** The model was implemented in MicroMath Scientist for Windows,
version 2.01. A detailed description is given in the ESI.{
{{ At thermodynamic equilibrium the model gives a ratio of 49 : 51 for
1A : 1B instead of 50 : 50 when [1]0 = 50 mM. The reason is that the
amount of hydrazide A involved in complex formation is subtracted from
the amount of free A. Consequently, the slightly higher concentration of B
shifts the equilibrium in favor of hydrazone 1B.
reach the thermodynamic equilibrium. This may pose limits to the
type of reversible reaction used for ‘‘tethering’’. With this respect, it
may be noteworthy that all successful (biological) applications of
the ‘‘tethering’’ strategy1–7 rely on disulfide formation. Disulfide
formation is a fast reaction and very compatible with polar
solvents. As a final comment, it should be pointed out that the
above limitations of the ‘‘tethering’’ strategy are present regardless
of the type of molecular receptor considered.
1 D. A. Erlanson, A. C. Braisted, D. R. Raphael, M. Randal, R. M. Stroud,
E. M. Gordon and J. A. Wells, Proc. Natl. Acad. Sci. USA, 2000, 97,
9367–9372.
2 D. A. Erlanson, J. A. Wells and A. C. Braisted, Annu. Rev. Biophys.
Biomol. Struct., 2004, 33, 199–223.
3 A. C. Braisted, J. D. Oslob, W. L. Delano, J. Hyde, R. S. McDowell,
N. Waal, C. Yu and M. R. Arkin, J. Am. Chem. Soc., 2003, 125,
3714–3715.
4 B. G. Szczepankiewicz, G. Liu, P. J. Hajduk, C. Abad-Zapatero,
Z. Pei, Z. Xin, T. H. Lubben, J. M. Trevillyan, M. A. Stashko,
S. J. Ballaron, H. Liang, F. Huang, C. W. Hutchins, S. W.
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Support by MIUR (contract 2003037580, PRIN 2003) and the
University of Padova (Project CPDA054893) is gratefully
acknowledged.
Notes and references
{ All new compounds were characterized by 1H, 13C and 31P NMR
spectroscopy, ESI-MS and HPLC. See ESI.{
§ The mixture composition was determined by integrating the respective
signals for hydrazones 1A and 1B in the 1H NMR spectrum. Throughout
this study, the absence of any further changes in the 1H NMR spectra upon
standing at 50 uC was taken as an indicator that the thermodynamic
equilibrium was reached. This was independently confirmed by a control
experiment in which an identical 1H NMR spectrum was obtained starting
from two different mixtures (either preformed 1A or preformed 1B) upon
standing at 50 uC (see ESI{).
" The observed amplification curve is not caused by a difference in ionic
strength in the mixtures. In fact, the addition of an excess of
tetramethylammonium chloride (TMACl, 100 mM) to mixture
1 : A : B = 2 : 12 : 12 mM caused only a minor decrease in the observed
amplification (from 70 to 67%). In addition, rerunning the amplification
experiment at constant ionic strength ([A] + TMACl = 50 mM) did not
significantly affect the observed profile.
I The formation of other species (for example dimer 1A?1A as the most
likely candidate) that may affect the final product distribution cannot be
ruled out. However, the 1H NMR spectra of hydrazone 1A recorded at 2
and 25 mM (see ESI{). were superimposable, showing no indication
whatsoever of dimerization. Therefore, we decided to use a minimal model
involving only the thermodynamic equilibria required to explain the
experimental observations.
5 T. Obita, T. Muto, T. Endo and D. Kohda, J. Mol. Biol., 2003, 328,
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6 Y. Krishnan-Ghosh and S. Balasubramanian, Angew. Chem., Int. Ed.,
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898–952.
8 For reviews on dynamic combinatorial chemistry, see: (a) P. T.
Corbett, J. Leclaire, L. Vial, K. R. West, J.-L. Wietor, J. K. M.
Sanders and S. Otto, Chem. Rev., 2006, 106, 3652–3711; (b)
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Nat. Rev. Drug Discovery, 2002, 1, 26–36.
9 (a) R. M. Bennes and D. Philp, Org. Lett., 2006, 8, 3651–3654; (b) I. Saur
and K. Severin, Chem. Commun., 2005, 1471–1473; (c) P. T. Corbett,
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P. T. Corbett, S. Otto and J. K. M. Sanders, Chem. Eur. J., 2004, 10,
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1342 | Chem. Commun., 2007, 1340–1342
This journal is ß The Royal Society of Chemistry 2007