Biocompatible hydrogels have a wide variety of potential applications in biotechnology and medicine, such as the controlled delivery and release of cells, cosmetics and drugs; and as supports for cell growth and tissue engineering1. Rational peptide design and engineering are emerging as promising new routes to such functional biomaterials2-4. Here we present the first examples of rationally designed and fully characterized self-assembling hydrogels based on standard linear peptides with purely α-helical structures, which we call hydrogelating self-assembling fibres (hSAFs). These form spanning networks of α-helical fibrils that interact to give self-supporting physical hydrogels of >99% water content. The peptide sequences can be engineered to alter the underlying mechanism of gelation and, consequently, the hydrogel properties. Interestingly, for example, those with hydrogen-bonded networks melt upon heating, whereas those formed via hydrophobic interactions strengthen when warmed. The hSAFs are dual-peptide systems that only gel on mixing, which gives tight control over assembly5. These properties raise possibilities for using the hSAFs as substrates in cell culture. We have tested this in comparison with the widely used Matrigel substrate, and demonstrate that, like Matrigel, hSAFs support both growth and differentiation of rat adrenal pheochromocytoma cells for sustained periods in culture.

Two stages in the rational redesign of a peptide‐based, self‐assembling fiber (SAF) are described. The SAF system comprises two peptides designed to form an offset α‐helical coiled‐coil heterodimer. The “sticky‐ends” are complementary and promote longitudinal assembly. Alone, the two peptides are unstructured, but co‐assemble upon mixing to form α‐helical fibrils, which bundle to form fibers 40–50 nm wide and tens of micrometers long. Assembly is controllable and occurs at pH 7 in water, making SAFs a potential scaffold for 3D cell culture. The purposes of the redesigns were 1) to investigate the fiber‐thickening process, and 2) to increase fiber stability for potential biological and biomedical applications. First, mutations were made to the original peptide designs to increase fibril–fibril interactions and so produce thicker and more‐stable fibers. The second iteration aimed to increase the primary peptide–peptide interactions by increasing the overlap in the offset dimer and so promote the initial step in fiber formation. As judged by circular dichroism spectroscopy and transmission electron microscopy, both iterations improved fiber assembly and stability: the critical peptide concentration for assembly improved from 60 μM to 4 μM; the midpoint of thermal unfolding increased from 22 °C to 65 °C; and the salt tolerance improved from 75 mM to greater than 250 mM KCl. These improvements bring closer applications of the SAF system under physiological conditions, for example as a biocompatible material for 3D cell culture. In addition, ordered surface features were observed in the second‐ and third‐generation fibers compared with the original design. This indicates improved internal order in the redesigned fibers. In turn, this suggests a molecular mechanism for the improved stability and sheds light on the fiber‐assembly process.

We describe a straightforward single-peptide design that self-assembles into extended and thickened nano-to-mesoscale fibers of remarkable stability and order. The basic chassis of the design is the well-understood dimeric alpha-helical coiled-coil motif. As such, the peptide has a heptad sequence repeat, abcdefg , with isoleucine and leucine residues at the a and d sites to ensure dimerization. In addition, to direct staggered assembly of peptides and to foster fibrillogenesisthat is, as opposed to blunt-ended discrete speciesthe terminal quarters of the peptide are cationic and the central half anionic with lysine and glutamate, respectively, at core-flanking e and g positions. This +,-,-,+ arrangement gives the peptide its name, MagicWand (MW). As judged by circular dichroism (CD) spectra, MW assembles to alpha-helical structures in the sub-micromolar range and above. The thermal unfolding of MW is reversible with a melting temperature >70 degrees C at 100 muM peptide concentration. Negative-stain transmission electron microscopy (TEM) of MW assemblies reveals stiff, straight, fibrous rods that extended for tens of microns. Moreover, different stains highlight considerable order both perpendicular and parallel to the fiber long axis. The dimensions of these features are consistent with bundles of long, straight coiled alpha-helical coiled coils with their axes aligned parallel to the long axis of the fibers. The fiber thickening indicates inter-coiled-coil interactions. Mutagenesis of the outer surface of the peptide i.e., at the b and f positionscombined with stability and microscopy measurements, highlights the role of electrostatic and cation-pi interactions in driving fiber formation, stability and thickening. These findings are discussed in the context of the growing number of self-assembling peptide-based fibrous systems.

Interest in the design of peptide-based fibrous materials is growing because it opens possibilities to explore fundamental aspects of peptide self-assembly and to exploit the resulting structures--for example, as scaffolds for tissue engineering. Here we investigate the assembly pathway of self-assembling fibers, a rationally designed alpha-helical coiled-coil system comprising two peptides that assemble on mixing. The dimensions spanned by the peptides and final structures (nanometers to micrometers), and the timescale over which folding and assembly occur (seconds to hours), necessitate a multi-technique approach employing spectroscopy, analytical ultracentrifugation, electron and light microscopy, and protein design to produce a physical model. We show that fibers form via a nucleation and growth mechanism. The two peptides combine rapidly (in less than seconds) to form sticky ended, partly helical heterodimers. A lag phase follows, on the order of tens of minutes, and is concentration-dependent. The critical nucleus comprises six to eight partially folded dimers. Growth is then linear in dimers, and subsequent fiber growth occurs in hours through both elongation and thickening. At later times (several hours), fibers grow predominantly through elongation. This kinetic, biomolecular description of the folding-and-assembly process allows the self-assembling fiber system to be manipulated and controlled, which we demonstrate through seeding experiments to obtain different distributions of fiber lengths. This study and the resulting mechanism we propose provide a potential route to achieving temporal control of functional fibers with future applications in biotechnology and nanoscale science and technology.

