RAPID COMMUNICATION
Inversion of Population Distribution of Felodipine Conformations at Increased Concentration in Dimethyl Sulfoxide Is a Prerequisite to Crystal Nucleation ILYA A. KHODOV,1 SERGEY V. EFIMOV,2 MICHAEL YU. NIKIFOROV,1 VLADIMIR V. KLOCHKOV,2 NIKOLAJ GEORGI3 1
G. A. Krestov Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo 153045, Russia Kazan Federal University, Kazan 420008, Russia 3 Max Planck Institute for Mathematics in the Sciences, Leipzig 04103, Germany 2
Received 31 October 2013; revised 2 December 2013; accepted 10 December 2013 Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/jps.23833 ABSTRACT: Knowledge of the preferred conformations of biologically active compounds is of the utmost importance for a better understanding of the structure–activity relationships underlying their biological activity, as well as their mechanism of action. Moreover, investigating the mechanism of nucleation from a saturated solution can facilitate the discovery and preparation of new polymorphic forms. To search regularities in the crystal nucleation of biologically active compounds (drugs) from a saturated solution, we studied the conformational preference of felodipine in dilute and saturated solution in dimethyl sulfoxide. The inversion of conformation distribution at increased concentration occurs: conformers that dominate in a dilute solution become the least abundant in the saturated one. Conformers C 2014 Wiley Periodicals, Inc. and that dominate in the saturated solution are of the same type as revealed in crystalline state by X-ray. the American Pharmacists Association J Pharm Sci Keywords: NMR spectroscopy; nucleation; structure; polymorphism; crystallization
INTRODUCTION Studies of the conformational preference of small molecules play an important role in understanding the nucleation of biologically active compounds from saturated solutions. In this work, conformations of felodipine in dilute and saturated solution in dimethyl sulfoxide (DMSO) were studied and fractions of conformers were determined. Felodipine [ethylmethyl-4-(2,3-dichlorophenyl)-1,4-dihydro2,6-dimethyl-3,5-pyridine-dicarboxylate] is widely used as antihypertensive and antianginal drug.1 The possible existence of different polymorphic forms connected with conformations of the phenyl ring was predicted in.2 Commercial felodipine has the melting point of 144◦ C and is a racemic mixture of enantiomers.3 Some research points to the existence of four crystal structures.4 Nuclear overhauser effect (NOE) nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for studying spatial structure and conformational detail for flexible small molecules.5,6 Other possible approaches include the analysis of scalar and dipolar couplings, which have some essential restrictions.7,8 According to Teberekidis and Sigalas,9 felodipine molecule can exist in six possible conformations (Fig. 1). The difference between conformation A, B, and E, on the one side, and C, D, and F, on the other side, consists in 180◦ rotation of the dichlorophenyl ring. The distance between protons 4 and 6 is thus the key feature that allows us to distinguish between the two types of conformations (A, B, and E vs. C, D, and F).
Correspondence to: Ilya A. Khodov (Telephone: +7-4932-336237; Fax: +74932-336237; E-mail: [email protected]) Journal of Pharmaceutical Sciences
C 2014 Wiley Periodicals, Inc. and the American Pharmacists Association
Additional information on proton–proton distances is necessary to study individual conformations, which differ from each other by orientation of the substituent at the position 3 in the 1,4dihydropyridine ring. Distances in proton pairs (3b–3c), (4–6 ) and (NH1–6 ) were determined earlier by the combination of 1D and 2D NOE spectroscopy.6 In this study, 2D NOESY experiment allowed us to conclude that conformations A, B, and E dominate in dilute DMSO solution. Results of 1D spectroscopy showed that the other three forms, C, D, and F, were also present in a noticeable concentration. However, it was difficult to make any quantitative conclusions because of experimental artifacts caused by spin diffusion.10 In this work, we implemented an experiment with suppression of spin diffusion, QUIET-NOESY.11 The pulse sequence QUIET-NOESY differs from that used in the 2D NOESY by inserting a doubly selective inversion pulse in the middle of the mixing time Jm . Simultaneous inversion of the longitudinal magnetization is achieved by modulated Gaussian cascade Q3. The so-called “quiet windows” then are formed in a 2D spectrum on the intersections of the two frequency bands. These windows contain cross-peaks caused by processes of direct magnetization transfer, whereas indirect pathways are removed provided the chemical shifts of intermediate spins do not fall into these quiet windows.
