Pushing the limits: A 200×106 Da structurally defined, linear macromolecule (PG5) has a molar mass, cross-section dimension, and cylindrical shape that are comparable to some naturally occurring objects, such as amyloid fibrils or certain plant viruses. The macromolecule is resistant against flattening out on a surface; the picture shows PG5 embracing the tobacco mosaic virus (TMV). |
Organic
chemists have always been trying to imitate biology. Although it is
possible to make many molecules that imitate biomolecules in terms of
structure and function, it remains a challenge to attain the size and
form of large biomolecules. An international team led by A. Dieter
Schlüter at the ETH Zurich (Switzerland) has now introduced a branched
polymer that resembles the tobacco mosaic virus in size and cylindrical
form. As the researchers outline in the journal Angewandte Chemie (“The Largest Synthetic Structure with Molecular Precision: Towards a Molecular Object”), this is the largest synthetic macromolecule with defined shape and atomic structure reported to date.
Previously,
the largest reported synthetic structures with a defined atomic
structure were polystyrene polymers with a molecular mass of about 40
million Daltons. However, this value corresponds to a small fraction of
the mass of large DNA molecules. Formation of a large synthetic molecule
that also has a defined form is much more difficult. For biologists,
however, it is routine. Even the simplest organism has a well-defined
form, such as the rod-shaped tobacco mosaic virus. For chemists it is a
model: a massive molecular ensemble with perfect control over its
chemical structure, function, size, and molecular form.
Schlüter
and co-workers have now presented a branched polymer that approximates
the size and form of the tobacco mosaic virus. Their complex synthesis,
which requires 170,000 bond-forming reactions in a single molecule, led
to a structurally defined, linear macromolecule with a diameter of about
10 nm and a molecular weight of 200 million Daltons. It thus has a
molar mass, cross section, and cylindrical form comparable to the
tobacco mosaic virus.
The
new macromolecule is a dendronized polymer: it consists of a linear
backbone with highly and regularly branched side chains. “This is the
biggest synthetic macromolecule with a defined chemical structure and
defined form to date,” according to Schlüter. “Our experiment is a first
step toward the synthesis of molecular objects.” A structure is
considered to be an object if it keeps its form regardless of its
environment, when its interior can be distinguished from the outer
environment, and when there is a clear boundary between the two. There
are many synthetic nano-objects, however these are not single molecules,
but are aggregates of several or many individual molecules.
SOURCE: Wiley
