pH as a Trigger of Peptide β-Sheet Self-Assembly and Reversible Switching between Nematic and Isotropic Phases

Abstract

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.

Lyotropic phase behaviour of dilute, aqueous hen lysozyme amyloid fibril dispersions

“…This was accomplished by varying the ionic strength of the dispersion medium [23,30], as well as by adjusting the pH, resulting in aggregation around the isoelectric point, at which fibrils carry no net electric charge [24]. Similarly, for the here investigated system it appears that at high acid concentration and thus presence of a high amount of chloride ions, similar screening effects or, potentially, salt bridges led to formation of heterogeneous structures; interactions, which in another report were found to become significant for peptide-based b-sheet fibrils at pH B 2 [16]. Hence, the tendency to develop heterogeneous textures only dominates at sufficiently low pH, i.e.…”

“…Conveniently, colloidal dispersions of high aspect ratio amyloid fibrils from polypeptides [15][16][17][18] and proteins such as insulin [19,20], lysozyme [21,22] or b-lactoglobulin [23,24] display lyotropic ordering in the form of phase-separated spherulitic droplets or nematic liquids. Clearly, the liquid-crystalline nature of such systems may lend itself as a powerful tool to order these materials, e.g.…”

The formation of spherulites by amyloid fibrils of bovine insulin

“…Because the literature indicates that spherulites could be prevalent in many other amyloid-containing systems (17,18,(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35), it is possible that the ability to form such structures is a property common to all, or at least most, amyloid fibril systems. From the data presented here, and from the wider protein literature, it appears that once biological control of the native structure of a protein has been lost for whatever reason, the assembly of proteins into organized structures can proceed via a pathway not dissimilar to that of typical synthetic polymers.…”

“…The possible significance of spherulitic deposits in the development of these diseases has not, however, been established. The formation of spherulites in solutions containing amyloid fibrils in vitro has been reported for a pathogenic Ig light chain (27), for several different synthetic peptides (28), including a helix-turn-helix peptide (29), for gels of ␤-lactoglobulin (30,31), and for peptide analogues of silkmoth eggshell proteins, chorion (32). Furthermore, spherical aggregates have been observed in solutions of the nonpathogenic protein ␣-L-iduronidase (33), in gels formed from solutions of a synthetic peptide derived from the ␤-sheet domain of platelet factor 4 (34) and in solutions of the 40-residue A␤ peptide implicated in Alzheimer's disease, where the spherical aggregates were termed ''␤-amy balls'' (35).…”

Failure of Aβ(1-40) amyloid fibrils under tensile loading

“…According to earlier experimental and theoretical studies these mechanical properties are related to their molecular structure [10,12]. The exceptional mechanical properties of amyloids make them good candidates for a wide range of potential technological applications, and specifically as new bionanomaterials utilizing them as nanowires [13][14][15][16], gels [17][18][19][20][21], scaffolds and biotemplates [13,[22][23][24][25][26][27], liquid crystals [28], adhesives [29] and biofilm materials [30]. These applications often imply the functionalization of the amyloid fibrils with the introduction of additional elements, including enzymes, metal ions, fluorophores, biotin or cytochromes.…”