Dendron-mediated self-assembly, disassembly, and self-organization of complex systems have been investigated. The most ideal building blocks would need to display shape persistence in solution and in the solid state, since only this feature provides access to the use of complementary methods of structural analysis. Most supramolecular dendrimers are chiral even when they are constructed from nonchiral building blocks and are equipped with mechanisms that amplify chirality. This poses additional challenges associated with the understanding of the structural origin of chirality in different supramolecular structures through combinations of structural analysis methods. While many supramolecular structures assembled from dendrimers and dendrons resemble some of the related morphologies generated from block-copolymers, they are much more complex and are not determined by the volume ratio between the dissimilar parts of the molecule.

Dendron-Mediated Self-Assembly, Disassembly, and Self-Organization of Complex Systems

This review discusses one-dimensional supramolecular polymers that form in aqueous media. First, naturally occurring supramolecular polymers are described, in particular, amyloid fibrils, actin filaments, and microtubules. Their structural, thermodynamic, kinetic, and nanomechanical properties are highlighted, as well as their importance for the advancement of biologically inspired supramolecular polymer materials. Second, five classes of synthetic supramolecular polymers are described: systems based on (1) hydrogen-bond motifs, (2) large π-conjugated surfaces, (3) host–guest interactions, (4) peptides, and (5) DNA. We focus on recent studies that address key challenges in the field, providing mechanistic understanding, rational polymer design, important functionality, robustness, or unusual thermodynamic and kinetic properties.

Supramolecular Polymers in Aqueous Media

The protein shell of the tobacco mosaic virus (TMV) provides a robust and practical tubelike scaffold for the preparation of nanoscale materials. To expand the range of applications for which the capsid can be used, two synthetic strategies have been developed for the attachment of new functionality to either the exterior or the interior surface of the virus. The first of these is accomplished using a highly efficient diazonium coupling/oxime formation sequence, which installs >2000 copies of a material component on the capsid exterior. Alternatively, the inner cavity of the tube can be modified by attaching amines to glutamic acid side chains through a carbodiimide coupling reaction. Both of these reactions have been demonstrated for a series of substrates, including biotin, chromophores, and crown ethers. Through the attachment of PEG polymers to the capsid exterior, organic-soluble TMV rods have been prepared. Finally, the orthogonality of these reactions has been demonstrated by installing different functional groups on the exterior and interior surfaces of the same capsid assemblies.

Dual-surface modification of the tobacco mosaic virus.

A short account is given of the physical and chemical studies that have led to an understanding of the structure of the tobacco mosaic virus particle and how it is assembled from its constituent coat protein and RNA. The assembly is a much more complex process than might have been expected from the simplicity of the helical design of the particle. The protein forms an obligatory intermediate (a cylindrical disk composed of two layers of protein units), which recognizes a specific RNA hairpin sequence. This extraordinary mechanism simultaneously fulfils the physical requirement for nucleating the growth of the helical particle and the biological requirement for specific recognition of the viral DNA.

The tobacco mosaic virus particle: structure and assembly

Protein subunits in the two layers of the disk of tobacco mosaic virus have very similar conformations. Much of the bonding between subunits is polar, including salt-bridge systems. Arginine residues play a prominent part here and elsewhere. Interactions within each layer involve groups whose contacts can be adjusted to allow the transition from disk to virus helix. Aromatic clusters within each molecule are linked in a hydrophobic girdle encircling each ring.

Protein disk of tobacco mosaic virus at 2.8 A resolution showing the interactions within and between subunits.

Self-assembly of the protein subunits of tobacco mosaic virus takes place by a series of steps. The natural state of aggregation of TMV protein is not the helix but the 20S disk, which consists of two paired rings of seventeen sub-units each. Movement of 5 A is involved in the pairing. Control of the aggregation and its consequences for virus assembly are described in the next two articles.

States of aggregation of tobacco mosaic virus protein.

Near neutral pH9 TMV protein sub-units polymerize into two-layer aggregates culminating in the 20S disk, whereas the helical rod forms cooperatively at acid pH. The mode of aggregation is controlled by the state of ionization of two abnormally titrating carboxyl groups.

A. C. H. Durham

Papers

States of aggregation of tobacco mosaic virus protein.

Polymerization of tobacco mosaic virus protein and its control.