We have used the pH-induced self-assembly of a peptide-amphiphile to make a nanostructured fibrous scaffold reminiscent of extracellular matrix. The design of this peptide-amphiphile allows the nanofibers to be reversibly cross-linked to enhance or decrease their structural integrity. After cross-linking, the fibers are able to direct mineralization of hydroxyapatite to form a composite material in which the crystallographic c axes of hydroxyapatite are aligned with the long axes of the fibers. This alignment is the same as that observed between collagen fibrils and hydroxyapatite crystals in bone.

Twelve derivatives of peptide-amphiphile molecules, designed to self-assemble into nanofibers, are described. The scope of amino acid selection and alkyl tail modification in the peptide-amphiphile molecules are investigated, yielding nanofibers varying in morphology, surface chemistry, and potential bioactivity. The results demonstrate the chemically versatile nature of this supramolecular system and its high potential for manufacturing nanomaterials. In addition, three different modes of self-assembly resulting in nanofibers are described, including pH control, divalent ion induction, and concentration. P reprogrammed noncovalent bonds, within and between molecules, build highly functional and dynamic structures in biology, which motivates our interest in self-assembly of synthetic systems. Over the past few decades a substantial amount of literature describing noncovalent self-assembly of nanostructures has accumulated (1-14). However, it is still difficult to design supramolecular structures, particularly if we want to start with designed molecules and form objects that measure between nanoscopic and macroscopic dimensions. Developing this ability will take us closer to the broad, bottom-up approach of selfassembly observed in biology.Our laboratory has studied over the past decade self-assembly of designed molecules into macromolecular structures of twodimensional (15, 16), one-dimensional (17, 18), and zerodimensional nature (19)(20)(21)(22). These self-assembled objects contain between 10 1 and 10 5 molecules and thus resemble synthetic and biological polymers in molar mass. The interactions that lead to the formation of these structures include chiral dipole-dipole interactions, -stacking, hydrogen bonds, nonspecific van der Waals interactions, hydrophobic forces, electrostatic interactions, and repulsive steric forces. All systems studied involved combinations of these forces that counterbalance the enormous translational and rotational entropic cost caused by polymolecular aggregation. In some cases the possibility of internally linking these self-assembled structures through covalent bonds has been explored (1,17,20,23). A cross-linking produces actual polymers whose various shapes and dimensionalities are controlled by self-assembly and are very different from the well-known ''beads-on-a-chain'' structures of traditional polymers.In our studies of self-assembling systems we also have explored self-organization at length scales much greater than those of the aggregates themselves, reaching into scales of microns, millimeters, and even centimeters. We also have been interested in functionalities that emerge from self-assembly at these largerlength scales. An interesting example was the layering and polar stacking of mushroom-shaped supramolecular structures each measuring about 5 nm. The stem-to-cap layers of these nanostructures result in centimeter-scale films that are spontaneously piezoelectric (24). The search for useful systems in the microscopic and macroscopic regime that take advantage of mole...

The role of hydrogen bonding and amphiphilic packing in the self-assembly of peptide-amphiphiles (PAs) was investigated using a series of 26 PA derivatives, including 19 N-methylated variants and 7 alanine mutants. These were studied by circular dichroism spectroscopy, a variety of Fourier transform infrared spectroscopies, rheology, and vitreous ice cryo-transmission electron microscopy. From these studies, we have been able to determine which amino acids are critical for the self-assembly of PAs into nanofibers, why the nanofiber is favored over other possible nanostructures, the orientation of hydrogen bonding with respect to the nanofiber axis, and the constraints placed upon the portion of the peptide most intimately associated with the biological environment. Furthermore, by selectively eliminating key hydrogen bonds, we are able to completely change the nanostructure resulting from self-assembly in addition to modifying the macroscopic mechanical properties associated with the assembled gel. This study helps to clarify the mechanism of self-assembly for peptide amphiphiles and will thereby help in the design of future generations of PAs.

Replicating the multi-hierarchical self-assembly of collagen has long-attracted scientists, from both the perspective of the fundamental science of supramolecular chemistry and that of potential biomedical applications in tissue engineering. Many approaches to drive the self-assembly of synthetic systems through the same steps as those of natural collagen (peptide chain to triple helix to nanofibres and, finally, to a hydrogel) are partially successful, but none simultaneously demonstrate all the levels of structural assembly. Here we describe a peptide that replicates the self-assembly of collagen through each of these steps. The peptide features collagen's characteristic proline-hydroxyproline-glycine repeating unit, complemented by designed salt-bridged hydrogen bonds between lysine and aspartate to stabilize the triple helix in a sticky-ended assembly. This assembly is propagated into nanofibres with characteristic triple helical packing and lengths with a lower bound of several hundred nanometres. These nanofibres form a hydrogel that is degraded by collagenase at a similar rate to that of natural collagen.

The general design criteria and synthesis of four new peptide-based solid-state tubular array structures are described. Peptide nanotubes, which are extended tubular -sheet-like structures, are constructed by the self-assembly of flat, ring-shaped peptide subunits made up of alternating D-and L-amino acid residues. Peptide self-assembly is directed by the formation of an extensive network of intersubunit hydrogen bonds. In the crystal structures, nanotubes are stabilized by intertubular hydrophobic packing interactions. Peptide nanotubes exhibit good mechanical and thermal stabilities in water and are stable for long periods of times in most common organic solvents including DMF and DMSO. The remarkable stability of peptide nanotubes can be attributed to the highly cooperative nature of the noncovalent interactions throughout the crystal lattice. Nanotube structures were characterized by cryoelectron microscopy, electron diffraction, Fourier-transform infrared spectroscopy, and crystal structure modeling. This study also serves to exemplify the predictive structural aspects of the peptide self-assembly process.