EXPERIMENTAL Felodipine and deuterated DMSO were purchased from Sigma– Aldrich Rus (Moscow, Russia) and used without further purification. Samples were prepared with concentration of 0.077 g/L (dilute solution) and 26.57 g/L (saturated solution). Solution volume was 0.6 mL. Khodov et al., JOURNAL OF PHARMACEUTICAL SCIENCES
1
2
RAPID COMMUNICATION
Figure 1. Representative structures of the felodipine molecule.9 The calibration distance is labeled rcalibr. ; measured distances used to calculate the fractions of conformers are labeled rexp. (the type of NOE experiment is shown by an additional index 1D or 2D). The scheme on the right shows the numbering of atoms.
All NMR experiments were performed on a Bruker Avance II-500 NMR spectrometer (Bruker BioSpin GmbH, Karlsruhe, Germany) equipped with a 5-mm probe using standard Bruker TOPSPIN software (Bruker BioSpin GmbH, Germany). Temperature control was performed using a Bruker variable temperature unit (BVT-2000) in combination with a Bruker cooling unit (BCU-05) to provide chilled air. Experiments were performed at 298 K without sample spinning. Two-dimensional nuclear overhauser effect spectroscopy (2D ge-NOESY)12 and transient overhauser effects spectroscopy with suppression of spin diffusion (QUIET-NOESY)13 experiments were performed with pulsed filtered gradient techniques.14 The spectra were recorded in a phase-sensitive mode with 2048 points in the F2 direction and 512 points in the F1 direction. Mixing time values were 0.30, 0.50, 0.70, and 0.90 s. The spectra were acquired with 24 scans and relaxation delay of 2 s. R
RESULTS AND DISCUSSION In this paper, we used two methods to overcome the influence of spin diffusion: elimination by concerted T-ROESY/NOESY15,16 and QUIET-NOESY. Results of both approaches were in agreement with each other. Proton–proton distances obtained by conventional NOESY method and by QUIET-NOESY are presented in Table 1. The method of estimating internuclear distances for different proton pairs in a molecule is based on the existence of a strong dependency of the cross-relaxation rate constant Fij on the distance rij between the interacting nuclear spins. Normally, such a dependency is approximated by the simple formula Fij ∼ 1/r6 ij , and then internuclear distances are obtained according to the expression rij = rcalibr. (Fcalibr. /Fij )1/6 , where rcalibr. and Fcalibr. refer to the proton pair chosen for calibration; in our
case, the distance that was the same in all six conformers was chosen for the purpose of calibration. Note that distance 3b–3c cannot be calculated straightforward, as it includes methyl and methylene group. Therefore we used averaging of internal molecular rotation according to Tropp:17 ⎡
reff
1 i 2 ⎤− 6 2 3 i θ Y ϕ 1 1 2k mol mol ⎦ =⎣ . 5 k=−2 3 i=1 ri3
(1)
here, Y2k are second rank spherical harmonics; θ and φ are polar coordinates of internuclear vectors connecting each nucleus pair subject to averaging over three jump sites (i = 1–3). In Khodov et al.,6 geometric restraints for felodipine molecule in dilute solution were determined. In this paper, measurements on the saturated solution were also added (see Table 1). Analysis of QUIET-NOESY data yields interproton distances that are larger than that obtained from NOESY data, which can be ascribed to suppression of spin diffusion. Use of this advanced technique is justified by a relatively high viscosity of solutions in DMSO. Combined use of experimental and calculated from quantum chemistry distances allowed us to determine the distribution of felodipine conformers in DMSO. According to Lee and Krishna,18 observed cross-relaxation rate depends on conformer fractions in the case of fast exchange (typically no less than 102 –103 s−1 ) as F=
Fi xi .
