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Molecular Modeling of β‐Cyclodextrin Complexes with Nootropic Drugs
Molecular Modeling of &C yclodextrin Complexes with Nootropic Drugs MARIAE. AMATO*', FLORENCE DJEDAi"I*, GIUSEPPE c. PAPPAIARDO', BRUNOPERLY*, AND GIUSEPPE SCARIATA* Received March 8, 1991, from the 'Dipartimento di Scienze Chimiche, Universitct di Catania, Wale A. Doria 6, 95725 Catania, Italy, *CEN Saclay, Service de Chimie Molbculaire, 91 191 Gif-sur-Yvette,France, and the 'Dipartimento di Scienze Chimiche, I1 Cattedra di Chimica Generale, Accepted for publication March 23, 1992. Fawltd di Farmacia, Universitct di Catania, Viale A. Doria 6, 95125 Catania, Italy.
Abstract 0 The geometry and structural features of the inclusion winplexes of Pcyclodextrin (pCD) with the chiral antiamnesic drugs (?)-l -benzyl-4-hydroxymethylpyrrolidin-2-one (WEB-1 868). (*)-1benzeneaulfonyl-5-etoxypyrrolidin-2-one (RU-35929), and (&)-1-(3pyridiniysuWc~nyl)-5ethoxypyrrolidin-2-one(RU-47010) were studied by the molecular modeling method (MacroModel interactive computer program). Docking proceduresyielded the most stable complexes, which showed the aromatic ring of the guests inside the cavity and the pyrrolidinone ring out from the side of the p C D secondary hydroxyl groups. The binding energies were essentially due to hydrogen-bonded structures involving the C=O group of the guests. Selective interactions allowed chiral discrimination, and accordingly, separate p C D complexes of the Rand S enantiomers of each guest compound were studied. The almost round g C D structure, in all the cases, assumed an elliptic shape on passing from the isolated molecule to the docked complex. The optimized structures and conformations of p C D and its inclusion compounds showed acceptable general agreement with information from proton nuclear magnetic resonance studies.
Experimental Methods P C D ( F l u b ) was used without purification. Compound 1(gif€from Boehringer-Ingelheim) was recrystallized from ethyl acetatecyclohexane. Compounds 2 and 3 (giftsfrom Roussel-Ucla were recrystallized from ethanol. The complexes were obtained by mixing equimolar solutions (10 mM) of guest and BCD. All samples were freeze dried twice from D,O. The 'H NMR spectra of the samples in D,O were recorded at 298 K with a Bruker WM500 spectrometer operating at 500.13 MHz. Chemical shifts are given relative to the external standard tetramethylsilane. Computational Methodology-caldations were performed with the MamModel program7 run on a VAX-VMS 3500 computer controlling a Tektronix 4111 high-resolution graphics system. The startingstructure of p C D was created by the Grow subroutine of the carbohydrate mode, and the structures of guest molecules were generated by the manualdrawing routine in MacroModel. The molecular mechanics (MM2) force field method' was used for energy minimization of geometries and conformations of the isolated host and guest molecules (the root mean squares of the gradients vectors were
Pharmaceutical applications of cyclodextrins (CDs) as additive and drug-complexing agents have been rapidly growing.1 The interaction of guest molecules with CDs may induce useful modificationsof the chemical and physical properties of the guest molecule, which may lead to improved stability, solubility in aqueous medium, and bioavailability.2 Poorly water soluble drugs, therefore, can be orally administered in the complexed form, by taking advantage of the wellestablished low toxicity of the CDs by the oral route.2 The value of inclusion complex formation is enhanced by potential analytical applications in the separation of structural, geometric, and optical isomers. Therefore, experimental and theoretical information on the geometry and structural features of the CD inclusion complexes, as well as on the topology of the host-guest interactions, is increasingly important. Molecular modeling methods by use of computers are valuable new tools for obtaining information on the structure and geometry of the inclusion compounds. This work reports a molecular-modeling study of pcyclodextrin (PCD) inclusion compounds formed by the chiral nootropic drugs (*)-1-benzyl-4-hydroxymethylpyrrolidin-2one (1; WEB-1868) (~)-1-benzenesulfonyl-5-ethoxypyrrolidin-2-one) (2; RU-35929), and (+)-1-(3-pyridinylsulfony1)-5ethoxypy~rolidin-2-one (3; RU-47010). Nootropic (mindacting) drugs are a new class of drugs for treatment of organic brain spdromes.3.4 The aim of this study was to get insight into both the stereochemical aspects of the complexation process and the interpretation of the proton NMR ('H NMR) spedra of the complexes in water solution. The conformational properties of 1,2, and 3 in the three aggregation states of matter have been deedbed in previous papers.6.6 OO22-3549/92/120@1 157$02.50/0 0 1992, American Pharmaceutical Association
