding hydrophobic side chains in the strands to pack into a collapsed U-shaped structure. This conformation is stabilized by hydrophobic interactions, not intramolecular hydrogen bonds. Intermolecular stacking of the collapsed peptidomimetics, enabled by intermolecular hydrogen bonding and hydrophobic interactions, affords 25-27 A wide protofilaments having a cross beta-sheet structure. Association of protofilaments, mediated by the dibenzofuran substructures and driven by the hydrophobic effect, affords 50-60 A wide filaments. These widths can be controlled by changing the length of the peptide strands. Further assembly of the filaments into fibrils or ribbons can be controlled by modification of the template, C-terminus, and buffer ion composition.     

Thermodynamic analysis of autonomous parallel beta-sheet formation in water

We report the first thermodynamic analysis of parallel beta-sheet formation in a model system that folds in aqueous solution. NMR chemical shifts were used to determine beta-sheet population, and van't Hoff anaysis provided thermodynamic parameters. Our approach relies upon the d-prolyl-1,1-dimethyl-1,2-diaminoethane unit to promote parallel beta-sheet formation between attached peptide strands. The development of a macrocyclic reference molecule to provide chemical shift data for the fully folded state was crucial to the quantitative anaylsis.     

An unnatural amino acid that induces beta-sheet folding and interaction in peptides

This paper introduces a unique amino acid that can readily be incorporated into peptides to make them fold into beta-sheetlike structures that dimerize through beta-sheet interactions. This new amino acid, Orn(i-PrCO-Hao), consists of an ornithine residue with the beta-strand-mimicking amino acid Hao [J. Am. Chem. Soc. 2000, 122, 7654-7661] attached to its side chain. When Orn(i-PrCO-Hao) is incorporated into a peptide, or appended to its N-terminus, the Hao group hydrogen bonds to the three subsequent residues to form a beta-sheetlike structure. The amino acid Orn(i-PrCO-Hao) is readily used in peptide synthesis as its Fmoc derivative, Fmoc-Orn(i-PrCO-Hao)-OH (3). Fmoc-Orn(i-PrCO-Hao)-OH behaves like a regular amino acid in peptide synthesis and was uneventfully incorporated into the peptide o-anisoyl-Val-Orn(i-PrCO-Hao)-Phe-Ile-Leu-NHMe (4) through standard automated Fmoc solid-phase peptide synthesis, with DIC and HOAt as the coupling agent for Fmoc-Orn(i-PrCO-Hao)-OH and o-anisic acid and HATU as the coupling agent for all other couplings. A second synthetic strategy was developed to facilitate the preparation of peptides with N-terminal Orn(i-PrCO-Hao) residues, which avoids the need for the preparation of Fmoc-Orn(i-PrCO-Hao)-OH. In this strategy, Boc-Orn(Fmoc)-OH is used as the penultimate amino acid in the peptide synthesis, and i-PrCO-Hao-OH (2) is used as the final amino acid. N-Terminal Orn(i-PrCO-Hao) peptide H-Orn(i-PrCO-Hao)-Phe-Ile-Leu-NHMe.TFA (5) was prepared in a fashion similar to that for 4, using DIC and HOAt as the coupling agent for i-PrCO-Hao-OH and HATU as the coupling agent for all other couplings. 1H NMR transverse-ROESY, coupling constant, and chemical shift studies establish that peptide 4 forms a dimeric beta-sheetlike structure in CDCl3 solution. The 1H NMR studies also suggest that the ornithine unit adopts a well-defined turn conformation. Analogous 1H NMR studies of peptide 5 indicate that this TFA salt folds but does not dimerize in CD3OD solution. Collectively, these synthetic and spectroscopic studies establish that the amino acid Orn(i-PrCO-Hao) induces beta-sheet structure and interactions in peptides in suitable organic solvents. Unlike the Hao amino acid, which acts as a prosthetic to replace three residues of the peptide strand, the Orn(i-PrCO-Hao) amino acid acts as a splint that helps enforce a beta-sheetlike structure without replacing the residues and their side chains. This feature of Orn(i-PrCO-Hao) is important, because it allows the creation of beta-sheet structure with minimal perturbation of the peptide sequence.     

