Conformational equilibria of pharmaceuticals in supercritical CO2, IR spectroscopy and quantum chemical calculations

Journal Pre-proof Conformational equilibria of pharmaceuticals in supercritical CO2, IR spectroscopy and quantum chemical calculations

R.D. Oparin, D.V. Ivlev, M.G. Kiselev PII:

S1386-1425(20)30049-4

DOI:

https://doi.org/10.1016/j.saa.2020.118072

Reference:

SAA 118072

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received date:

27 October 2019

Revised date:

18 December 2019

Accepted date:

13 January 2020

Please cite this article as: R.D. Oparin, D.V. Ivlev and M.G. Kiselev, Conformational equilibria of pharmaceuticals in supercritical CO2, IR spectroscopy and quantum chemical calculations, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy(2018), https://doi.org/10.1016/j.saa.2020.118072

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2018 Published by Elsevier.

Journal Pre-proof Conformational Equilibria of Pharmaceuticals in Supercritical CO2. IR spectroscopy and Quantum Chemical Calculations

R.D. Oparin, D.V. Ivlev, M.G. Kiselev G. A. Krestov Institute of Solution Chemistry of the Russian Academy of Sciences Ivanovo, Russia [email protected]

Abstract In this work we demonstrate a self-consistent effective technique of analyzing the conformational

of

equilibria of active pharmaceutical ingredient (API) molecules dissolved in supercritical carbon

ro

dioxide in a wide range of thermodynamic parameters of state. This approach can be useful for pharmaceutics when the crystalline forms of pharmaceuticals with a high purity degree and desirable

-p

polymorphism are produced using CO2-based supercritical fluids technologies. Within this approach we use a combination of quantum chemical calculations and in situ IR spectroscopy. Quantum

re

chemical calculations allow us to perform the initial conformational search and to determine the

lP

energy characteristics of the most stable conformers of API and the energy barriers of transitions between them. IR spectroscopy gives the information on the equilibrium of the most stable conformers

na

of pharmaceuticals dissolved in scCO2 in the thermodynamic parameter range of interest. Finally we validate our approach by applying it to the study of carbamazepine dissolved in scCO2 being in permanent contact with an excess of crystalline carbamazepine as an example. The conformational

Jo ur

search for carbamazepine molecules in scCO2 was also performed using molecular dynamics simulation for comparison with the results obtained by the technique presented in this paper.

Keywords: Supercritical CO2; Carbamazepine; Quantum Chemical Calculations; IR spectroscopy; Conformational Equilibria.

Journal Pre-proof 1. Introduction The studies of conformational equilibria of drug molecules are of great importance because this phenomenon is closely related to polymorphism of crystalline forms of such compounds (see, e.g., [1, 2]). The role of polymorphism of active pharmaceutical ingredients (API) is well known [3]. For example, the compressibility or compactibility of crystalline API at tablet formation may strongly depend on its polymorphism. On the other hand, different polymorphic forms of one API may demonstrate different pharmacological properties of the target pharmaceutical product, such as: bioavailability, bio-activity or solubility in human body liquids. Special attention should be paid to the study of polymorphism and conformational equilibria of API in terms of applying supercritical fluid (SCF) technologies based on the use of supercritical carbon dioxide (scCO2) as a solvent in production

of

of pharmaceuticals. Their application allows producing API with a high degree of purity and desirable

ro

polymorphism.

In a number of our works (see, e.g., Refs. [4-6]), we developed and successfully used a self-

-p

consistent approach that allows controlling the polymorphism of pharmaceuticals through variation of their conformational manifold in supercritical CO2 (scCO2) phase being in permanent contact with the

re

crystalline forms of these pharmaceuticals. In these works we studied the drugs that possess different

lP

types of polymorphism (paracetamol as a representative of packing polymorphism, ibuprofen and mefenamic acid as representatives of conformational polymorphism), whereas their molecules have

na

high conformational lability. Thus, we demonstrated that there is a correlation between the conformational manifold of API dissolved in the scCO2 phase and polymorphic forms of its crystalline phase on a solid-fluid interface. We showed that these findings may be useful for selecting the

Jo ur

thermodynamic parameters of a SCF solution necessary for producing the final pharmaceutical product with a specified polymorphism in the technological process. In the above-mentioned works we successfully demonstrated the applicability of conformational analysis based on a combination of the IR data and computer simulation of molecules in the scCO2 phase. In these studies we applied the quantum chemical calculations (QCC) to perform the initial conformational search for drug molecules and to simulate the IR spectra of the obtained conformers. Then, we used the modeling spectra of the conformers in the procedure of the experimental IR spectra deconvolution into individual bands that correspond to the existing conformers in scCO2. Finally we obtained information on the conformational manifold in the phase diagram range of interest. The application and reasonability of an analogous approach based on a combination of in situ IR spectroscopy and ab initio calculations for studying specific and non-specific solid-solvent interactions in paracetamol – CO2-expanded organic solutions was also considered in Ref. [7]. The authors of Ref. [8] also showed the validity of using a combination of experimental and computational methods for investigating polymer swelling by scCO2 as a function of temperature and pressure. Successful applications of such combination of these methods were demonstrated in Refs. [9, 10], where the

