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