Peptide strand length controls the energetics of self-assembly and morphology of ß-sheet fibrils.

Self-assembling peptides can be used as versatile, natural, and multifunctional building blocks to produce a variety of well-defined nanostructures, materials and devices for applications in medicine and nanotechnology. Here, we concentrate on the 1D self-assembly of de novo designed Px-2 peptide ß-strands into anti-parallel ß-sheet tapes and higher order aggregates. We study six members of the Px-2 family, ranging from 3 amino acids (aa) to 13 aa in length, using a range of complementary experimental techniques, computer simulation and theoretical statistical mechanics. The critical concentration for self-assembly (c*) is found to increase systematically with decreasing peptide length. The shortest peptide found to self-assemble into soluble ß-tapes in water is a 5 amino acid residue peptide. These investigations help decipher the role of the peptide length in controlling self-assembly, aggregate morphology, and material properties. By extracting free energies from these data using a statistical mechanical analysis and combining the results with computer simulations at the atomistic level, we can extract the entropy of association for individual ß-strands.

Self-assembling peptides as injectable lubricants for osteoarthritis.

The self-assembly of peptides is explored as an alternative route towards the development of new injectable joint lubricants for osteoarthritis (OA). The versatility of the peptide chemistry allows the incorporation of behavior reminiscent of hyaluronic acid (HA), while the triggered in situ self-assembly provides easy delivery of the samples by injection due to the low viscosity of the peptide solutions (that are initially monomeric). Using design criteria based on the chemical properties of HA, a range of de novo peptides were prepared with systematic alterations of charge and hydrophilicity that self-assembled into nematic fluids and gels in physiological solution conditions. The frictional characteristics of the peptides were evaluated using cartilage on cartilage sliding contacts along with their rheological characteristics. Peptide P(11)-9, whose molecular, mesoscopic, and rheological properties most closely resembled HA was found to be the most effective lubricant amongst the peptides. In healthy static and dynamic friction testing (corresponding to healthy joints) P(11)-9 at 20-40 mg/mL performed similar to HA at 10 mg/mL. In friction tests with damaged cartilage (corresponding to early stage OA) P(11)-9 was a less efficient lubricant than HA, but still the best among all the peptides tested. The results indicate that de novo self-assembling peptides could be developed as an alternate therapeutic lubricant for early stage OA.

pH as a trigger of peptide beta-sheet self-assembly and reversible switching between nematic and isotropic phases.

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.