Journal Pre-proof authors studied the complexes of aromatic primary amines and epoxides with CO2 and their stability at different) pressures in their binary mixtures with scCO2. This gives us grounds to conclude that it is quite reasonable to apply these methods in combination for studying a wide range of CO2-based binary systems, in particular, drug – scCO2 mixtures. Nowadays, there are a lot of pharmaceuticals whose crystalline forms can exist in different polymorphic forms. However, the molecules of such compounds possess low conformational lability. Normally, such APIs are representatives of the packing type of polymorphism. Carbamazepine (CBZ) is one of such drugs and it can exist as four anhydrous polymorphs and one dehydrate [11, 12]. In these forms the molecules of CBZ create hydrogen-bonded cyclic dimers, with the only difference between the polymorphs being the packing of these dimers [13]. Moreover, based on the theoretical

of

calculations, we predicted the probability of existence of the CBZ solid form, for which the hydrogen-

ro

bonded molecules create chainlet structures [14]. Nevertheless, the CBZ molecule having a relatively rigid aromatic base is characterized by constrained conformational lability owing only to the

-p

carboxamide group that can participate in conformational transitions. All these facts make CBZ an interesting subject of an inquiry, aimed, on the one hand, at checking the applicability of our approach

re

to such kind of molecules with low conformational lability, and, on the other hand, at determining the

lP

effect of thermodynamic parameters of state on the conformational equilibria of such molecules in the SCF phase.

na

Thus, in the present work, we will describe in detail our approach and the experimental technique we use, including QCC and IR spectra measurement and processing. Further, studying CBZ as an example, we will demonstrate the validity of our approach to analyzing conformational equilibria

Jo ur

of its molecules in the scCO2 phase and, thus, the approach applicability to drugs, whose molecules have constrained conformational lability. At the first stage, using QCC we will determine the possible CBZ conformers and calculate the IR spectra, which will be further used in the procedure of the experimental IR spectra deconvolution. Then, on the basis of the IR spectra analysis we will determine the effect of thermodynamic parameters of the SCF solution on the conformational equilibria of the CBZ molecules dissolved in scCO2.

2. Methods and techniques 2.1. Quantum chemical calculations In all our works, using the GAUSSIAN 09 suite of programs [15] we calculate the electronic structure of an API molecule. Further, within the density functional theory with different functional and basis sets we optimize the molecular geometry, and determine the vibration frequencies of certain functional groups that can be involved in conformational transitions. The search for all possible conformers is based on analyzing the potential energy surface scan. In turn, the potential energy profiles are calculated along certain dihedral angles whose variation determines the probable

Journal Pre-proof conformational transitions. Finally the structure of conformers corresponding to the local minima of potential energy is confirmed by frequency calculations. For the found conformers, we make calculations of the parameters of probable intramolecular hydrogen bonds along with the transition energy barriers between the conformational states that are also determined by scanning the potential energy surface. In order to apply the QCC data to the analysis of the IR spectroscopy results, we calculate the IR spectra for the conformers. Then we choose the analytical spectral bands or group of bands related to the vibrations of the functional groups involved in the estimated conformational transitions. In our studies we usually use spectral domains related to the vibrations of the API molecule aromatic system as the analytical one. This choice is explained by the fact that conformational

of

transitions of API molecules indirectly affect the vibration of the atoms of the aromatic fragments. In

ro

this case, because of their close positions to the API functional groups (in particular, C=O, N−H, O−H, O=C−O−H) that are sensitive to the conformation changes, electronic density redistribution changes

-p

the vibrational parameters of the aromatic system atoms.

One more advantage of using this spectral domain as the analytical one is related to the fact that

re

the aromatic system does not form strong specific bonds and, as a consequence, the integral absorption

lP

coefficient of the corresponding spectral band is not sensitive to changes in the thermodynamic parameters and remains almost constant over a wide range of temperatures and pressures. Thus, the

na

integral intensity of such spectral band depends only on the API concentration in scCO2 if the sample optical path length is constant. Consequently, the emergence of a new spectral contribution or a sharp change in the spectral band dispersion, which is induced, for instance, by the change in the

Jo ur

thermodynamic conditions, is an indicator of the API molecule conformational transition. Other spectral bands can also be used as analytical ones. These bands are related to the vibrations of functional groups that can be potentially involved in the creation of specific interactions (O−H, N−H, C=O, O=C−O−H). However, the use of these bands as analytical ones is possible only if there are no such interactions. Thus, in one of our studies (see Ref. [6]) we also made an attempt to use as analytical spectral bands those related to the vibrations of the N−H group (stretching and rocking vibration modes) that is directly involved in the conformational transition, but due to the extremely low concentration of API dissolved in scCO2 the probability of hydrogen bonding between the molecules of this API involving this group can be neglected.