(2)
i
Taking into account the correlation between cross-relaxation rate and internuclear distance, it is easy to see that for the case of conformational equilibrium I ↔ II, the following correlation
˚ Determined by NOE Measurements with and Without Suppression of Spin Diffusion and Corresponding Table 1. Interproton Distances (A) Distances from Quantum-Chemical Calculations9 NOESY Atom Pair 3b–3c 4–6
QUIET-NOESY
Dilute
Saturation
Dilute
Saturation
A
B
C
D
E
F
Calibration 2.21 ± 0.03
Calibration 2.98 ± 0.08
Calibration 2.31 ± 0.06
Calibration 3.08 ± 0.09
2.69 2.12
2.69 2.12
2.69 2.73
2.69 2.73
2.69 2.12
2.69 2.73
Khodov et al., JOURNAL OF PHARMACEUTICAL SCIENCES
DOI 10.1002/jps.23833
RAPID COMMUNICATION
3
REFERENCES ˚ 1. Edgar B, Collste P, Haglund K, Regardh CG. 1987. Pharmacokinetics and haemodynamic effects of felodipine as monotherapy in hypertensive patients. Clin Invest Med 10:388–394. ˇ 2. Srˇciˇc S, Kerˇc J, Urleb U, Zupanˇciˇc I, Lahajnar G, Kofler B, Smid-
Figure 2. Conformer distribution for (a and c) dilute and (b and d) saturated solutions of felodipine in DMSO based on data with (c and d) and without (a and b) suppression of spin diffusion.
takes place: 1 6 rexp
=
x1 1 − x1 + . 6 r1 r26
(3)
By solving Eq. (3), we obtain the fraction x1 of conformers A, B, and E; x2 (C,D,F) = 1–x1 . Results of distribution determination for dilute and saturated solutions based on data with and without suppression of spin diffusion are given in Figure 2. It is worth noting that suppression of spin diffusion leads finally to a refinement of conformer fractions of up to approximately 20%. Crystal structure of felodipine was established earlier in Surov et al.4 by X-ray. It was found that the conformation with chlorine atoms oriented in the same direction as the four protons form the crystalline state (see, for example, conformation D in Fig. 1), and conformers A, B, and E are absent. Our results show that conformations of this type become dominant on increasing the solution concentration, whereas the other type of conformations prevails in dilute solution. Thus, experimentally revealed inversion of conformer distribution is responsible for the nucleation of crystalline state of felodipine containing the conformer resembling C, D, and F by the orientation of the dichlorobenzene ring. The inversion of conformer distribution in the saturated solution is because of both the increased intermolecular interactions felodipine–felodipine and felodipine– DMSO.
ACKNOWLEDGMENTS We thank Dr. A.O. Surov, Prof. G.L. Perlovich, Prof. G. A. Alper, and Prof. M.G. Kiselev for helpful discussions. Financial support was from the Russian Foundation for Basic Research (project no. 13-03-97041 r povolzh’e a) and from the Marie Curie International Research Staff Exchange Scheme PIRSES-GA-2009-247500.
DOI 10.1002/jps.23833
Korbar J. 1992. Investigation of felodipine polymorphism and its glassy state. Int J Pharm 87:1–10. 3. Rollinger JM, Burger A. 2001. Polymorphism of racemic felodipine and the unusual series of solid solutions in the binary system of its enantiomers. J Pharm Sci 90:949–959. 4. Surov AO, Solanko KA, Bond AD, Perlovich GL, Bauer-Brandl A. 2012. Crystallization and polymorphism of felodipine. Cryst Growth Des 12:4022–4030. 5. Butts CP, Jones CR, Harvey JN. 2011. High precision NOEs as a probe for low level conformers—A second conformation of strychnine. Chem Commun 47:1193–1195. 6. Khodov IA, Nikiforov MYu, Alper GA, Blokhin DS, Efimov SV, Klochkov VV, Georgi N. 2013. Spatial structure of felodipine dissolved in DMSO by 1D and 2D NOESY NMR spectroscopy. J Mol Struct 1035:358–362. ´ 7. Navarro-Vazquez A. 2012. MSpin-RDC. A program for the use of residual dipolar couplings for structure elucidation of small molecules. Magn Reson Chem 50(S1):S73–S79. 8. Butts CP, Jones CR, Song Zh, Simpson TJ. 2012. Accurate NOEdistance determination enables the stereochemical assignment of a flexible molecule–arugosin C. Chem Commun 48:9023–9025. 9. Teberekidis VI, Sigalas MP. 2007. Theoretical study of hydrogen bond interactions of felodipine with polyvinylpyrrolidone and polyethyleneglycol. J Mol Struct (Theochem) 803:29–38. 10. Jones CR, Butts CP, Harvey JN. 2011. Accuracy in determining interproton distances using nuclear overhauser effect data from a flexible molecule. Beilstein J Org Chem 7:145–150. 11. Vincent SJF, Zwahlen C, Bodenhausen G. 1996. Suppression of spin diffusion in selected frequency bands of nuclear overhauser spectra. J Biomol NMR 7:169–172. 