Diacid linkers that promote parallel beta-sheet secondary structure in water

We report the development of diacid units that promote formation of a two-stranded parallel beta-sheet secondary structure between peptide segments attached via their N-termini. These linker units are formed by attaching glycine to one carboxyl group of cis-1,2-cyclohexanedicarboxylic acid (CHDA). Parallel sheet formation in water is observed when l-residue strands are attached to the CHDA-Gly unit with either of the two absolute configurations.     

A noncovalent approach to antiparallel beta-sheet formation

Four tripeptide chains, when attached to the same end of a hydrogen-bonded duplex (1.2) with the unsymmetrical, complementary sequences of ADAA/DADD, have been brought into proximity, leading to the formation of four hybrid duplexes, 1a.2a, 1a.2b, 1b.2a, and 1b.2b, each of which contains a two-stranded beta-sheet segment. The extended conformations of the peptide chains were confirmed by 1D and 2D NMR. The peptide strands stay registered through hydrogen bonding and the beta-sheets are stabilized by side chain interactions. Two-dimensional NMR data also indicate that the duplex template prevents further aggregation in the peptide segment. When the peptide chains are attached to the two different termini of the duplex template, NMR studies show the presence of a mixture with no clearly defined conformations. In the absence of the duplex template, the tripeptides are found to associate randomly. Finally, isothermal titration calorimetry studies revealed that the hybrid duplex 1a.2a was more stable than either the duplex template or the peptides alone.     

Elasticity of crystalline beta-sheet monolayers

Designed amphiphilic beta-sheet peptides with the sequence Pro-Glu-(Phe-Glu)(n)-Pro (n = 2-7) were previously shown by grazing incidence X-ray diffraction (GIXD), to form ordered two-dimensional (2-D) monolayer structures at interfaces induced by the proline residues at peptide termini. The GIXD diffraction pattern was modeled with two coexisting lattice arrangements, suggesting structural flexibility exhibited in the multiple ways by which beta-strands and their amino acid side chains pack into ordered 2-D structures. Here, we find by in-situ GIXD measurements that the ordered beta-sheet assemblies may undergo a quasi-reversible compression and expansion cycle at the air-water interface. The diffraction measurements indicate that on compression the repeat distance that corresponds to the long axes of the peptide strands may decrease by up to 37% in length. Upon expansion the compressed beta-sheet assemblies revert elastically to their original conformation. The interstrand repeat distance along the peptide hydrogen bonds apparently does not change along the film compression and expansion. Based on the GIXD data, at surface pressures higher than approximately 3 mN/m, beyond the peptide limiting area per molecule, the compressibility is 7.4 +/- 0.6 m/N. The out-of-plane Bragg rod diffraction patterns imply that in the compressed state the beta-strands buckle up in reaction to the increase in surface pressure. At low surface pressure, the 2-D compressibility of the crystalline beta-sheet was estimated at approximately 32 m/N attributed to interdomain rearrangements.     

Structural analysis of alanine tripeptide with antiparallel and parallel beta-sheet structures in relation to the analysis of mixed beta-sheet structures in Samia cynthia ricini silk protein fiber using solid-state NMR spectroscopy

The structural analysis of natural protein fibers with mixed parallel and antiparallel beta-sheet structures by solid-state NMR is reported. To obtain NMR parameters that can characterize these beta-sheet structures, (13)C solid-state NMR experiments were performed on two alanine tripeptide samples: one with 100% parallel beta-sheet structure and the other with 100% antiparallel beta-sheet structure. All (13)C resonances of the tripeptides could be assigned by a comparison of the methyl (13)C resonances of Ala(3) with different [3-(13)C]Ala labeling schemes and also by a series of RFDR (radio frequency driven recoupling) spectra observed by changing mixing times. Two (13)C resonances observed for each Ala residue could be assigned to two nonequivalent molecules per unit cell. Differences in the (13)C chemical shifts and (13)C spin-lattice relaxation times (T(1)) were observed between the two beta-sheet structures. Especially, about 3 times longer T(1) values were obtained for parallel beta-sheet structure as compared to those of antiparallel beta-sheet structure, which could be explicable by the difference in the hydrogen-bond networks of both structures. This very large difference in T(1) becomes a good measure to differentiate between parallel or antiparallel beta-sheet structures. These differences in the NMR parameters found for the tripeptides may be applied to assign the parallel and antiparallel beta-sheet (13)C resonances in the asymmetric and broad methyl spectra of [3-(13)C]Ala silk protein fiber of a wild silkworm, Samia cynthia ricini.     