WEB-1868 (1)
RU-35929 (2)
RU-47010 (3) Journal of Pharmaceutical Sciences I 1157 Vol. 81, No. 12, December 1992
lidinone ring and the side chain may interact with the edge of the host. Before the modeling procedure, a round-shaped structure was obtained for the &CD molecule by MM2 calculations. The 0(2),-0(3),+ intramolecular distances of hydroxyl groups of adjacent glycosyl residues were in the range of 2.77-3.11 A, with an average of 2.89 indicating that they are linked by hydrogen bonds that stabilize the round conformation of &CD.Ss 12-14 The calculated glycosidic angles (in the range of 116.4-117.2", with a n average of 116.8") were in good agreement with experimental data from X-ray analysis (117.4")12 and previous MM2 calculations (116.9916 p C D and slightly lower than the data from neutron diffraction of p-CD * 11H 2 0 (119").13 Docking calculations were performed for both R and S enantiomers of 1, 2, and 3 to study the specific interactions between such asymmetric guests and the chiral cavity of &CD. The formation of complexes with y-cyclodextrin was excluded on the basis of both docking calculations and NMR evidence. Molecular modeling for complexes with a-cyclodextrin show that host-guest interactions are very weak. This finding is in line with the small NMR shift of the H-3 resonance of the a-cyclodextrin, suggesting partial inclusion of the guests from the side of the secondary hydroxyl groups. The approach of the pyrrolidinone moiety of the guests toward &CD produced very weak interactions for 1,and quite energetically unfavorable interactions were observed for both 2 and 3. It was not surprising to find steric repulsive interactions between &CD and the pyrrolidinone rings of 2 and 3, which bear the bulky C(5)-ethoxy substituent. The relatively weak interactions calculated for 1 could be attributed to the less bulky -CH,OH substituent at C(4), which emerges from the hydrophobic cavity at the hydroxyl edge level and, consequently, determines the effective hydrogen bond with the primary hydroxyls of &CD. Table I shows the gas-phase binding energies as calculated by subtracting the
calculations were performed excluding water; solvation effects and hydrophobic-hydrophilic interactions were both explicitly and implicitly ignored. The calculated overall binding energy included van der Waals and charge interactions as attractive forces producing a complex.^ To check the nonpolar character of the cavity of the optimized PCD, preliminary trial calculations were made by inserting a number of water molecules (3-10) inside the cavity. About 100 water molecules were placed in a pseudospheric disposition outside the PCD structure. The energy minimization procedure pushed all the water moleculesout of the cavity and close to the hydroxyl groups of the edges of the P C D basket. Hydrophobic interactions are considered to be mainly responsible for inclusion complex formation, and the lack of a suitable model of the solvent effect may cause the structures to appear more flexible than they are in reality.
Results and Discussion Previously reported 'H NMR results provide evidence for the formation of 1:l complexes in solution for all the drugs investigated.10 In all complexes, the characteristic upfield shift of the H-3 and H-5 protons of PCD was obeerved.10 An effect is also observed on guest molecules: upon inclusion, some proton signals appear in duplicate, as expeded for diastereoisomeric pairs arising from a racemic mixture of a guest in the presence of the chiral CD.11 Doubling of aromatic proton resonances of 1 and 2 produced unresolved multiplets. Except for the H-2' resonance, all the pyrrolidinone ring proton signals of 1 were duplicated; the H-6 and H-6' protons showed the greatest separation of chiral resonances (Figure 1).Pyridine proton resonances of 3 gave rise to three superimposed, well-resolved seta of signals that could reasonably arise from the free guest and the two diastereoisomeric complexed forms (Figure 2). Complex, unresolved multiplets were observed for aliphatic ring protons of 3, except the side-chain ethoxy protons that were well resolved and in duplicate form. These results indicate that the guest molecules penetrate the &CD cavity at the wide secondary hydroxyl side of the aromatic ring first, and both the pyrro-
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.
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3.2
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2.8
2.6
.
