Figure 7.1 shows the schematic representation of the general process of bio -mimetic self- assembly. The primary building units, that is, the monomers, of bio-mimetic self-assembly come with a variety of structural and compositional diversities. Details on this will be found in the third section of this chapter. In most cases, hydrogen bonding is largely responsible for the primary self-assembly step, which makes the aggregates have a unique morphology and a fixed aggregation number. Rod-shaped dimers and disk-shaped oligomers are among the abundant forms. However, depending on the structure and functionality of the monomers and environmental conditions around them, that is, the conditions that can induce too loose or too tight packing at this step, conventional forms of self-assembled aggregates such as normal micelles, vesicles, and others are often rod ribbon rod (dimer)
ribbon double helix
tube sheet helix double helix sheet needle needle
fiber primary building unit primary self-aggregates secondary self-aggregates nm-|im tertiary self-aggregates
Figure 7.1. General scheme for bio-mimetic self-assembly.
nm assembled, too, even though hydrogen bonding is still involved. For a certain class of bio-mimetic self-assembly, the process occurs with only this first step. A typical example is avidin-biotin binding, which is a highly specific but strong one-way self-assembly with a strong action of hydrogen bonding with structural matching. Binding based on the cyclodextrin host can also be classified within this type of self-assembly. "Lock-and-key" or "nut-and-bolt" self-assembly might be a good name for this.
For most of the building units of bio-mimetic self-assembly, these primary self-aggregates possess enough intermolecular bonding capability to go through the secondary self-assembly step. The secondary self-aggregates formed at this step show a wide variety of structural diversities and a wide range of length scales at the same time. These include rod, hollow tube, ribbon, twisted tape, sheet, helix, and more, with their length ranging from nanometer to micrometer. Many of the unique functionalities of the bio-mimetic self- assembled aggregates are believed to have originated from the morphological uniqueness acquired at this step via this structural diversity. Tubular aggregates that beautifully serve as transmembrane channels (or pores) are one good example that can be found in living systems. Additional hydrogen bonding and n-n interaction are the forces that are responsible for the directionality of the secondary self-aggregates, their orientationality, and their highly asymmetric morphology.
Also, depending on the individual systems, the weak intermolecular forces such as van der Waals attraction take an important part in the force balance process . The morphology of each secondary self- aggregate is a unique result of force balancing between each primary building unit. It is also highly sensitive in each environmental condition, such as pH, counterion, ionic strength, concentration, temperature, and so forth. However, these are still self-assembled aggregates formed solely by intermolecular forces without the formation of any permanent bonding such as a covalent bond. Thus, for the most part, they are highly susceptible to subtle structural transition due to a change in any of these conditions.
Modern research has not reached the point where these unique structural features can be controlled at will, at least, at the level of structure/phase control of surfactant self-assembled aggregates. It will take the total understanding of the complete balance between all the forces involved. Geometrical matching should be included in the force balance, as well. Sometimes, this cooperative primary self- assembly and secondary self-assembly are involved with a fixed aggregation number. A typical example in living systems is the formation of microtubules. a- and P-tubulin subunits first form a,P-tubulin dimer by hydrogen bonding, and these dimers are self-assembled into helical microtubules with an exact aggregation number of 13 dimers in each 13 columns. This is a spontaneous thermodynamic process. Ferritin is another good example that always has a fixed aggregation number. Also, there can be a process without the formation of dimers or oligomers. One example is the formation of actin filaments. This linear helix is formed by chiral self-assembly of actin subunits, which is also a spontaneous thermodynamic process. As the concentration of filaments is increased, they are assembled into fiberlike or networked gel- l ike structures with the help of other proteins, fodrin and filamin.
Secondary self-aggregates can undergo a further self-assembly step, in many cases, by change of temperature and by the increase of the concentration of the building units. This forms tertiary self-aggregates. As is the case for the secondary ones, these tertiary structures also show a variety of diversities including double helix, needle, fibril, and superstructures such as vesicle-in-tube, and have much larger sizes that can easily pass beyond the micrometer scale. Directional hydrogen bonding and n-n interaction are not significantly involved in this step of self-assembly. The role of weak intermolecular forces becomes more important. A further step of assembly, whenever it is induced by another action of weak intermolecular forces, induces an even higher degree and larger size of self-assembled aggregates. Fiberlike structures whose size often reaches the millimeter range is a typical example. For example, in living organisms, the self-assembly of collagen all the way to the functional tissues fits with this scheme. Beyond this point, gelation is often the case for the next step of bio-mimetic self-assembly. Formation of gels depends, of course, on the intermolecular bonding capability between building units, but also on the type of solvents, that is, their intermolecular interaction with solvent molecules as well. This will be further described in the fifth section of this chapter.
Throughout the overall self-assembly processes of bio-mimetic building units, there almost always is chirality, whether it is molecular or morphological. It can be generic, and it can be induced, as well. Also, when it comes to the higher
TABLE 7.1. Bio-mimetic self-assembly.
self-assembly versus conventional (non-bio-mimetic)
Building unit Packing mode Directional force Intermolecular force Assembly rate Aggregate structure Aggregate size Stability
Monomer exchange Disassembly
High: isolation possible Slow
Not likely, slow
Weak or none
Low: isolation not possible Fast
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