Macrocyclic beta-sheet peptides that mimic protein quaternary structure through intermolecular beta-sheet interactions

This paper reports the design, synthesis, and characterization of a family of cyclic peptides that mimic protein quaternary structure through beta-sheet interactions. These peptides are 54-membered-ring macrocycles comprising an extended heptapeptide beta-strand, two Hao beta-strand mimics [JACS 2000, 122, 7654] joined by one additional alpha-amino acid, and two delta-linked ornithine beta-turn mimics [JACS 2003, 125, 876]. Peptide 3a, as the representative of these cyclic peptides, contains a heptapeptide sequence (TSFTYTS) adapted from the dimerization interface of protein NuG2 [PDB ID: 1mio]. 1H NMR studies of aqueous solutions of peptide 3a show a partially folded monomer in slow exchange with a strongly folded oligomer. NOE studies clearly show that the peptide self-associates through edge-to-edge beta-sheet dimerization. Pulsed-field gradient (PFG) NMR diffusion coefficient measurements and analytical ultracentrifugation (AUC) studies establish that the oligomer is a tetramer. Collectively, these experiments suggest a model in which cyclic peptide 3a oligomerizes to form a dimer of beta-sheet dimers. In this tetrameric beta-sheet sandwich, the macrocyclic peptide 3a is folded to form a beta-sheet, the beta-sheet is dimerized through edge-to-edge interactions, and this dimer is further dimerized through hydrophobic face-to-face interactions involving the Phe and Tyr groups. Further studies of peptides 3b-3n, which are homologues of peptide 3a with 1-6 variations in the heptapeptide sequence, elucidate the importance of the heptapeptide sequence in the folding and oligomerization of this family of cyclic peptides. Studies of peptides 3b-3g show that aromatic residues across from Hao improve folding of the peptide, while studies of peptides 3h-3n indicate that hydrophobic residues at positions R3 and R5 of the heptapeptide sequence are important in oligomerization.     

Assembly of triple-stranded beta-sheet peptides at interfaces

A 30-residue peptide, BS30, which incorporates two proline residues to induce reverse turns, was designed to form a triple-stranded beta-sheet monolayer at the air-water interface. To discern the structural role of proline, a second peptide, BS30G, identical to BS30 but with glycine residues replacing proline, was prepared and examined in parallel fashion. Surface pressure-molecular area isotherms indicated a limiting area per molecule (ca. 460 A(2)) for BS30 that corresponds well to that estimated from the known dimensions of crystalline beta-sheet monolayers (492 A(2)). Comparable measurements on BS30G yielded a smaller molecular area (380 A(2)). Grazing incidence X-ray diffraction measurements performed on the BS30 monolayer at nominal area per molecule of 500 A(2), exhibited two Bragg peaks corresponding to 4.79 and 34.9 A spacings, consistent with formation of triple-stranded beta-sheet structures that assemble into two-dimensional crystallites at the air-water interface. Visualized by Brewster angle microscopy, BS30 monolayers displayed uniform, solidlike domains, whereas BS30G appeared to be disordered.     

Self-assembling diblock copolymers of poly[N-(2-hydroxypropyl)methacrylamide] and a beta-sheet peptide

The self-assembly of hybrid diblock copolymers composed of poly(HPMA) and beta-sheet peptide P11 (CH(3)CO-QQRFQWQFEQQ-NH(2)) blocks was investigated. Copolymers were synthesized via thiol-maleimide coupling reaction, by conjugation of semitelechelic poly(HPMA)-SH with maleimide-modified beta-sheet peptide. As expected, CD and CR binding studies showed that the peptide block imposed its beta-sheet structural arrangement on the structure of diblock copolymers. TEM and AFM proved that peptide and these copolymers had the ability to self-assemble into fibrils.     