2.4
Figure l-tligh-field region of the 500-MHz 'H NMR spectra of 1 in D,O: (a) no additive; (b) after addition of an equimolar amount of PCD. 1158 I Journal of Pharmaceutical Sciences Vol. 81, No. 12,December 1992
8
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9.0 8.5 8.0 Flgure 2-Low-field region of the 500-MHz 'H NMR spectra of S p C D complex in D20. energies of the components from the energy of the complexes. Electrostatic and van der Waals forces are the main contributors to the total energy of the complexes. The contribution of hydrogen bonding is less than that of electrostatic or van der Waals forces. In all cases, the binding energies increased when the phenyl ring approached the cavity. The most favorable interactions, ensuring the formation of more stable complexes, were found as the aromatic ring approached P C D at the side of the secondary hydroxyl groups (Table I). Despite the long computer time required, calculations were repeated to check the safe location of the absolute energy minimum inside the torus cavity. This search was accomplished both through small movements along the docking axis about the minimum position and random rotations of the guest molecule relative to the host. Fully optimized conformations and structures of the most favorable inclusion complexes derived from these docking procedures were compared with those of the isolated host and guest molecules (Table 11). The final structures of the host-guest complexes are shown along two projections in Figures 3-5. Inspection of Figures 3-5 shows that the complexes with lowest energy have the aromatic ring of the guests approximately accommodated in the center of the cavity and the pyrrolidinone ring that protrudes from the side of the secondary hydroxyl groups.
b
03
9
,
Flgure *Top view (Left) and side view (Rlght) of the computergenerated structures of the inclusion complexes of the (A) Rand (B) S enantiomers of 1.
Because of their angular bridged structures, the guests were inserted into the host cavity with the aromatic ring inclined with respect to the oxygen glycosidicplane. For 1,the phenyl plane was tilted by -10" and 16" in the R and S isomers, respectively. By contrast, the aromatic rings of the R enantiomers of 2 and 3 were more inclined (-26" and 18",
Table M a c P h a w Blndlng Energles of &CD lnclublon Complexes wlth 1, 2, and 3 Attalned by Docklng Procedure
Binding Energy (kcal/mol)of:
Compound
Configuration
1
Pyrrolidinone Ring Entering from p C D Secondary Hydroxyl Side
S R S R
3
Aromatic Ring Entering from p C D Secondary Hydroxyl Side
-8.29
-23.37 -22.50 - 18.20 - 19.38 -18.0 -20.42
-11.57 -12.8 24.53 14.84 7.21 3.55
R
2
Aromatic Ring Entering from p C D Primary Hydroxyl . . Side
S
- 14.00 -5.51 -13.92 -5.40 - 12.6
Optimized
Structure -27.90 -27.36 -24.28 -26.04 -24.60 -26.53
Table IkDlhedral Angles of Isolated and &CD-Included 1,2, and 3"
Dihedral Angle, ' Compound 1
Configuration R
S 2
R
3
R
S S
C(2)-N(l)-X(6)4(7)
C(8)-C(7)-X(6)-N(l)
C(3)-C(4)-C(5)-N(l)
N(l)-C(2)-C(3)-C(4)
Isolated
Included
Isolated
Included
Isolated
Included
Isolated
Included
-69.2 69.2 34.9 -34.9 32.0 -32.0
-69.6 64.4 56.8 -58.6 58.4 -58.9
-64.1 64.1 - 125.5 125.5 - 125.0 125.9
-57.0 64.1 -144.9 132.2 - 150.4 150.1
-33.2 33.2 30.9 -30.9 30.9 -30.9
-31.2 34.4 32.9 -27.8 32.2 -28.0
-21.0 21 .o 1.7 -1.7 1.6 -1.6
- 18.0 21.2 1.5 2.3 0.0 3.0
' X = C (in 1) or S (in 2 and 3).