12. Thrippleton MJ, Keeler J. 2003. Elimination of zero-quantum interference in two-dimensional NMR spectra. Angew Chem Int Ed 42:3938– 3941. 13. Zwahlen C, Vincent SJF, Di Bari L, Levitt MH, Bodenhausen G. 1994. Quenching spin diffusion in selective measurements of transient overhauser effects in nuclear magnetic resonance. Applications to oligonucleotides. J Am Chem Soc 116:362–368. 14. Wagner R, Berger S. 1996. Gradient-selected NOESY—A fourfold reduction of the measurement time for the NOESY experiment. J Magn Reson (Ser A) 123:119–121. 15. Bax A, Davis DG. 1985. Practical aspects of two-dimensional transverse NOE spectroscopy. J Magn Reson 63:207–213. ¨ en E, Stenutz R, Widmalm G. 2009. Conformational 16. Olsson U, Saw´ flexibility and dynamics of two (1→6)-linked disaccharides related to an oligosaccharide epitope expressed on malignant tumour cells. Chem Eur J 15:8886–8894. 17. Tropp J. 1980. Dipolar relaxation and nuclear overhauser effects in nonrigid molecules: The effect of fluctuating internuclear distances. J Chem Phys 72:6035–6043. 18. Lee W, Krishna N. 1992. Influence of conformational exchange on the 2D NOESY spectra of biomolecules existing in multiple conformations. J Magn Reson 98:36–48.
Khodov et al., JOURNAL OF PHARMACEUTICAL SCIENCES
Inversion of Population Distribution of Felodipine Conformations at Increased Concentration in Dimethyl Sulfoxide Is a Prerequisite to Crystal Nucleation ILYA A. KHODOV,1 SERGEY V. EFIMOV,2 MICHAEL YU. NIKIFOROV,1 VLADIMIR V. KLOCHKOV,2 NIKOLAJ GEORGI3 1
G. A. Krestov Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo 153045, Russia Kazan Federal University, Kazan 420008, Russia 3 Max Planck Institute for Mathematics in the Sciences, Leipzig 04103, Germany 2
Received 31 October 2013; revised 2 December 2013; accepted 10 December 2013 Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/jps.23833 ABSTRACT: Knowledge of the preferred conformations of biologically active compounds is of the utmost importance for a better understanding of the structure–activity relationships underlying their biological activity, as well as their mechanism of action. Moreover, investigating the mechanism of nucleation from a saturated solution can facilitate the discovery and preparation of new polymorphic forms. To search regularities in the crystal nucleation of biologically active compounds (drugs) from a saturated solution, we studied the conformational preference of felodipine in dilute and saturated solution in dimethyl sulfoxide. The inversion of conformation distribution at increased concentration occurs: conformers that dominate in a dilute solution become the least abundant in the saturated one. Conformers C 2014 Wiley Periodicals, Inc. and that dominate in the saturated solution are of the same type as revealed in crystalline state by X-ray. the American Pharmacists Association J Pharm Sci Keywords: NMR spectroscopy; nucleation; structure; polymorphism; crystallization
INTRODUCTION Studies of the conformational preference of small molecules play an important role in understanding the nucleation of biologically active compounds from saturated solutions. In this work, conformations of felodipine in dilute and saturated solution in dimethyl sulfoxide (DMSO) were studied and fractions of conformers were determined. Felodipine [ethylmethyl-4-(2,3-dichlorophenyl)-1,4-dihydro2,6-dimethyl-3,5-pyridine-dicarboxylate] is widely used as antihypertensive and antianginal drug.1 The possible existence of different polymorphic forms connected with conformations of the phenyl ring was predicted in.2 Commercial felodipine has the melting point of 144◦ C and is a racemic mixture of enantiomers.3 Some research points to the existence of four crystal structures.4 Nuclear overhauser effect (NOE) nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for studying spatial structure and conformational detail for flexible small molecules.5,6 Other possible approaches include the analysis of scalar and dipolar couplings, which have some essential restrictions.7,8 According to Teberekidis and Sigalas,9 felodipine molecule can exist in six possible conformations (Fig. 1). The difference between conformation A, B, and E, on the one side, and C, D, and F, on the other side, consists in 180◦ rotation of the dichlorophenyl ring. The distance between protons 4 and 6 is thus the key feature that allows us to distinguish between the two types of conformations (A, B, and E vs. C, D, and F).
