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Molecular self-assembly

physical science Maturity 9-11 Next article →

Molecular self-assembly is a fundamental process in chemistry and materials science. It occurs when molecules adopt a specific, organized arrangement on their own. This happens without any guidance or management from an outside source. Scientists categorize this process into two distinct types. The first is intermolecular self-assembly, where separate molecules join together. The second is intramolecular self-assembly, which is more commonly referred to as folding. This concept is central to supramolecular chemistry, which studies how complex systems form through molecular interactions.

Molecular self-assembly
Molecular self-assembly

The mechanism of self-assembly relies on various non-covalent interactions. These forces act as the "instructions" that guide molecules into place. Common forces include hydrogen bonding and metal coordination. Other important drivers are hydrophobic forces, van der Waals forces, and electrostatic interactions. Molecules may also use pi-stacking to achieve their final structure. Jehle observed that when molecules are immersed in a liquid, charge fluctuation forces play a role. These forces favor the association of identical molecules as nearest neighbors. Through these specific interactions, molecules can build highly complex and challenging topologies.

NTCDI AFM2a
NTCDI AFM2a

Self-assembly creates many different types of structures and phases. In liquid environments, it can lead to the formation of colloids, micelles, and vesicles. It is also responsible for creating liquid crystal phases and Langmuir monolayers. Surfactant molecules often form these single-layer structures at interfaces. One remarkable example of a complex topology is the Borromean rings. These are three interlocking rings where removing one ring causes the others to unlock. While DNA can be used to create a biological version of these rings, scientists have recently used non-biological building blocks to make them as well.

In biological systems, self-assembly is essential for the function of cells. It is the process that allows lipids to assemble into cell membranes. It also drives the formation of the double helical structure of DNA through hydrogen bonding between strands. Furthermore, proteins use self-assembly to form complex quaternary structures. When multiple copies of a polypeptide from a single gene assemble, they form a protein multimer. Interestingly, if two different mutant alleles produce polypeptides that form a mixed multimer, it can show greater activity. This specific phenomenon is known as intragenic complementation.

Molecular self-assembly
Molecular self-assembly

However, self-assembly can sometimes result in errors within living organisms. If proteins fold incorrectly, they can self-assemble into insoluble amyloid fibers. This specific type of incorrect assembly is responsible for certain infectious prion-related neurodegenerative diseases. On a more functional level, self-assembly creates amazing biological tools. For example, the growth of nanoscale β-keratin structures allows geckos to climb walls. These structures, known as lamellae, setae, and spatulae, enable them to adhere to ceilings and rock overhangs.

Historical research has helped define how these molecular interactions work. In 1964, researchers F.H. Crick and L.E. Orgel published work on the theory of inter-allelic complementation. Later, in 1965, H. Bernstein and colleagues studied these concepts regarding temperature-sensitive mutants of bacteriophage T4D. These studies contributed to our understanding of how genes and proteins interact. Today, these principles are applied in nanotechnology through a "bottom-up" manufacturing approach. This differs from "top-down" methods like lithography, where structures are carved from larger blocks. In a bottom-up approach, the desired final structure is programmed into the shape and functional groups of the molecules themselves.

DNA nanotechnology is a major area of current research using these bottom-up methods. Instead of using DNA just to carry biological information, scientists use it as a structural material. By utilizing the molecular recognition properties of nucleic acids, researchers create branched DNA complexes. They can build complex 2D and 3D lattices using methods like DNA origami. These structures can take the shape of polyhedra. Furthermore, these DNA assemblies can serve as templates for other molecules, such as streptavidin proteins or gold nanoparticles. This ability to build at the nanoscale offers a vision for the future of microchip manufacturing.

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