Journal of Pharmaceutical Sciences I 1159 Vol. 87. No. 12, December 7992
Flgure &Top
view (Left) and side view (Rlght) of the computergenerated structures of the inclusion complexes of the (A) Rand (B)S enantiomers of 2. respectively) than the same rings of the S isomers (-13" and 8",respectively) with respect to the O(4) plane. The less inclined aromatic rings were also more deeply inserted into the cavity. These observed locations allowed the pyrrolidinone ring to lie very close to the P C D secondary hydroxyls and the carbonyl groups and, in all cases, to form a hydrogen bond with one of them (Table 111). Hydrogen bonding determines specific interaction sites, although it contributes little to the binding energy of the complexes. This finding was important in the exploitation of the chiral selectivity of p-CD toward asymmetric guests. Because of the symmetrically cylindrical shape of the F C D cavity, the three-point interaction required for enantiomer recognition must be verified by close contacts between the two hydroxyl groups of the p-CD rim and the two substituents of the chiral center of the guest, in addition to the tight inclusion of the aromatic ring. In the complex of 1, the carbonyl group was hydrogen bonded with the O(3)H hydroxyl of the G-1 glucose unit of p-CD in both R and S isomers. For the S enantiomer of 1, a hydrogen bond was also formed between the hydroxyl of the -CH,OH substituent and the O(2)H of the same glucose unit, corresponding to the earlier-mentioned O(3)H .*..0 = C hydrogen bond. In the R enantiomer, the O(3)H oxygen of the G-3 glucose unit was involved in the hydrogen bond with the -CH,OH moiety of the guest 1. In the p-CD complexes with 2 and 3, the carbonyl groups of 2 and 3 were at one hydrogen bond distance from one secondary hydroxyl of PCD, the O(2)H of p-CD was involved with the R enantiomers, and the O(3)H of p-CD was involved with the S enantiomers. Additional hydrogen bond interactions were not detectable for the ethoxy substituent at the chiral carbons of 2 and 3. However, the more inclined axis of the aromatic ring of the R isomers caused their out-of-center shifting in the cavity. As a consequence, the ethoxy group of the R enantiomers of 2 and 3 was closer to the wall of P C D than that of the corresponding S isomers (Figures 4 and 5). As a common feature, the macrocyclic 8-CD ring, which has 1160 I Journal of Pharmaceutical Sciences Vol. 81, No. 12, December 1992
Figure S T O P view (Left) and side view (Rlght) of the computergenerated structures of the inclusion complexes of the (A) Rand (B)S enantiomers of 3.
a round shape when empty, markedly distorted during complex formation to a n approximately elliptic geometry on which lie the O(4) atoms. In this geometry, the plane of the aromatic ring of the guests was closely perpendicular to the short axis of the ellipse. The largest distortion was observed for the inclusion complexes of the R and S enantiomers of 1. The ellipse-shaped structure of the complexed P C D agrees with general views based on X-ray analyses of solid P C D complexes with aromatic and aliphatic guests12J6 and indicates that the host molecule undergoes conformational modifications to ensure the fitting of the guest into the cavity. However, all our calculations were referred to the gas phase, and packing forces in the crystal phase or solvation effects could well determine the preferred binding mode with real systems.
References and Notes 1. Szejtli, J. In Cyclodeztrin Technology; Kluwer Academic: Dordrecht, The Netherlands, 1988; pp 186-293. 2. Duchhe, D. In P m . Fourth Int. Symp. on Cyclodextrins; Huber, 0.;Szejtli, J., Eds.;Kluwer Academic: Dordrecht, The Netherlands, 1988; pp 265-275. 3. Tallal, P. J. Clin. Psychopharmacol. 1985, 5, 272. 4. Kabes, J. J. Int. Med. Res. 1985, 13, 185. 5. Amah, M. E.; Bandoli, G.;Djedaini, F.; Dolmella, A.; Grassi, A.; Pappalardo, G. C. J. Mol. Struct. 1990,222,487-502. Table Ill-Hydrogen Bond Dlstances between Carbonyl Groups of the Guests and p C D Secondary Hydroxyls
Compounds
Configuration
Hydrogen Bond Distance, A C=O HO-CH,OH .* OH *-.a
~
1
R
S 2
R
3
R S
S
a-,
Not applicable.
1.865 1.93 1.849 1.a25 1.so1 1.808
2.119 2.076
-a -
-
6. Amato, M. E.; Bandoli, G.; Cadlato, U.; Pappalardo, G. C.; Toja,
E.J. Mol. Strud. 1990,238, 413-427.
7. MacroModel version 2.5,Department of Chemistry, Columbia University, New York, NY. 8. Harata, K. Bull. Chem. Soc. Jpn. 1988,61,1939-1944. 9. Tabushi, I.; Ki osuke, Y.; Sugimoto,T.; Yamamura, K. J. Am. Chem. SOC. I~~'&Ioo, 9~-919. 10. Amato, M. .E.;D'epfiini, F.; Perly, B. Minutes, Fifth Int. Symp. on Cyclodextnns; dhtione de Sans: Paris, 1990;pp 138-141. 11. Casy, A.F.;Mercer, A. D.Magn.Reson. Chem. 1988,26, 765774. 12. Harata, K. Bull. Chem.Soc. Jpn. 1982,55,2315-2320. 13. Stainer, T.; Maeon Sax, A.; Saenger, W.J. Am. Chem. Soc. 1990,
112,6184-6190. 14. Betzel, C.; Saenger, W.;Hingerty, B. E.; Brown, G. M.J. Am. Chem. Soc 1984,106,7545-7557. 15. Lu,T.-X.; Zhang, D.-B.; Dong,S.J. J. Chem. Soc., Faraday Trans. 2 1989,85,1439-1445. 16. Allen, F.H.; Kennard, 0.; Taylor, R. Acc. Chem. Res. 1983,16, 146-153.
