A number of strategies exist to design molecular materials based on self-assembled peptides and their derivatives. [1] These include soft materials based on a variety of structural motifs including coiled-coils, [2,3] b-sheets, [4,5] b-hairpins, [6] and peptide amphiphiles. [7][8][9] In these systems, the peptide chains usually contain at least ten amino acids. It has been known for some time that using aromatic components in conjunction with peptides allows the use of much smaller peptides by taking advantage of p-stacking interactions. [10][11][12][13][14][15] One system that has been illustrated is that of N-fluorenylmethoxycarbonyl diphenylalanine (Fmoc-FF) which forms a hydrogel under physiological conditions. This example and other closely related aromatic short peptide derivatives are known to form fibrous hydrogels that have found applications in biological sensing [16] and cell culture. [13,17] Understanding of the supramolecular structures formed by these molecules will aid the rational design of new architectures tailored to the needs of specific biological and non-biological applications. However, to date a complete structure has not been proposed for any member of this class of self-assembly systems. Here we apply a number of spectroscopic techniques to Fmoc-FF and construct a model based on the data obtained comprising a new nanocylindrical molecular architecture based on p-p interlocked b-sheets. Transmission electron microscopy (TEM) and wide angle X-ray scattering (WAXS) was used to confirm the proposed model. Hydrogels of Fmoc-FF were prepared as described previously utilizing a sequential change in pH.[13] As shown in Figure 1a self-supporting gels were formed. The viscoelastic properties of the gels were assessed using oscillatory rheology. Figure 1b shows the mechanical spectrum obtained at room temperature for a Fmoc-FF (20 mmol L -1 ) gel. The storage modulus (G') is found to be approximately an order of magnitude larger than the loss modulus (G''), indicative of an elastic rather than viscous material. Both G' and G'' were found to be essentially independent of frequency over four decades (Fig. 1b). Such rheological behavior is characteristic of solid like gel materials. Light microscopy ( Fig. 1c) revealed a network of fine fibers with microscopic widths. Cryo Scanning Electron Microscopy (cryoSEM) revealed a dense network of flat ribbons with dimensions in the order of tens of nanometers (Fig. 1d). Circular dichroism (CD) was used to investigate the backbone orientation of the dipeptide within the hydrogel. CD analysis of peptide-based supramolecular materials is prone to artifacts. Usually only a narrow concentration range, where the hydrogel forms, can be used reliably to present a detectable CD signal, while showing no or little light scattering ef-

A number of short peptide amphiphiles consisting of dipeptides linked to fluorenylmethoxycarbonyl spontaneously form fibrous hydrogels under physiological conditions (see figure). The structural and physical properties of these gels are dictated by the amino acid sequence of the peptide building blocks, and the gels support the three‐dimensional cell culture of chondrocytes.

We report the effect of pH on the self-assembly process of Fmoc-diphenylalanine (Fmoc-FF) into fibrils consisting of antiparallel beta-sheets, and show that it results in two apparent pKa shifts of approximately 6.4 and approximately 2.2 pH units above the theoretical pKa (3.5). Using Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), wide angle X-ray scattering (WAXS), and oscillatory rheology, these two transitions were shown to coincide with significant structural changes. An entangled network of flexible fibrils forming a weak hydrogel dominates at high pH, while nongelling flat rigid ribbons form at intermediate pH values. Overall, this study provides further understanding of the self-assembly mechanism of aromatic short peptide derivatives.

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We have investigated the self-assembly behavior of fluorenyl-9-methoxycarbonyl (Fmoc)-FG, Fmoc-GG, and Fmoc-GF and compared it to that of Fmoc-FF using potentiometry, fluorescence and infrared spectroscopy, transmission electron microscopy, wide-angle X-ray scattering, and oscillatory rheometry. Titration experiments revealed a substantially shifted apparent pK(a) transition for Fmoc-FG, Fmoc-GG, and Fmoc-GF. The apparent pK(a) values observed correlated with the hydrophobicity (log P) of the Fmoc-dipeptide molecules. Fmoc-GG and Fmoc-GF were found to self-assemble only in their protonated form (below their apparent pK(a)), while Fmoc-FG formed self-assembled structures above and below its apparent pK(a). Fmoc-GG and Fmoc-FG were found to form hydrogels below their apparent pK(a) transitions in agreement with the entangled fibers morphologies revealed by TEM. Unlike Fmoc-FF and Fmoc-GG, Fmoc-FG showed unusual gelation behavior as gels were found to form upon heating. Fmoc-GF formed precipitates instead of a hydrogel below its apparent pK(a) in agreement with the formation of micrometer scale sheetlike structures observed by TEM. The fact that all four Fmoc-dipeptides were found to self-assemble suggests that the main driving force behind the self-assembly process is a combination of the hydrophobic and π-π interactions of the fluorenyl moieties with a secondary role for hydrogen bonding of the peptidic components. The nature of the peptidic tail was found to have a pronounced effect on the type of self-assembled structure formed. This work indicates that the substitution of phenylalanine by glycine significantly impacts on the mode of assembly and illustrates the versatility of aromatic peptide amphiphiles in the formation of structurally diverse nanostructures.