Table 1 Comparison of the experimentally determined diffusion coefficients (Dexp) and hydrodynamic radii (rsexp) with the calculated ones (Dcalc).
Estimated hydrodynamic radii (rs), radius of gyration (Rg) and the maximal distance of an atom from the center of geometry (Rmax) are also listed
Dexp Â10À10/m2 sÀ1
rsexp/A
Dcalc Â10À10/m2 sÀ1
rs/A
Rg/A
Rmax/A
System
4
1.38
1.23
0.91
7.98
8.90
12.16
1.22
1.10
0.81
9.06
10.03
13.61
6.90
6.50
8.07
10.78
10.48
12.97
2À
2À
4 + SO4
5 + SO4
smaller (B8 A) than the experimental one (12 A). However,
the value Rmax, defined as the maximal distance of an atom
from the center of geometry (see ESIw for more information),
gives a better approximation for the hydrodynamic radius,
2À
namely for 5-SO4 (exp: 12.16 A, calc: 12.97 A). The values
2À
of Rmax obtained for 4 and 4-SO4 are similar and are also
slightly bigger than the experimental rs value of 4.
In conclusion we have shown that anion templation can
be used to synthesise a novel sulfate-encapsulated triply
interlocked capsule. Structural assignment is aided by 1H
NMR, ESI-MS and DOSY spectra. The experimentally
determined diffusion coefficients are in good agreement with
molecular dynamic simulations confirming the formation
of an interlocked structure with a hydrodynamic radius of
B12 A. Further studies into the anion binding properties of 4
and 5 are continuing in our laboratories.
2À
Fig. 2 Snapshot of 5-SO4 in DMSO solution taken after 20 ns of
simulation, showing the sulfate anion inside of the interlocked struc-
ture. Only urea N–H hydrogen binding groups are shown for clarity.
The DMSO solvent molecules are omitted.
The molecular dynamics simulations support entirely the
formation and the thermodynamic stability of the interlocked
structure as can be seen in Fig. S8 (ESIw), where the time
evolution of the N–HÁ Á ÁS distances for 20 ns of NPT data
collection run are shown. All urea N–H protons are kept
bonded to sulfate with NÁ Á ÁS distances varying between 2.13 A
and 4.15 A. Furthermore, the oxygen donors are shared with
more than one urea group, leading to different ureaÁ Á Ásulfate
environments with weaker (shared) or stronger (not shared)
interactions. A snapshot taken at the end of the simulation,
Y.L. thanks WEILUN and COSF for the financial support.
KMM thanks the EPSRC for a postdoctoral fellowship. P.J.C.
acknowledges the FCT for grant SFRH/BPD/27082/2006.
V. F. acknowledges the FCT with co-participation of the
European Community funds FEDER, for their financial
support under project PTDC/QUI/68582/2006.
Notes and references
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Rotaxanes and Knots. A Journey Through the World of Molecular
Topology, Wiley-VCH, Weinheim, Germany, 1999.
2À
illustrating 5-SO4 anion binding association, is shown in
Fig. 2.
2À
Evidence for the interlocked capsule formation (5-SO4
)
using the diffusion coefficients obtained from the DOSY NMR
experiments can also be corroborated by the molecular
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2À
dynamics simulations. The diffusion coefficients for 4, 4-SO4
2À
and 5-SO4 were estimated from the 10 ns trajectories of
NVE simulations using the Einstein relation (see ESIw
for details) and are given in Table 1. Although the diffusion
coefficient (Dcalc) calculated for 4 is slightly underestimated
when compared with the experimental one, leading to a bigger
value for the hydrodynamic radii (9.06 A) when compared
with the experimental one (7.98 A), agreement can be
considered to be good. The Dcalc values obtained for
2À
2À
4-SO4
and 5-SO4
are also in close agreement with
experimental values and, therefore, the calculated radii using
the Stokes–Einstein relation compare well with the experimentally
determined ones. Indeed, the ratio rs(5-SO42À)/rs(4-SO42À) is 1.35
for the experimental values and 1.36 for the calculated ones,
which is excellent. The calculated rs(5-SO42À) 13.61 A is only
B1.5 A bigger than the experimental one (12.16 A). We
also decided to check whether the theoretical values for
hydrodynamic radii are correlated or not with the radius of
gyration (Rg) of the systems. The estimated values of Rg for 4
and 4-SO42À are very similar (B6 A), but are smaller than the
experimentally determined hydrodynamic radii (rsexp), B8 A
8 J. W. Lee, B. K. Kim, J. H. Kim, W. S. Shin and S. H. Jin, Bull.
Korean Chem. Soc., 2005, 26, 1790–1794.
2À
and B9 A, respectively. The Rg value for 5-SO4 is much
ꢀc
This journal is The Royal Society of Chemistry 2009
7136 | Chem. Commun., 2009, 7134–7136