Acknowledgments This work was supported by MPI of Italy. Our thanks are 'ven to bussel-Uclaf and to Boehringer-lngelheim for the kind g i a o f the samples.
Journal of Pharmaceutical Sciences I 1161 Vol. 81, No. 12, December 1992
Abstract 0 The geometry and structural features of the inclusion winplexes of Pcyclodextrin (pCD) with the chiral antiamnesic drugs (?)-l -benzyl-4-hydroxymethylpyrrolidin-2-one (WEB-1 868). (*)-1benzeneaulfonyl-5-etoxypyrrolidin-2-one (RU-35929), and (&)-1-(3pyridiniysuWc~nyl)-5ethoxypyrrolidin-2-one(RU-47010) were studied by the molecular modeling method (MacroModel interactive computer program). Docking proceduresyielded the most stable complexes, which showed the aromatic ring of the guests inside the cavity and the pyrrolidinone ring out from the side of the p C D secondary hydroxyl groups. The binding energies were essentially due to hydrogen-bonded structures involving the C=O group of the guests. Selective interactions allowed chiral discrimination, and accordingly, separate p C D complexes of the Rand S enantiomers of each guest compound were studied. The almost round g C D structure, in all the cases, assumed an elliptic shape on passing from the isolated molecule to the docked complex. The optimized structures and conformations of p C D and its inclusion compounds showed acceptable general agreement with information from proton nuclear magnetic resonance studies.
Experimental Methods P C D ( F l u b ) was used without purification. Compound 1(gif€from Boehringer-Ingelheim) was recrystallized from ethyl acetatecyclohexane. Compounds 2 and 3 (giftsfrom Roussel-Ucla were recrystallized from ethanol. The complexes were obtained by mixing equimolar solutions (10 mM) of guest and BCD. All samples were freeze dried twice from D,O. The 'H NMR spectra of the samples in D,O were recorded at 298 K with a Bruker WM500 spectrometer operating at 500.13 MHz. Chemical shifts are given relative to the external standard tetramethylsilane. Computational Methodology-caldations were performed with the MamModel program7 run on a VAX-VMS 3500 computer controlling a Tektronix 4111 high-resolution graphics system. The startingstructure of p C D was created by the Grow subroutine of the carbohydrate mode, and the structures of guest molecules were generated by the manualdrawing routine in MacroModel. The molecular mechanics (MM2) force field method' was used for energy minimization of geometries and conformations of the isolated host and guest molecules (the root mean squares of the gradients vectors were
Pharmaceutical applications of cyclodextrins (CDs) as additive and drug-complexing agents have been rapidly growing.1 The interaction of guest molecules with CDs may induce useful modificationsof the chemical and physical properties of the guest molecule, which may lead to improved stability, solubility in aqueous medium, and bioavailability.2 Poorly water soluble drugs, therefore, can be orally administered in the complexed form, by taking advantage of the wellestablished low toxicity of the CDs by the oral route.2 The value of inclusion complex formation is enhanced by potential analytical applications in the separation of structural, geometric, and optical isomers. Therefore, experimental and theoretical information on the geometry and structural features of the CD inclusion complexes, as well as on the topology of the host-guest interactions, is increasingly important. Molecular modeling methods by use of computers are valuable new tools for obtaining information on the structure and geometry of the inclusion compounds. This work reports a molecular-modeling study of pcyclodextrin (PCD) inclusion compounds formed by the chiral nootropic drugs (*)-1-benzyl-4-hydroxymethylpyrrolidin-2one (1; WEB-1868) (~)-1-benzenesulfonyl-5-ethoxypyrrolidin-2-one) (2; RU-35929), and (+)-1-(3-pyridinylsulfony1)-5ethoxypy~rolidin-2-one (3; RU-47010). Nootropic (mindacting) drugs are a new class of drugs for treatment of organic brain spdromes.3.4 The aim of this study was to get insight into both the stereochemical aspects of the complexation process and the interpretation of the proton NMR ('H NMR) spedra of the complexes in water solution. The conformational properties of 1,2, and 3 in the three aggregation states of matter have been deedbed in previous papers.6.6 OO22-3549/92/120@1 157$02.50/0 0 1992, American Pharmaceutical Association
WEB-1868 (1)
RU-35929 (2)
RU-47010 (3) Journal of Pharmaceutical Sciences I 1157 Vol. 81, No. 12, December 1992
