1H NMR and Molecular Modeling of â-Cyclodextrin-Indomethacin
J . Org. Chem., Vol. 61, No. 3, 1996 913
in the range 8.71 to 2.19 Å and 8.54 to 4.11 Å, respec-
tively. The NOE buildup curves shown in Figure 3
provide experimental evidence that the distance H7-H3′
is greater than H7-H5′, as correctly reproduced by the
geometry optimization. A more interesting picture comes
out if we compare the distance H4-H3′ for both com-
plexes, namely vii and iii: in the former it ranges from
8
.76 to 5.35 Å, therefore outside a suitable interval of
distances for the buildup of NOE, and in the latter spans
the range 6.40-2.60 Å. The presence of a significant
amount of the complex indomethacin Z-â-cyclodextrin
in solution would be consequently proved by an intense
dipolar correlation between H4 and H3′ in the ROESY
spectrum. Experimentally no intermolecular ROE be-
tween H4 and H3′ was observed.
F igu r e 5. Left: energy-minimized structure of the complex
1
2
E-2. Right: energy-minimized structure of the complex 1Z-
.
favourable interaction energies are obtained for the
topologies Cl up and N up for both Z and E isomers.
Hence, the Z isomer interacts with 2 mainly through the
indole ring (conformer iii), while for the E isomer the
p-chlorobenzoyl ring penetrates inside the torus of the
host molecule 2 (conformer vii). The graphical output
for the calculations of the geometries of complexes iii and
vii is displayed in Figure 5. The remaining two low-
energy conformations, ii and viii, are not expected to be
appreciably populated at room temperature as they differ
Con clu sion s
Free indomethacin 1 exists in solution as a mixture of
E and Z isomers, the latter being the prevalent form, as
demonstrated by low-temperature NMR spectra carried
out on the methyl ester 3. Complexation of 1 with âCD
(2) leads to a selective stabilization of the E isomer with
respect to Z; hence the major complex in solution is 1E-
âCD. This conformational change was experimentally
proven by chemical shift varations on the guest molecule
upon complexation, intermolecular NOEs and ROEs, and
energy calculations. The solution conformation of the
inclusion complex is characterized by the p-chlorobenzoyl
ring of 1E deeply inserted into the lipophylic cavity of
the host, the entry occurring through the larger rim of
the âCD truncated cone.
-
1
by 3.62 and 2.80 kcal mol
from the most stable
conformations, iii and vii, respectively. The most impor-
tant feature arising from the energy calculation is that
the complexation of indomethacin 1 with â-cyclodextrin
causes a reversal in the thermodynamical stability of the
two isomers of the guest. CVFF calculations on the
isolated guest gave a total energy for indomethacin Z 0.77
kcal lower than for indomethacin E, in good agreement
with our solution state measurements at low temperature
1
2
and with reported solid state and gas phase investiga-
tions.13 Conversely, Tables 2 and 3 show that the energy
Exp er im en ta l Section
of the complex host-1E (conformer vii) is 2.04 kcal more
stable than the corresponding adduct of 1Z (conformer
iii). van der Waals interactions appear to be the most
important stabilizing factor of the complex of indometha-
cin E (EvdW (vii) - EvdW (iii) ) -9.86 kcal); these favorable
interactions are only partially compensated by the major
ability of the carboxylate group of the Z isomer of
indomethacine to establish H-bonds with the secondary
OH groups located on the larger rim of the cyclodextrin
torus, as demonstrated by the coulombic contribution to
the total energy (ECoulomb (vii) - ECoulomb (iii) ) +6.47 kcal).
These data lead therefore to the conclusion that free 1
exists in solution mainly as the Z isomer, while 1
complexed with 2 is mainly in the E form. However,
since the energy data are obtained by in vacuo calcula-
tions they can only be used in a semiquantitative way.
Any attempt to derive a more quantitative picture (e.g.
Boltzmann populations) should be avoided since solva-
tation effects, not taken into account in the calculations,
are expected to play a role. Calculations also provide a
rationale for the unexpectedly large deshielding experi-
enced by H7 of 1 upon complexation: the low-field shift
is due to two different effects that coherently sum up,
the complexation and the stabilization of the E form of 1
in the complex.
1H NMR spectra were recorded at 200.13, 300.13, and 400.13
MHz. The ROESY experiment on the complex was performed
using a mixing time of 500 ms.
The variable temperature spectra were carried out on
indomethacin methyl ester 3 in the temperature range 298-
1
2 2 2
70 K in CH Cl -d (Merck Co. Italy).
1
-(p-Ch lor oben zoyl)-5-m eth oxy-2-m eth ylin d ole-3-a ce-
tic Acid Sod iu m Sa lt (1). Indomethacin was obtained from
Sigma Co. (Italy) and converted into sodium salt 1 by NaOH
titration. The water solution was then freeze-dried, dissolved
in D O, and freeze-dried two times. The NMR characterization
2
is reported in Table 1.
Com p lex of th e In d om eth a cin Sod iu m Sa lt w ith â-Cy-
clod extr in . A 5.3 mM solution of 1 in water was added to a
5
.3 mM solution of 2. The reaction mixture was allowed to
stand overnight and then freeze-dried. The solid was in turn
redissolved in D O and freeze-dried two times. A selection of
2
1
the H NMR data for the complex are reported in Table 1.
1-(p-Ch lor oben zoyl)-5-m eth oxy-2-m eth ylin d ole-3-a ce-
tic a cid Meth yl Ester (3). 1-(p-Chlorobenzoyl)-5-methoxy-
2
-methylindole-3-acetic acid (206 mg) dissolved in 5 mL of 2,2-
dimethoxypropane was added of 0.5 mL of 37% HCl. The
solution was stirred at room temperature until the reaction
was complete (ca. 2 h), as monitored by TLC. Successively
the solvent was evaporated at reduced pressure and the crude
material suspended in water and extracted several times with
ethyl acetate. After flash cromatography (eluant hexane-
EtOAc ) 75:25) 192 mg of a pale yellow solid was obtained,
1
with an overall yield of 90%: mp 92-93 °C; H NMR (CH
2
-
Interproton distances between H3′ and H5′ of the seven
glucose units of â-cyclodextrin and H7, H6, and H4 of 1
measured on the minimized geometries of vii and iii are
consistent with the ROEs and transient NOE data (we
assumed 4.5 Å as cutoff distance for detectable Over-
hauser enhancements). The complex with the lowest
energy, 1E-âCD (vii), showed nonbonded distances of H7
of indomethacin with the seven H3′ and H5′ of the host
Cl
2
-d
2
) δ 7.65 (m, 2H), 7.49 (m, 2H), 6.94 (d, J ) 2.6 Hz, 1H),
6
3
.93 (d, J ) 9.1 Hz, 1H), 6.66 (dd, J ) 2.6 and 9.1 Hz, 1H),
.81 (s, 3H), 3.68 (s, 3H), 3.67 (s, 2H), 2.32 (s, 3H).
Com p u ta tion a l Deta ils
Molecular modeling and dynamics simulations were
carried out on a Silicon Graphics Personal IRIS 4D-35