Nanobiotics
Conventional anti-biotics such as penicillins etc function metabolically,
by interfering with bacterial synthesis of the cell wall. Many bacteria
have evolved to develop resistance to common anti-biotics, and some
bacterial are resistent to most. In 1993, a new approach to developing such
agents was pioneered. Reza
Ghadiri at the Scripps Research Institute in California developed a
series of cyclic peptides using a combination of the natural (l) form with
the un-natural (d) amino acid enantiomer. It has been known for some time
that the stacking of crystals composed entirely of molecules of one chiral
form (all in a so called R or an S configuration) can be quite different
from the racemic form of the same molecule, which contains equal amounts of
R and S isomers. For example, the racemic form (R,S) of the Pirkle reagent
forms an infinite chain of hydrogen bonded interactions in the solid state
(left structure), whereas the enantiomerically pure S form forms
self-recognising dimers of a quite different type (right structure). [To view the
structures, install Chime in your browser: http://www.mdli.com/ ]
Exactly the same happens with the the cyclic peptides synthesised by
Ghadiri's group. This time, the R and S forms are created by coupling
different amino acids, and not simply taking a racemic mixture of each. The
monomeric structures of these peptides are shown below. Notice particularly
how water of crystallisation occupies the central cavity, and how
water-hating (lipophilic) groups occupy the outside rim
In the solid state, just like the (R,S) Pirkle reagent, these cyclic
peptides form an infinite chain, this time with the rings interacting via
hydrogen bonds by stacking on top of each other. This results in alignment
of the central cavities to form a tube or channel, filled with water
molecules. Adjacent tubes interact via dispersion forces aligning the
lipophilic rims. The result is a membrane-like "nanostructure". If you
rotate the structure below, you will eventually see five channels occupied
by the (red) water molecules, but you have to get the orientation exactly
orthogonal to see the effect.
When added to a culture of penicillin-resistant bacteria, these nanobiotics
are thought to insert into the lipophilic cell membrane. Here, surrounding
solvating water is removed, and the peptide rings instead stack into the
ziggurat, in a manner analogous to the crystal structures shown above. The
resulting channels allow water and other hydophilic species (e.g. glucose)
to drain out of the cell, killing it in around 15-20 seconds. By altering
the precise nature of the amino acids comprising the cyclic peptide,
various specific nanobiotics can be engineered. Unlike conventional
anti-biotics therefore, these substances do not function chemically but in
essence mechanically, at a true nanomolecular scale.
For further reading, see
- New
Scientist Feature by Trisha Gura
- "Peptide Nanotubes and Beyond", Jeffrey D. Hartgerink, Thomas D.
Clark, M. Reza Ghadiri,
Chemistry, 1998, 4,1367-1372.
- J. Sanchez-Quesada, M. R. Ghadiri, H. Bayley, and O. Braha,
J. Am. Chem. Soc., 2000, 122, in press.
- H. Rapaport, H. Sun Kim, K. Kjaer, P. B. Howes, S. Cohen, J.
Als-Nielsen, M. R. Ghadiri, L. Leiserowitz and M. Lahav,
J. Am. Chem. Soc, 1999, 121, 1186.
- T.D.Clark, J.M.Buriak, K.Kobayashi, M.P.Isler, D.E.McRee and
M.R.Ghadiri,
J.Am.Chem.Soc., 1998, 120, 8949.
- M.R.Ghadiri, K.Kobayashi, J.R.Granja, R.K.Chadhaand D.E.McRee,
Angew.Chem.,Int.Ed.Engl.,1995, 34, 93.
- "Photoswitchable Hydrogen-Bonding in Self-Organized Cylindrical
Peptide Systems", M. S. Vollmer, T. D. Clark, C. Steinem, M. R. Ghadiri
Angew. Chemie Int. Ed. 1999, 38, 1598-1601
- For details of the Pirkle reagent, H.S. Rzepa, M. L. Webb, A. M. Z.
Slawin and D. J. Williams, J. Chem. Soc., Chem. Commun., 1991,
765.
- Another cyclic peptide related to the ones described here is Gramicidin S. View
its structure here.