lidinone ring and the side chain may interact with the edge of the host. Before the modeling procedure, a round-shaped structure was obtained for the &CD molecule by MM2 calculations. The 0(2),-0(3),+ intramolecular distances of hydroxyl groups of adjacent glycosyl residues were in the range of 2.77-3.11 A, with an average of 2.89 indicating that they are linked by hydrogen bonds that stabilize the round conformation of &CD.Ss 12-14 The calculated glycosidic angles (in the range of 116.4-117.2", with a n average of 116.8") were in good agreement with experimental data from X-ray analysis (117.4")12 and previous MM2 calculations (116.9916 p C D and slightly lower than the data from neutron diffraction of p-CD * 11H 2 0 (119").13 Docking calculations were performed for both R and S enantiomers of 1, 2, and 3 to study the specific interactions between such asymmetric guests and the chiral cavity of &CD. The formation of complexes with y-cyclodextrin was excluded on the basis of both docking calculations and NMR evidence. Molecular modeling for complexes with a-cyclodextrin show that host-guest interactions are very weak. This finding is in line with the small NMR shift of the H-3 resonance of the a-cyclodextrin, suggesting partial inclusion of the guests from the side of the secondary hydroxyl groups. The approach of the pyrrolidinone moiety of the guests toward &CD produced very weak interactions for 1,and quite energetically unfavorable interactions were observed for both 2 and 3. It was not surprising to find steric repulsive interactions between &CD and the pyrrolidinone rings of 2 and 3, which bear the bulky C(5)-ethoxy substituent. The relatively weak interactions calculated for 1 could be attributed to the less bulky -CH,OH substituent at C(4), which emerges from the hydrophobic cavity at the hydroxyl edge level and, consequently, determines the effective hydrogen bond with the primary hydroxyls of &CD. Table I shows the gas-phase binding energies as calculated by subtracting the
calculations were performed excluding water; solvation effects and hydrophobic-hydrophilic interactions were both explicitly and implicitly ignored. The calculated overall binding energy included van der Waals and charge interactions as attractive forces producing a complex.^ To check the nonpolar character of the cavity of the optimized PCD, preliminary trial calculations were made by inserting a number of water molecules (3-10) inside the cavity. About 100 water molecules were placed in a pseudospheric disposition outside the PCD structure. The energy minimization procedure pushed all the water moleculesout of the cavity and close to the hydroxyl groups of the edges of the P C D basket. Hydrophobic interactions are considered to be mainly responsible for inclusion complex formation, and the lack of a suitable model of the solvent effect may cause the structures to appear more flexible than they are in reality.
Results and Discussion Previously reported 'H NMR results provide evidence for the formation of 1:l complexes in solution for all the drugs investigated.10 In all complexes, the characteristic upfield shift of the H-3 and H-5 protons of PCD was obeerved.10 An effect is also observed on guest molecules: upon inclusion, some proton signals appear in duplicate, as expeded for diastereoisomeric pairs arising from a racemic mixture of a guest in the presence of the chiral CD.11 Doubling of aromatic proton resonances of 1 and 2 produced unresolved multiplets. Except for the H-2' resonance, all the pyrrolidinone ring proton signals of 1 were duplicated; the H-6 and H-6' protons showed the greatest separation of chiral resonances (Figure 1).Pyridine proton resonances of 3 gave rise to three superimposed, well-resolved seta of signals that could reasonably arise from the free guest and the two diastereoisomeric complexed forms (Figure 2). Complex, unresolved multiplets were observed for aliphatic ring protons of 3, except the side-chain ethoxy protons that were well resolved and in duplicate form. These results indicate that the guest molecules penetrate the &CD cavity at the wide secondary hydroxyl side of the aromatic ring first, and both the pyrro-
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3.2
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2.8
2.6
.
2.4
Figure l-tligh-field region of the 500-MHz 'H NMR spectra of 1 in D,O: (a) no additive; (b) after addition of an equimolar amount of PCD. 1158 I Journal of Pharmaceutical Sciences Vol. 81, No. 12,December 1992
8
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9.0 8.5 8.0 Flgure 2-Low-field region of the 500-MHz 'H NMR spectra of S p C D complex in D20. energies of the components from the energy of the complexes. Electrostatic and van der Waals forces are the main contributors to the total energy of the complexes. The contribution of hydrogen bonding is less than that of electrostatic or van der Waals forces. In all cases, the binding energies increased when the phenyl ring approached the cavity. The most favorable interactions, ensuring the formation of more stable complexes, were found as the aromatic ring approached P C D at the side of the secondary hydroxyl groups (Table I). Despite the long computer time required, calculations were repeated to check the safe location of the absolute energy minimum inside the torus cavity. This search was accomplished both through small movements along the docking axis about the minimum position and random rotations of the guest molecule relative to the host. Fully optimized conformations and structures of the most favorable inclusion complexes derived from these docking procedures were compared with those of the isolated host and guest molecules (Table 11). The final structures of the host-guest complexes are shown along two projections in Figures 3-5. Inspection of Figures 3-5 shows that the complexes with lowest energy have the aromatic ring of the guests approximately accommodated in the center of the cavity and the pyrrolidinone ring that protrudes from the side of the secondary hydroxyl groups.
