Friday, September 4, 2009

Supramolecular Chemistry.

What is supramolecular chemistry?

It has been described as ‘chemistry beyond the molecule’, whereby a ‘supermolecule’ is a
species that is held together by non-covalent interactions between two or more
covalent molecules or ions. It can also be described as ‘lego™ chemistry’ in which
each lego™ brick represents a molecular building block and these blocks are held
together by intermolecular interactions (bonds), of a reversible nature, to form
a supramolecular aggregate. These intermolecular bonds include electrostatic
interactions, hydrogen bonding, – interactions, dispersion interactions and
hydrophobic or solvophobic effects.

Supramolecular Chemistry: The study of systems involving aggregates of molecules
or ions held together by non-covalent interactions, such as electrostatic interactions,
hydrogen bonding, dispersion interactions and solvophobic effects.


Supramolecular chemistry is a multidisciplinary field which impinges on
various other disciplines, such as the traditional areas of organic and inorganic
chemistry, needed to synthesise the precursors for a supermolecule, physical
chemistry, to understand the properties of supramolecular systems and computational
modelling to understand complex supramolecular behaviour. A great
deal of biological chemistry involves supramolecular concepts and in addition
a degree of technical knowledge is required in order to apply supramolecular
systems to the real world, such as the development of nanotechnological devices.


Supramolecular chemistry can be split into two broad categories;
  • host–guest chemistry
  • self-assembly


The host component is defined as an organic molecule or ion whose binding sites converge
in the complex. The guest component is any molecule or ion whose binding sites
diverge in the complex.1 A binding site is a region of the host or guest that is
of the correct size, geometry and chemical nature to interact with the other species.


Host–guest complexes include biological systems, such as enzymes and their substrates, with enzymes being the host and the substrates the guest. In terms of coordination chemistry,
metal–ligand complexes can be thought of as host–guest species, where large
(often macrocyclic) ligands act as hosts for metal cations. If the host possesses a
permanent molecular cavity containing specific guest binding sites, then it will
generally act as a host both in solution and in the solid state and there is a
reasonable likelihood that the solution and solid state structures will be similar
to one another. On the other hand, the class of solid state inclusion compounds
only exhibit host–guest behaviour as crystalline solids since the guest is bound
within a cavity that is formed as a result of a hole in the packing of the host
lattice. Such compounds are generally termed clathrates from the Greek klethra,
meaning ‘bars’.


Where there is no significant difference in size and no species is acting as a host for another, the non-covalent joining of two or more species is termed self-assembly. Strictly, self-assembly is an equilibrium between two or more molecular components to produce an aggregate with a
structure that is dependent only on the information contained within the chemical
building blocks.


Nature itself is full of supramolecular systems, for example, deoxyribonucleic
acid (DNA) is made up from two strands which self-assemble via hydrogen
bonds and aromatic stacking interactions to form the famous double helical
structure.


Host–Guest Chemistry: The study of large ‘host’ molecules that are capable of
enclosing smaller ‘guest’ molecules via non-covalent interactions.


Self-Assembly: The spontaneous and reversible association of two or more
components to form a larger, non-covalently bound aggregate.


Binding Site: A region of a molecule that has the necessary size, geometry
and functionalities to accept and bind a second molecule via non-covalent
interactions.

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