(part of Steps Toward Molecular Manufacturing)
- storage of MBBs:
The central idea why using an AFM to assemble large
macromolecular structures might be so very exciting lies in the
selectivity that is provided by positioning the reactive groups of a
MBB-to-be-added at the construction site with atomic precision, which
means that an estimated concentration difference between this reaction
site and the solution background of a factor of about 100,000,000
should be achievable. Consequently, the reaction rates should differ
by the same order of magnitude. So, even though the individual MBB in
solution has the same potential reactivity all the time, the
background of unwanted random reactions is negligibly low because the
concentration of MBBs in solution is in the µM range.
But by ordinary synthetic chemistry
standards this represents a pathologically low concentration of
material and the synthesis and storage of MBBs is almost certainly
going to occur at quite higher concentrations, very likely in the mM
range. But in turn this then means, if the MBBs have the same
potential reactivity all the time, that the rate of random reactions
jumps by a similar factor, namely about thousandfold. And so the raw
materials would get degraded at an uncomfortably high rate by mere
storage, before they ever even get to the scene of action. Clearly,
this problem has to be addressed. Several potential solutions are
thinkable.
Besides techniques like cryogenic storage and
storage in an inactive or protected form, the presumably simplest
method is the storage of only compatible classes of chemical
polarities.
As the stability problem in storage
arises mainly because MBBs will in the usual case have link-formation
functionalities of both polarities (as e.g. amino acids do too), which
enables them to react with each other and polymerize spontaneously,
the problem could be solved by somewhat restricting the freedom of
functionalization patterns. MBBs could be separated into different
distinct classes of linking-chemistries, which contain functional
groups of only one type of polarity that is not able to dimerize. This
segregation of MBBs into classes of mutually compatible chemical
polarities will limit the flexibility of inverting the polarity of
links and produces more complicated constraints for designing and
assembling macromolecules. But the big benefit will be on the
practical side with much easier handling of the raw
materials. Essentially no special precautions would have to be
followed.
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last updated Oct. 5 1996
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