b
03
9
,
Flgure *Top view (Left) and side view (Rlght) of the computergenerated structures of the inclusion complexes of the (A) Rand (B) S enantiomers of 1.
Because of their angular bridged structures, the guests were inserted into the host cavity with the aromatic ring inclined with respect to the oxygen glycosidicplane. For 1,the phenyl plane was tilted by -10" and 16" in the R and S isomers, respectively. By contrast, the aromatic rings of the R enantiomers of 2 and 3 were more inclined (-26" and 18",
Table M a c P h a w Blndlng Energles of &CD lnclublon Complexes wlth 1, 2, and 3 Attalned by Docklng Procedure
Binding Energy (kcal/mol)of:
Compound
Configuration
1
Pyrrolidinone Ring Entering from p C D Secondary Hydroxyl Side
S R S R
3
Aromatic Ring Entering from p C D Secondary Hydroxyl Side
-8.29
-23.37 -22.50 - 18.20 - 19.38 -18.0 -20.42
-11.57 -12.8 24.53 14.84 7.21 3.55
R
2
Aromatic Ring Entering from p C D Primary Hydroxyl . . Side
S
- 14.00 -5.51 -13.92 -5.40 - 12.6
Optimized
Structure -27.90 -27.36 -24.28 -26.04 -24.60 -26.53
Table IkDlhedral Angles of Isolated and &CD-Included 1,2, and 3"
Dihedral Angle, ' Compound 1
Configuration R
S 2
R
3
R
S S
C(2)-N(l)-X(6)4(7)
C(8)-C(7)-X(6)-N(l)
C(3)-C(4)-C(5)-N(l)
N(l)-C(2)-C(3)-C(4)
Isolated
Included
Isolated
Included
Isolated
Included
Isolated
Included
-69.2 69.2 34.9 -34.9 32.0 -32.0
-69.6 64.4 56.8 -58.6 58.4 -58.9
-64.1 64.1 - 125.5 125.5 - 125.0 125.9
-57.0 64.1 -144.9 132.2 - 150.4 150.1
-33.2 33.2 30.9 -30.9 30.9 -30.9
-31.2 34.4 32.9 -27.8 32.2 -28.0
-21.0 21 .o 1.7 -1.7 1.6 -1.6
- 18.0 21.2 1.5 2.3 0.0 3.0
' X = C (in 1) or S (in 2 and 3).
Journal of Pharmaceutical Sciences I 1159 Vol. 87. No. 12, December 7992
Flgure &Top
view (Left) and side view (Rlght) of the computergenerated structures of the inclusion complexes of the (A) Rand (B)S enantiomers of 2. respectively) than the same rings of the S isomers (-13" and 8",respectively) with respect to the O(4) plane. The less inclined aromatic rings were also more deeply inserted into the cavity. These observed locations allowed the pyrrolidinone ring to lie very close to the P C D secondary hydroxyls and the carbonyl groups and, in all cases, to form a hydrogen bond with one of them (Table 111). Hydrogen bonding determines specific interaction sites, although it contributes little to the binding energy of the complexes. This finding was important in the exploitation of the chiral selectivity of p-CD toward asymmetric guests. Because of the symmetrically cylindrical shape of the F C D cavity, the three-point interaction required for enantiomer recognition must be verified by close contacts between the two hydroxyl groups of the p-CD rim and the two substituents of the chiral center of the guest, in addition to the tight inclusion of the aromatic ring. In the complex of 1, the carbonyl group was hydrogen bonded with the O(3)H hydroxyl of the G-1 glucose unit of p-CD in both R and S isomers. For the S enantiomer of 1, a hydrogen bond was also formed between the hydroxyl of the -CH,OH substituent and the O(2)H of the same glucose unit, corresponding to the earlier-mentioned O(3)H .*..0 = C hydrogen bond. In the R enantiomer, the O(3)H oxygen of the G-3 glucose unit was involved in the hydrogen bond with the -CH,OH moiety of the guest 1. In the p-CD complexes with 2 and 3, the carbonyl groups of 2 and 3 were at one hydrogen bond distance from one secondary hydroxyl of PCD, the O(2)H of p-CD was involved with the R enantiomers, and the O(3)H of p-CD was involved with the S enantiomers. Additional hydrogen bond interactions were not detectable for the ethoxy substituent at the chiral carbons of 2 and 3. However, the more inclined axis of the aromatic ring of the R isomers caused their out-of-center shifting in the cavity. As a consequence, the ethoxy group of the R enantiomers of 2 and 3 was closer to the wall of P C D than that of the corresponding S isomers (Figures 4 and 5). As a common feature, the macrocyclic 8-CD ring, which has 1160 I Journal of Pharmaceutical Sciences Vol. 81, No. 12, December 1992
Figure S T O P view (Left) and side view (Rlght) of the computergenerated structures of the inclusion complexes of the (A) Rand (B)S enantiomers of 3.