Correspondence to: Ilya A. Khodov (Telephone: +7-4932-336237; Fax: +74932-336237; E-mail: [email protected]) Journal of Pharmaceutical Sciences
C 2014 Wiley Periodicals, Inc. and the American Pharmacists Association
Additional information on proton–proton distances is necessary to study individual conformations, which differ from each other by orientation of the substituent at the position 3 in the 1,4dihydropyridine ring. Distances in proton pairs (3b–3c), (4–6 ) and (NH1–6 ) were determined earlier by the combination of 1D and 2D NOE spectroscopy.6 In this study, 2D NOESY experiment allowed us to conclude that conformations A, B, and E dominate in dilute DMSO solution. Results of 1D spectroscopy showed that the other three forms, C, D, and F, were also present in a noticeable concentration. However, it was difficult to make any quantitative conclusions because of experimental artifacts caused by spin diffusion.10 In this work, we implemented an experiment with suppression of spin diffusion, QUIET-NOESY.11 The pulse sequence QUIET-NOESY differs from that used in the 2D NOESY by inserting a doubly selective inversion pulse in the middle of the mixing time Jm . Simultaneous inversion of the longitudinal magnetization is achieved by modulated Gaussian cascade Q3. The so-called “quiet windows” then are formed in a 2D spectrum on the intersections of the two frequency bands. These windows contain cross-peaks caused by processes of direct magnetization transfer, whereas indirect pathways are removed provided the chemical shifts of intermediate spins do not fall into these quiet windows.
EXPERIMENTAL Felodipine and deuterated DMSO were purchased from Sigma– Aldrich Rus (Moscow, Russia) and used without further purification. Samples were prepared with concentration of 0.077 g/L (dilute solution) and 26.57 g/L (saturated solution). Solution volume was 0.6 mL. Khodov et al., JOURNAL OF PHARMACEUTICAL SCIENCES
1
2
RAPID COMMUNICATION
Figure 1. Representative structures of the felodipine molecule.9 The calibration distance is labeled rcalibr. ; measured distances used to calculate the fractions of conformers are labeled rexp. (the type of NOE experiment is shown by an additional index 1D or 2D). The scheme on the right shows the numbering of atoms.
