A generic statistical mechanical model is presented for the selfassembly of chiral rod-like units, such as ␤-sheet-forming peptides, into helical tapes, which with increasing concentration associate into twisted ribbons (double tapes), fibrils (twisted stacks of ribbons), and fibers (entwined fibrils). The finite fibril width and helicity is shown to stem from a competition between the free energy gain from attraction between ribbons and the penalty because of elastic distortion of the intrinsically twisted ribbons on incorporation into a growing fibril. Fibers are stabilized similarly. The behavior of two rationally designed 11-aa residue peptides, P 11-I and P11-II, is illustrative of the proposed scheme. P11-I and P11-II are designed to adopt the ␤-strand conformation and to selfassemble in one dimension to form antiparallel ␤-sheet tapes, ribbons, fibrils, and fibers in well-defined solution conditions. The energetic parameters governing self-assembly have been estimated from the experimental data using the model. The 8-nm-wide fibrils consist of eight tapes, are extremely robust (scission energy Ϸ200 kBT), and sufficiently rigid (persistence length lfibril Ϸ 20 -70 m) to form nematic solutions at peptide concentration c Ϸ 0.9 mM (volume fraction Ϸ0.0009 vol͞vol), which convert to self-supporting nematic gels at c > 4 mM. More generally, these observations provide a new insight into the generic self-assembling properties of ␤-sheet-forming peptides and shed new light on the factors governing the structures and stability of pathological amyloid fibrils in vivo. The model also provides a prescription of routes to novel macromolecules based on a variety of self-assembling chiral units, and protocols for extraction of the associated energy changes.P rospects for the large-scale production of low-cost peptides by genetic engineering (1) open up new opportunities for exploiting protein-like self-assembly as a route to novel biomolecular materials (2-5). In this context, the small-oligopeptide route has distinct processing advantages over the use of longer polypeptides. Previously, we have demonstrated that oligopeptides can be designed to self-assemble into micrometer-long ␤-sheet tapes (6). We now wish to show that, as a consequence of the amino acid chirality, an entire hierarchy of twisted self-assembling macromolecular structures is accessible, with tapes as the most primitive form: ribbons, fibrils, and fibers. These polymers are shown to give rise to nematic fluids and gels at concentrations determined by the characteristic flexibility and length of each type of polymer.The type of molecular assembly we discuss and exemplify here arises not only in the context of desirable engineered biomaterials, but also in pathological self-assembly of mis-folded proteins, when the aggregated assemblies are referred-to as ''amyloids.'' A very wide class of proteins may be induced into producing the tapefibril-fiber sequence of structures (7) We present a theoretical model that enables the morphology and properties of thes...

Molecular self-assembly is becoming an increasingly popular route to new supramolecular structures and molecular materials. The inspiration for such structures is commonly derived from self-assembling systems in biology. Here we show that a biological motif, the peptide beta-sheet, can be exploited in designed oligopeptides that self-assemble into polymeric tapes and with potentially useful mechanical properties. We describe the construction of oligopeptides, rationally designed or based on segments of native proteins, that aggregate in suitable solvents into long, semi-flexible beta-sheet tapes. These become entangled even at low volume fractions to form gels whose viscoelastic properties can be controlled by chemical (pH) or physical (shear) influences. We suggest that it should be possible to engineer a wide range of properties in these gels by appropriate choice of the peptide primary structure.

The hierarchical self-assembly of rationally designed synthetic peptides into beta-sheet tapes, ribbons, fibrils, and fibers opens up potentially useful routes to soft-solidlike materials such as hydrogels, organogels, or liquid crystals. Here, it is shown how incorporation of Glu (-CH(2)CH(2)COOH) or Orn (-CH(2)CH(2)CH(2)NH(2)) into the primary structure of an 11 amino acid peptide enables self-assembly to be rapidly (seconds) and reversibly controlled by simply changing pH. Solutions of monomeric peptide, typically at concentrations in excess of 0.003 v/v, can be switched within seconds to, for example, nematic gel states comprised of interconnected orientationally ordered arrays of fibrils or vice versa. This is to be compared with the lyophilized peptide dissolution route to nematic fluids and gels which is impracticably long, taking many hours or even days. An important design principle, that stabilization of fibrillar dispersions requires of the order of one unit of net positive or negative charge per peptide molecule, is first demonstrated and then used to design an 11 amino acid peptide P(11)-3 (CH(3)CO-Gln-Gln-Arg-Phe-Gln-Trp-Gln-Phe-Gln-Gln-Gln-NH(2)) whose self-assembly behavior is independent of pH (1 < pH < 10). pH control is then incorporated by appropriately positioning Glu or Orn side chains so that the peptide-peptide free energy of interaction in the tapelike substructure is strongly influenced by direct electrostatic forces between gamma-COO(-) in Glu(-) or delta-NH(3)(+) in Orn(+), respectively. This design principle is illustrated by the behavior of two peptides: P(11)-4 (CH(3)CO-Gln-Gln-Arg-Phe-Glu-Trp-Glu-Phe-Glu-Gln-Gln-NH(2)) which can be switched from its nematic to its isotropic fluid state by increasing pH and P(11)-5 (CH(3)CO-Gln-Gln-Orn-Phe-Orn-Trp-Orn-Phe-Gln-Gln-Gln-NH(2)) designed to exhibit the converse behavior. Acid-base titrations of fibrillar dispersions reveal deprotonation of the gamma-COOH of Glu or of the delta-NH(3)(+) of Orn(+) occurs over wide bands of up to 5 pH units, a feature of polyelectrolytes. The values of the energy parameters controlling self-assembly can therefore be smoothly and continuously varied by changing pH. This enables isotropic fluid-to-nematic transitions to be triggered by relatively small additions of acid or base, typically 1 part in 10(3) by volume of 1 M HCl or NaOH.

Rationally designed l3-sheet-forming peptides that spontaneously form three-dimensional fibrillar scaffolds in response to specific environmental triggers may potentially be used in skeletal tissue engineering, including the treatment/prevention of dental caries, via bioactive surface groups. We hypothesized that infiltration of caries lesions with monomeric low-viscosity peptide solutions would be followed by in situ polymerization triggered by conditions of pH and ionic strength, providing a biomimetic scaffold capable of hydroxyapatite nucleation, promoting repair. Our aim was to determine the effect of an anionic peptide applied to caries-like lesions in human dental enamel under simulated intra-oral conditions of pH cycling. Peptide treatment significantly increased net mineral gain by the lesions, due to both increased remineralization and inhibition of demineralization over a five-day period. The assembled peptide was also capable of inducing hydroxyapatite nucleation de novo. The results suggest that self-assembling peptides may be useful in the modulation of mineral behavior during in situ dental tissue engineering.

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