a round shape when empty, markedly distorted during complex formation to a n approximately elliptic geometry on which lie the O(4) atoms. In this geometry, the plane of the aromatic ring of the guests was closely perpendicular to the short axis of the ellipse. The largest distortion was observed for the inclusion complexes of the R and S enantiomers of 1. The ellipse-shaped structure of the complexed P C D agrees with general views based on X-ray analyses of solid P C D complexes with aromatic and aliphatic guests12J6 and indicates that the host molecule undergoes conformational modifications to ensure the fitting of the guest into the cavity. However, all our calculations were referred to the gas phase, and packing forces in the crystal phase or solvation effects could well determine the preferred binding mode with real systems.
References and Notes 1. Szejtli, J. In Cyclodeztrin Technology; Kluwer Academic: Dordrecht, The Netherlands, 1988; pp 186-293. 2. Duchhe, D. In P m . Fourth Int. Symp. on Cyclodextrins; Huber, 0.;Szejtli, J., Eds.;Kluwer Academic: Dordrecht, The Netherlands, 1988; pp 265-275. 3. Tallal, P. J. Clin. Psychopharmacol. 1985, 5, 272. 4. Kabes, J. J. Int. Med. Res. 1985, 13, 185. 5. Amah, M. E.; Bandoli, G.;Djedaini, F.; Dolmella, A.; Grassi, A.; Pappalardo, G. C. J. Mol. Struct. 1990,222,487-502. Table Ill-Hydrogen Bond Dlstances between Carbonyl Groups of the Guests and p C D Secondary Hydroxyls
Compounds
Configuration
Hydrogen Bond Distance, A C=O HO-CH,OH .* OH *-.a
~
1
R
S 2
R
3
R S
S
a-,
Not applicable.
1.865 1.93 1.849 1.a25 1.so1 1.808
2.119 2.076
-a -
-
6. Amato, M. E.; Bandoli, G.; Cadlato, U.; Pappalardo, G. C.; Toja,
E.J. Mol. Strud. 1990,238, 413-427.
7. MacroModel version 2.5,Department of Chemistry, Columbia University, New York, NY. 8. Harata, K. Bull. Chem. Soc. Jpn. 1988,61,1939-1944. 9. Tabushi, I.; Ki osuke, Y.; Sugimoto,T.; Yamamura, K. J. Am. Chem. SOC. I~~'&Ioo, 9~-919. 10. Amato, M. .E.;D'epfiini, F.; Perly, B. Minutes, Fifth Int. Symp. on Cyclodextnns; dhtione de Sans: Paris, 1990;pp 138-141. 11. Casy, A.F.;Mercer, A. D.Magn.Reson. Chem. 1988,26, 765774. 12. Harata, K. Bull. Chem.Soc. Jpn. 1982,55,2315-2320. 13. Stainer, T.; Maeon Sax, A.; Saenger, W.J. Am. Chem. Soc. 1990,
112,6184-6190. 14. Betzel, C.; Saenger, W.;Hingerty, B. E.; Brown, G. M.J. Am. Chem. Soc 1984,106,7545-7557. 15. Lu,T.-X.; Zhang, D.-B.; Dong,S.J. J. Chem. Soc., Faraday Trans. 2 1989,85,1439-1445. 16. Allen, F.H.; Kennard, 0.; Taylor, R. Acc. Chem. Res. 1983,16, 146-153.
Acknowledgments This work was supported by MPI of Italy. Our thanks are 'ven to bussel-Uclaf and to Boehringer-lngelheim for the kind g i a o f the samples.
Journal of Pharmaceutical Sciences I 1161 Vol. 81, No. 12, December 1992