All NMR experiments were performed on a Bruker Avance II-500 NMR spectrometer (Bruker BioSpin GmbH, Karlsruhe, Germany) equipped with a 5-mm probe using standard Bruker TOPSPIN software (Bruker BioSpin GmbH, Germany). Temperature control was performed using a Bruker variable temperature unit (BVT-2000) in combination with a Bruker cooling unit (BCU-05) to provide chilled air. Experiments were performed at 298 K without sample spinning. Two-dimensional nuclear overhauser effect spectroscopy (2D ge-NOESY)12 and transient overhauser effects spectroscopy with suppression of spin diffusion (QUIET-NOESY)13 experiments were performed with pulsed filtered gradient techniques.14 The spectra were recorded in a phase-sensitive mode with 2048 points in the F2 direction and 512 points in the F1 direction. Mixing time values were 0.30, 0.50, 0.70, and 0.90 s. The spectra were acquired with 24 scans and relaxation delay of 2 s. R
RESULTS AND DISCUSSION In this paper, we used two methods to overcome the influence of spin diffusion: elimination by concerted T-ROESY/NOESY15,16 and QUIET-NOESY. Results of both approaches were in agreement with each other. Proton–proton distances obtained by conventional NOESY method and by QUIET-NOESY are presented in Table 1. The method of estimating internuclear distances for different proton pairs in a molecule is based on the existence of a strong dependency of the cross-relaxation rate constant Fij on the distance rij between the interacting nuclear spins. Normally, such a dependency is approximated by the simple formula Fij ∼ 1/r6 ij , and then internuclear distances are obtained according to the expression rij = rcalibr. (Fcalibr. /Fij )1/6 , where rcalibr. and Fcalibr. refer to the proton pair chosen for calibration; in our
case, the distance that was the same in all six conformers was chosen for the purpose of calibration. Note that distance 3b–3c cannot be calculated straightforward, as it includes methyl and methylene group. Therefore we used averaging of internal molecular rotation according to Tropp:17 ⎡
reff
1 i 2 ⎤− 6 2 3 i θ Y ϕ 1 1 2k mol mol ⎦ =⎣ . 5 k=−2 3 i=1 ri3
(1)
here, Y2k are second rank spherical harmonics; θ and φ are polar coordinates of internuclear vectors connecting each nucleus pair subject to averaging over three jump sites (i = 1–3). In Khodov et al.,6 geometric restraints for felodipine molecule in dilute solution were determined. In this paper, measurements on the saturated solution were also added (see Table 1). Analysis of QUIET-NOESY data yields interproton distances that are larger than that obtained from NOESY data, which can be ascribed to suppression of spin diffusion. Use of this advanced technique is justified by a relatively high viscosity of solutions in DMSO. Combined use of experimental and calculated from quantum chemistry distances allowed us to determine the distribution of felodipine conformers in DMSO. According to Lee and Krishna,18 observed cross-relaxation rate depends on conformer fractions in the case of fast exchange (typically no less than 102 –103 s−1 ) as F=
Fi xi .
(2)
i
Taking into account the correlation between cross-relaxation rate and internuclear distance, it is easy to see that for the case of conformational equilibrium I ↔ II, the following correlation
˚ Determined by NOE Measurements with and Without Suppression of Spin Diffusion and Corresponding Table 1. Interproton Distances (A) Distances from Quantum-Chemical Calculations9 NOESY Atom Pair 3b–3c 4–6
QUIET-NOESY
Dilute
Saturation
Dilute
Saturation
A
B
C
D
E
F
Calibration 2.21 ± 0.03
Calibration 2.98 ± 0.08
Calibration 2.31 ± 0.06
Calibration 3.08 ± 0.09
2.69 2.12
2.69 2.12
2.69 2.73
2.69 2.73
2.69 2.12
2.69 2.73
Khodov et al., JOURNAL OF PHARMACEUTICAL SCIENCES
DOI 10.1002/jps.23833
RAPID COMMUNICATION
3
REFERENCES ˚ 1. Edgar B, Collste P, Haglund K, Regardh CG. 1987. Pharmacokinetics and haemodynamic effects of felodipine as monotherapy in hypertensive patients. Clin Invest Med 10:388–394. ˇ 2. Srˇciˇc S, Kerˇc J, Urleb U, Zupanˇciˇc I, Lahajnar G, Kofler B, Smid-
Figure 2. Conformer distribution for (a and c) dilute and (b and d) saturated solutions of felodipine in DMSO based on data with (c and d) and without (a and b) suppression of spin diffusion.
takes place: 1 6 rexp
=
x1 1 − x1 + . 6 r1 r26
(3)
By solving Eq. (3), we obtain the fraction x1 of conformers A, B, and E; x2 (C,D,F) = 1–x1 . Results of distribution determination for dilute and saturated solutions based on data with and without suppression of spin diffusion are given in Figure 2. It is worth noting that suppression of spin diffusion leads finally to a refinement of conformer fractions of up to approximately 20%. Crystal structure of felodipine was established earlier in Surov et al.4 by X-ray. It was found that the conformation with chlorine atoms oriented in the same direction as the four protons form the crystalline state (see, for example, conformation D in Fig. 1), and conformers A, B, and E are absent. Our results show that conformations of this type become dominant on increasing the solution concentration, whereas the other type of conformations prevails in dilute solution. Thus, experimentally revealed inversion of conformer distribution is responsible for the nucleation of crystalline state of felodipine containing the conformer resembling C, D, and F by the orientation of the dichlorobenzene ring. The inversion of conformer distribution in the saturated solution is because of both the increased intermolecular interactions felodipine–felodipine and felodipine– DMSO.
