DNA carries the blueprint for all living things-and now it could also serve as
the model for "dead" inorganic materials. However, the mechanism will be
completely different from the usual "transcription" of the code by base pairs;
instead, it is DNA's spatial structure that counts in this case. Japanese
researchers have used the genetic molecule as a "scaffold" for the construction
of tiny silicate rods and rings.
Looking at diatoms under a microscope is like looking into a bizarre microcosm:
the skeletons of the tiny creatures consist of surprisingly precise silicate
structures. Interest in such highly ordered inorganic
nanostructures, which are
considered to be potential
catalysts or nanotech components, is strong; yet
replication of these fossil structures has thus far seemed impossible. Whereas
organic materials can be assembled into defined superstructures by the
self-organization of smaller, tailored components, this is not possible for
inorganic substances. The
solution may be to use organic aggregates or large
biomolecules as a starting point for the
synthesis of inorganic structures.
A team led by Seiji Shinkai chose to use a plasmid DNA from
bacteria as a matrix
for the construction of tiny silicate structures. To pull this off, they needed
to overcome two obstacles: The silicate precursors they used are negatively
charged molecules, which will only stick to a positive matrix, and DNA also
carries negative charges. In addition, the silicate precursor requires an
organic solvent, while DNA is only soluble in
water. The team was able to solve
both problems in one elegant stroke: by attaching to the DNA a special ion, a
hydrocarbon chain with positively charged groups at both its head and tail. The
unusual thing is that the group at the chain's head is a guanidinium group, in
which two
nitrogen atoms "share" a positive charge so that they can
simultaneously dock onto two
oxygen atoms of a negatively charged phosphate
group on the DNA backbone, a particularly favorable arrangement. The DNA
phosphates' attraction to the guanidinium groups is thus much stronger than that
for the second charge at the "tail" of the chain. This group thus remains free
and gives the DNA a positive charge to attract and hold the silicate precursor.
The hydrocarbon chains also impart the necessary solubility of the DNA in the
organic solvent. Prepared in this manner, the plasmid DNA proved to be an ideal
matrix for silicate
Formation. The silicate precursors attach to the DNA and
then fuse together to form the silicate material. The DNA can subsequently be
removed by
heating. Plasmid DNA is usually twisted into a rodlike shape.
However,
enzymes can be used to relax it into a ring-shaped molecule, which
makes it possible to obtain both rods and rings of silicate. Different DNA types
may be used to create other nanosized "artificial fossils".