Effects of turn stability on the kinetics of refolding of a hairpin in a beta-sheet

As part of our continuing study of the effects of the turn sequence on the conformational stability as well as the mechanism of folding of a beta-sheet structure, we have undertaken a parallel investigation of the solution structure, conformational stability, and kinetics of refolding of the beta-sheet VFIVDGOTYTEV(D)PGOKILQ. The latter peptide is an analogue of the original Gellman beta-sheet VFITS(D)PGKTYTEV(D)PGOKILQ, wherein the TS(D)PGK turn sequence in the first hairpin has been replaced by VDGO. Thermodynamics studies revealed comparable conformational stability of the two peptides. However, unlike the Gellman peptide, which showed extremely rapid refolding of the first hairpin, early kinetic events associated with the refolding of the corresponding hairpin in the VDGO mutant were found to be significantly slower. A detailed study of the conformation of the modified peptide suggested that hydrophobic interactions might be contributing to its stability. Accordingly, we surmise that the early kinetic events are sensitive to whether the formation of the hairpin is nucleated at the turn or by sequestering of the hydrophobic residues across the strand, before structural rearrangements to produce the nativelike topology. Nucleation of the hairpin at the turn is expected to be intrinsically rapid for a strong turn. However, if the process must involve collapse of hydrophobic side chains, the nucleation should be slower as solvent molecules must be displaced to sequester the hydrophobic residues. These findings reflect the contribution of different forces toward nucleation of hairpins in the mechanism of folding of beta-sheets.     

Vibrational coupling, isotopic editing, and beta-sheet structure in a membrane-bound polypeptide

The N-acetylated hexapeptide WLLLLL (AcWL5) partitions into lipid membranes and is believed to assemble into an antiparallel beta-sheet. As a test of this structural assignment, the peptide bonds of residues 2-6 were labeled with (13)C and allowed to adsorb onto a supported lipid membrane. Peptides bound to the membrane were examined for evidence of coupling between the labeled vibrational modes in adjacent beta-strands with internal reflection infrared spectroscopy. Experimental results indicate that the amide I absorption band in D(2)O (i.e., amide I') attributable to (13)C is selectively enhanced when the label is at any one of several positions along the peptide backbone. Simulations employing an excitonic model with through-bond and through-space interactions were performed on AcWL5 models in parallel and antiparallel beta-sheet configurations. The simulations yield spectra in good agreement with the experimental results, accounting for the enhancement of both (13)C band intensities and band frequencies. They also yield insight into the physical origin and structure selectivity of the distinctive amide I' band shapes that arise in isotopically edited spectra. It is concluded that the beta-sheet formed by membrane-bound AcWL5 is indeed antiparallel, and the enhancement of (13)C bands in the infrared spectra of these peptides is caused by both interstrand and intrastrand coupling to (12)C modes.     

Cyclic modular beta-sheets

The development of peptide beta-hairpins is problematic, because folding depends on the amino acid sequence and changes to the sequence can significantly decrease folding. Robust beta-hairpins that can tolerate such changes are attractive tools for studying interactions involving protein beta-sheets and developing inhibitors of these interactions. This paper introduces a new class of peptide models of protein beta-sheets that addresses the problem of separating folding from the sequence. These model beta-sheets are macrocyclic peptides that fold in water to present a pentapeptide beta-strand along one edge; the other edge contains the tripeptide beta-strand mimic Hao [JACS 2000, 122, 7654] and two additional amino acids. The pentapeptide and Hao-containing peptide strands are connected by two delta-linked ornithine (deltaOrn) turns [JACS 2003, 125, 876]. Each deltaOrn turn contains a free alpha-amino group that permits the linking of individual modules to form divalent beta-sheets. These "cyclic modular beta-sheets" are synthesized by standard solid-phase peptide synthesis of a linear precursor followed by solution-phase cyclization. Eight cyclic modular beta-sheets 1a-1h containing sequences based on beta-amyloid and macrophage inflammatory protein 2 were synthesized and characterized by 1H NMR. Linked cyclic modular beta-sheet 2, which contains two modules of 1b, was also synthesized and characterized. 1H NMR studies show downfield alpha-proton chemical shifts, deltaOrn delta-proton magnetic anisotropy, and NOE cross-peaks that establish all compounds but 1c and 1g to be moderately or well folded into a conformation that resembles a beta-sheet. Pulsed-field gradient NMR diffusion experiments show little or no self-association at low (相似文献   