Protein:protein interactions are becoming increasingly significant as potential drug targets; however, the rational identification of small molecule inhibitors of such interactions remains a challenge. Pharmacophore modelling is a popular tool for virtual screening of compound libraries, and has previously been successfully applied to the discovery of enzymatic inhibitors. However, the application of pharmacophore modelling in the field of protein:protein interaction inhibitors has historically been considered more of a challenge and remains limited. In this review, we explore the interaction mimicry by known inhibitors that originate from in vitro screening, demonstrating the validity of pharmacophore mapping in the generation of queries for virtual screening. We discuss the pharmacophore mapping methods that have been successfully employed in the discovery of first-in-class inhibitors. These successful cases demonstrate the usefulness of a "tool kit" of diverse strategies for application across a range of situations depending on the available structural information.

Even the simplest organisms are too complex to have spontaneously arisen fully formed, yet precursors to first life must have emerged ab initio from their environment. A watershed event was the appearance of the first entity capable of evolution: the Initial Darwinian Ancestor. Here, we suggest that nucleopeptide reciprocal replicators could have carried out this important role and contend that this is the simplest way to explain extant replication systems in a mathematically consistent way. We propose short nucleic acid templates on which amino-acylated adapters assembled. Spatial localization drives peptide ligation from activated precursors to generate phosphodiester-bond-catalytic peptides. Comprising autocatalytic protein and nucleic acid sequences, this dynamical system links and unifies several previous hypotheses and provides a plausible model for the emergence of DNA and the operational code.

and slow-release drug-delivery through the directed packing of small molecules into the core. [7,8] Whereas traditional, inorganic, nanotube production leads to tubes that are highly uniform and therefore challenging to functionalize, designed proteins allow modular assembly of nanostructures containing multiple regions each with specific structure and function, into easily controllable smart materials and a number of protein nanotubes have been constructed. [9][10][11] Previously we demonstrated the selfassembly of a modular protein nanotube with a high aspect ratio (Figure 1). [12] The tubes are constructed from a mutant form of trp RNA-binding attenuation protein (TRAP) from Geobacillus stearothermophilus. [13][14][15][16] This small protein 11-mer forms rings ≈8.5 nm in diameter with a central pore ≈2 nm in diameter. In further experiments, TRAP has proved to be a versatile bionano building block, [17,18] and is also capable of assembling into a spherical cage. [19,20] The TRAP ring resembles a truncated cone with one face being slightly wider than the other. We arbitrarily name the narrow face "Face A" and the wide face "Face B" (Figure 1a). Each of the 11 subunits consists of 74 amino acids, many of which can be altered without significantly affecting the overall structure or stability of the fully folded and assembled 11-mer. Using the crystal structure [21] as a guide, two TRAP (trp RNA-binding attenuation protein) is a stable, ring-shaped protein that has proven useful in the development of artificial, self-assembled biological structures including a protein nanotube that assembles from a cysteine-containing mutant. While the structure of the nanotube is known in some detail, the nature of the interactions that connect the protein rings together to form tubes is not known. Here, evidence is presented that the two faces of the ring bind together using mixed interactions: On one face cysteine side chains are linked to the same face of a corresponding ring via a dithio linker molecule while the other face does not require covalent interactions and can assemble using protein-protein interactions alone. A coherent 3D model is constructed to explain the observed results, which are ultimately due to specific structural features that modulate the types of bonding that can occur. This detailed understanding offers the prospect of engineering the TRAP nanotube to endow it with bespoke properties.

Background.  There is no ideal substitute for the extracellular matrix (ECM) in tissue engineering. We aim to develop peptide‐based fibrous and hydrogel materials as potential scaffolds for 2‐/3‐D culture in vitro and in vivo. Methods.  Unlike other peptide scaffolds, we employed a system using α‐helical units with two complementary peptides, which combine to yield a sticky ended building block for fibre assembly (self‐assembly fibres, SAFs). This allows control over assembly, and brings utility as either peptide can be complemented or replaced by targeted bioactive peptides to alter morphology and/or add function. Results.  Our first designs, standard SAFs, exclusively rendered thickened, lengthened and stiff fibres (dimensions ≅ 50 nm × 10 μm) that settled out of solution. In recent designs (hSAFs), the solvent‐exposed surfaces of the α‐helices were engineered to promote fibre–fibre interactions and gel formation. Conclusions.  We have produced a series of responsive materials with properties of interest in the search for an ideal ECM replacement. These designs give hSAFs great promise for future human application, such as laryngeal nerve repair, tissue engineering support or vocal lamina propria replacement.