Nucleation and growth of elastin-like peptide fibril multilayers: an in situ atomic force microscopy study

Nanoscale Spatial Control over the Self-Assembly of Small Molecule Hydrogelators

Spatial control over molecular self-assembly at the nano scale offers great potential for many high-tech applications, yet remains a challenging task. Here, we report a polymer brush-mediated strategy to confine the self-assembly of hydrazone-based hydrogelators exclusively at nanoparticle surfaces. The surfaces of these nanoparticles are grafted with negatively charged polyacrylic acid, which enrich protons that can catalyze the in situ formation and self-assembly of hydrazone-based gelators. We found that, with respect to the polymer lengths, the concentration of the nanoparticles presents more significant effects on the self-assembly process and the properties of the resultant hydrogels, including gelation time, stiffness, and network morphology. More interestingly, the hydrogel fibers are found to be formed specifically around the nanoparticles, demonstrating the directed nanoscale molecular self-assembly. This work demonstrates that triggering molecular self-assembly using catalysis can serve as an effective way to realize directed molecular self-assembly at the nano scale, which may serve as a powerful approach to improve many material properties, such as the mechanical properties of supramolecular materials as we found in this work.

Molecular-level studies of extracellular matrix proteins conducted using atomic force microscopy

Extracellular matrix (ECM) proteins provide anchorage and structural strength to cells and tissues in the body and, thus, are fundamental molecular components for processes of cell proliferation, growth, and function. Atomic force microscopy (AFM) has increasingly become a valuable approach for studying biological molecules such as ECM proteins at the level of individual molecules. Operational modes of AFM can be used to acquire the measurements of the physical, electronic, and mechanical properties of samples, as well as for viewing the intricate details of the surface chemistry of samples. Investigations of the morphology and properties of biomolecules at the nanoscale can be useful for understanding the interactions between ECM proteins and biological molecules such as cells, DNA, and other proteins. Methods for preparing protein samples for AFM studies require only basic steps, such as the immersion of a substrate in a dilute solution or protein, or the deposition of liquid droplets of protein suspensions on a flat, clean surface. Protocols of nanolithography have been used to define the arrangement of proteins for AFM studies. Using AFM, mechanical and force measurements with tips that are coated with ECM proteins can be captured in ambient or aqueous environments. In this review, representative examples of AFM studies are described for molecular-level investigations of the structure, surface assembly, protein-cell interactions, and mechanical properties of ECM proteins (collagen, elastin, fibronectin, and laminin). Methods used for sample preparation as well as characterization with modes of AFM will be discussed.

Morphology and aggregation of RADA-16-I peptide Studied by AFM, NMR and molecular dynamics simulations

RADA-16-I is a self-assembling peptide which forms biocompatible fibrils and hydrogels. We used molecular dynamics simulations, atomic-force microscopy, NMR spectroscopy, and thioflavin T binding assay to examine size, structure, and morphology of RADA-16-I aggregates. We used the native form of RADA-16-I (H-(ArgAlaAspAla)4 -OH) rather than the acetylated one commonly used in the previous studies. At neutral pH, RADA-16-I is mainly in the fibrillar form, the fibrils consist of an even number of stacked β-sheets. At acidic pH, RADA-16-I fibrils disassemble into monomers, which form an amorphous monolayer on graphite and monolayer lamellae on mica. RADA-16-I fibrils were compared with the fibrils of a similar peptide RLDL-16-I. Thickness of β-sheets measured by AFM was in excellent agreement with the molecular dynamics simulations. A pair of RLDL-16-I β-sheets was thicker (2.3 ± 0.4 nm) than a pair of RADA-16-I β-sheets (1.9 ± 0.1 nm) due to the volume difference between alanine and leucine residues.