First crystallographic signature of amyloid-like fibril forming beta-sheet assemblage from a tripeptide with non-coded amino acids

The model tripeptide Boc-beta-Ala(1)-Aib(2)-beta-Ala(3)-OMe 1 [beta-Ala: beta-alanine, Aib: alpha-aminoisobutyric acid] forms an infinite parallel beta-sheet structure through intermolecular interactions in single crystals and from the SEM diagram of this peptide, it is evident that the compound has fibrillar morphology, a characteristic of neurodegenerative disease causing amyloid aggregate.     

Characterization of intermolecular beta-sheet peptides by mass spectrometry and hydrogen isotope exchange

The self-assembly of beta-sheet peptide domains resulting in the formation of fibrillar aggregates (amyloids) is a feature of various neurodegenerative disorders. In order to evaluate mass spectrometric methods for the characterization of intermolecular beta-sheet structures the hydrogen/deuterium exchange behaviour of model peptides DPKGDPKG-(VT)(n)-GKGDPKPD-amide (n = 3,4,5,6,7,8), (VT)(n)-peptides, composed of a central beta-sheet-forming domain and N- and C-terminal nonstructured octapeptide sequences, was measured by electrospray ionization mass spectrometry (ESI-MS) and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). The kinetic analysis of the hydrogen/deuterium exchange (HX) shows that intermolecular beta-sheet structures contain slowly exchanging protons (k 相似文献   

A Designed Three‐Stranded β‐Sheet in an α/β Hybrid Peptide

The incorporation of β‐amino acid residues into the antiparallel β‐strand segments of a multi‐stranded β‐sheet peptide is demonstrated for a 19‐residue peptide, Boc‐LV^βFV^DPGL^βFVVL^DPGLVL^βFVV‐OMe (BBH19). Two centrally positioned ^DPro–Gly segments facilitate formation of a stable three‐stranded β‐sheet, in which β‐phenylalanine (^βPhe) residues occur at facing positions 3, 8 and 17. Structure determination in methanol solution is accomplished by using NMR‐derived restraints obtained from NOEs, temperature dependence of amide NH chemical shifts, rates of H/D exchange of amide protons and vicinal coupling constants. The data are consistent with a conformationally well‐defined three‐stranded β‐sheet structure in solution. Cross‐strand interactions between ^βPhe3/^βPhe17 and ^βPhe3/Val15 residues define orientations of these side‐chains. The observation of close contact distances between the side‐chains on the N‐ and C‐terminal strands of the three‐stranded β‐sheet provides strong support for the designed structure. Evidence is presented for multiple side‐chain conformations from an analysis of NOE data. An unusual observation of the disappearance of the Gly NH resonances upon prolonged storage in methanol is rationalised on the basis of a slow aggregation step, resulting in stacking of three‐stranded β‐sheet structures, which in turn influences the conformational interconversion between type I′ and type II′ β‐turns at the two ^DPro–Gly segments. Experimental evidence for these processes is presented. The decapeptide fragment Boc‐LV^βFV^DPGL^βFVV‐OMe (BBH10), which has been previously characterized as a type I′ β‐turn nucleated hairpin, is shown to favour a type II′ β‐turn conformation in solution, supporting the occurrence of conformational interconversion at the turn segments in these hairpin and sheet structures.     