ACKNOWLEDGMENTS We thank Dr. A.O. Surov, Prof. G.L. Perlovich, Prof. G. A. Alper, and Prof. M.G. Kiselev for helpful discussions. Financial support was from the Russian Foundation for Basic Research (project no. 13-03-97041 r povolzh’e a) and from the Marie Curie International Research Staff Exchange Scheme PIRSES-GA-2009-247500.
DOI 10.1002/jps.23833
Korbar J. 1992. Investigation of felodipine polymorphism and its glassy state. Int J Pharm 87:1–10. 3. Rollinger JM, Burger A. 2001. Polymorphism of racemic felodipine and the unusual series of solid solutions in the binary system of its enantiomers. J Pharm Sci 90:949–959. 4. Surov AO, Solanko KA, Bond AD, Perlovich GL, Bauer-Brandl A. 2012. Crystallization and polymorphism of felodipine. Cryst Growth Des 12:4022–4030. 5. Butts CP, Jones CR, Harvey JN. 2011. High precision NOEs as a probe for low level conformers—A second conformation of strychnine. Chem Commun 47:1193–1195. 6. Khodov IA, Nikiforov MYu, Alper GA, Blokhin DS, Efimov SV, Klochkov VV, Georgi N. 2013. Spatial structure of felodipine dissolved in DMSO by 1D and 2D NOESY NMR spectroscopy. J Mol Struct 1035:358–362. ´ 7. Navarro-Vazquez A. 2012. MSpin-RDC. A program for the use of residual dipolar couplings for structure elucidation of small molecules. Magn Reson Chem 50(S1):S73–S79. 8. Butts CP, Jones CR, Song Zh, Simpson TJ. 2012. Accurate NOEdistance determination enables the stereochemical assignment of a flexible molecule–arugosin C. Chem Commun 48:9023–9025. 9. Teberekidis VI, Sigalas MP. 2007. Theoretical study of hydrogen bond interactions of felodipine with polyvinylpyrrolidone and polyethyleneglycol. J Mol Struct (Theochem) 803:29–38. 10. Jones CR, Butts CP, Harvey JN. 2011. Accuracy in determining interproton distances using nuclear overhauser effect data from a flexible molecule. Beilstein J Org Chem 7:145–150. 11. Vincent SJF, Zwahlen C, Bodenhausen G. 1996. Suppression of spin diffusion in selected frequency bands of nuclear overhauser spectra. J Biomol NMR 7:169–172. 12. Thrippleton MJ, Keeler J. 2003. Elimination of zero-quantum interference in two-dimensional NMR spectra. Angew Chem Int Ed 42:3938– 3941. 13. Zwahlen C, Vincent SJF, Di Bari L, Levitt MH, Bodenhausen G. 1994. Quenching spin diffusion in selective measurements of transient overhauser effects in nuclear magnetic resonance. Applications to oligonucleotides. J Am Chem Soc 116:362–368. 14. Wagner R, Berger S. 1996. Gradient-selected NOESY—A fourfold reduction of the measurement time for the NOESY experiment. J Magn Reson (Ser A) 123:119–121. 15. Bax A, Davis DG. 1985. Practical aspects of two-dimensional transverse NOE spectroscopy. J Magn Reson 63:207–213. ¨ en E, Stenutz R, Widmalm G. 2009. Conformational 16. Olsson U, Saw´ flexibility and dynamics of two (1→6)-linked disaccharides related to an oligosaccharide epitope expressed on malignant tumour cells. Chem Eur J 15:8886–8894. 17. Tropp J. 1980. Dipolar relaxation and nuclear overhauser effects in nonrigid molecules: The effect of fluctuating internuclear distances. J Chem Phys 72:6035–6043. 18. Lee W, Krishna N. 1992. Influence of conformational exchange on the 2D NOESY spectra of biomolecules existing in multiple conformations. J Magn Reson 98:36–48.
Khodov et al., JOURNAL OF PHARMACEUTICAL SCIENCES