Design and construction of an open multistranded beta-sheet polypeptide stabilized by a disulfide bridge

The design and characterization of an open eight-stranded beta-sheet in a synthetic, 2-fold symmetric 70-residue peptide is described. The design strategy involves the generation of a 35-residue four-stranded beta-sheet peptide in which successive hairpins are nucleated by appropriately positioned (D)Pro-Xxx sequences. Oxidative dimerization using a single Cys residue positioned at the center of the C-terminal strand results in a disulfide-bridged eight-stranded structure. Nuclear Overhauser effects firmly establish an eight-stranded beta-sheet in methanol. In water, the outer strands are frayed, but a well-defined four-stranded beta-sheet stabilized by a disulfide bridge and a hydrophobic cluster is determined from NMR data. Comparison of the precursor peptide with the disulfide-bridged dimer reveals considerable enhancement of beta-sheet content in the latter, suggesting that the disulfide cross-link is an effective strategy for the stabilization of beta-sheets.     

New heterocyclic beta-sheet ligands with peptidic recognition elements

A detailed and comprehensive overview is presented about the design, modeling, and synthesis, as well as spectroscopic characterization, of a new class of beta-sheet ligands. The characteristic feature of these compounds is a peptidic chimeric structure formed from a specific combination of aminopyrazolecarboxylic acids with naturally occurring alpha-amino acids. These hybrid peptides are designed with the aid of molecular modeling to exist mainly in an extended conformation. All their hydrogen bond donors and acceptors can be aligned at the bottom face in such a way that a perfect complementarity toward beta-sheets is obtained. Thus the aminopyrazoles impart rigidity and a highly efficient DAD sequence for the recognition of whole dipeptide fragments, whereas the natural alpha-amino acids are designed to mimick recognition sites in proteins, ultimately leading to sequence-selective protein recognition. The synthetic protocols either rely upon solution phase peptide coupling with a PMB protecting group strategy or solid-phase peptide coupling based on the Fmoc strategy, using the same protecting group. In solution, a key building block was prepared by catalytic reduction of a nitropyrazolecarboxylic acid precursor. Subsequently, it was (N-1)-protected with a PMB group, and elongated by HCTU- or T3P-assisted peptide coupling with dipeptide fragments, followed by PyClop-assisted coupling with another nitropyrazolecarboxylic acid building block. Final simultaneous deprotection of all PMB groups with hot TFA completed the high-yield protocol, which works racemization-free. After preparing a similar key building block with an Fmoc protection at N-3, we developed a strategy suitable for automated synthesis of larger hybrid ligands on a peptide synthesizer. Attachment of the first amino acid to a polystyrene resin over the Sieber amide linker is followed by an iterative sequence consisting of Fmoc deprotection with piperidine and subsequent coupling with natural alpha-amino acid via HATU/HOAt. High yields of free hybrid peptides are obtained after mild acidic cleavage from the resin, followed by deprotection of the PMB groups with hot TFA. The new aminopyrazole peptide hybrid compounds were characterized by various spectroscopic measurements including CD spectra, VT, and ROESY NMR experiments. All these accumulated data indicate the absence of any intramolecular hydrogen bonds and strongly support an extended conformation in solution, ideal for docking on to solvent-exposed beta-sheets in proteins. Initial results from aggregation tests of pathological proteins with these and related ligands look extremely promising.     

Laminated morphology of nontwisting beta-sheet fibrils constructed via peptide self-assembly

A synthetic peptide has been de novo designed that self-assembles into beta-sheet fibrils exhibiting a nontwisted, stacked morphology. The stacked morphology is constituted by 2.5 nm wide filaments that laterally associate to form flat fibril laminates exceeding 50 nm in width and micrometers in length. The height of each fibril is limited to the length of exactly one peptide monomer in an extended beta-strand conformation, approximately 7 nm. Once assembled, these highly ordered, 2-D structures are stable over a wide range of pH and temperature and exhibit characteristics similar to those of amyloid fibrils. Furthermore, the rate of assembly and degree of fibril lamination can be controlled with kinetic parameters of pH and temperature. Finally, the presence of a diproline peptide between two beta-sheet-forming strands in the peptide sequence is demonstrated to be an important factor in promoting the nontwisting, laminated fibril morphology.