2.2. MD simulation In this simulation, one CBZ molecule was surrounded by 1024 scCO2 molecules. We used the Gromos force field to model the CBZ molecule [16], and for CO2 - the model developed by Z. Zhang and Z. Duan [17]. For Lennard-Jones cross site-site interactions, the geometric mean mixing rule was applied. The temperature and pressure were controlled by a Nose-Hoover thermostat [18, 19] and a

Journal Pre-proof Parrinello-Rahman barostat [20], respectively. The leap-frog scheme was applied for the integration of equations of motion [21], the particle mesh Ewald method [22] was used for the long-range electrostatic interactions. For the conformational manifold calculations, the metadynamic method as implemented in the PLUMED package was applied [23]. The free energy hypersurface of conformers, as calculated by the metadynamic method, was determined by scanning along a pair of dihedral angles as collective variables, associated with the rotation of the amide fragment of the CBZ molecule carboxamide group. The probability density of the CBZ molecule existence along the dihedral angles (τ1,τ2) was calculated from the Boltzmann distribution: exp( F ( 1, 2) / RT )

 d 1 d 2exp( F ( 1, 2) / RT )

of

 ( 1, 2) 

ro

where F(τ1,τ2) is the CBZ free energy.

(1),

Pi 

 1iout



 2iout

d 1



d 2   ( 1, 2)

 2in i

re

 1in i

-p

As a result, the following formula was applied to calculate the conformer populations:

(2),

lP

where Pi is the population of the i-th conformer,  1ini , 2ini  ,  1iout , 2iout  are the pairs of dihedral

energy values cross zero).

2.3. In situ IR spectroscopy

na

angles on the boundary of the probability density maximum of the i-th conformer (where the free

Jo ur

To measure the IR spectra of the API dissolved in scCO2 we apply an universal experimental setup (see Fig. 1). We also use a specially designed homemade high-pressure-high-temperature (HPHT) optical cell with a variable optical path length. In our works (see, e.g., Refs.[4, 5, 24, 25]) we used several versions of this cell. The last improved version of the cell was used in Ref. [6], where its detailed description was presented. Fig. 2 shows photos of this cell. Such cell allows recording high resolution spectra with a high signal-to-noise ratio in a wide range of thermodynamic parameters of state (temperature up to 360°C, pressure up to 700 bar). Specially designed optical windows and a window holder allow varying the optical path length from 50 μm to 5 mm and working at the high pressure. The increased volume of the solid sample holder allows working in a wide range of API concentrations in the scCO2 phase. Due to this design of the cell and the module that includes an x-y positioning system it is possible to use it with spectrometers equipped with a standard connection port for the sample holder. In order to prepare the binary system [API−scCO2] to be studied, a sample of crystalline API is placed into a sample holder in the bottom part of the cell. To avoid the influence of the residuals of atmospheric components (in particular, water and oxygen) on the initial sample, the HPHT cell is

Journal Pre-proof pumped out to a residual pressure of 0.1 mbar. Then the cell is filled with dry CO2 up to the necessary pressure through a stainless steel capillary connected to a high pressure setup. The temperature inside the cell is reached by means of four cartridge heaters located in its body corners, and it is controlled by three thermocouples. One of them is located in the vicinity of one of the four cartridge heaters and is connected to a proportional-integral-derivative (PID) controller that controls the temperature with an accuracy of about ±1°C. The second one is located in the upper part of the cell body close to the fluid solution phase and the third one is located in the bottom part of the cell containing the API crystalline form. Such positioning of the thermocouples allows controlling the temperature gradient between the

Jo ur

na

lP

re

-p

ro

of

bottom and the fluid phases.

Figure 1. Schematic representation of the experimental setup.

In our studies the measurements of the IR absorption spectra of the binary [API-scCO2] mixture, which correspond to the CO2 phase are made on a FTIR spectrometer Bruker Vertex 80 equipped with a DTGS detector and a vacuum sample chamber with the residual pressure of 0.1 mbar, which helps to avoid the effects of the atmosphere components (in particular, water vapor) on the sample spectra. For each thermodynamic point of the phase diagram within the range of interest the spectra of the empty cell and of the cell filled only with CO2 are recorded. Then the spectra of the CO2 rich phase of the API–scCO2 binary mixture are recorded at the same thermodynamic parameters. Finally, the resulting spectra of the API dissolved in scCO2 are calculated by a direct subtraction of the spectra measured for the cell filled only with CO2 from the spectra of the binary mixture. We normally use various sample

Journal Pre-proof optical path lengths (from 2 mm to 0.1 mm) depending on the selected phase diagram range. This value is optimized to avoid oversaturation of the API spectral bands in the analytical spectral domain

-p

ro

of

and depends on the API solubility in scCO2 that can vary in the range of 10-6–10-2 mole fractions.

2.4. Experimental spectra processing

lP

inside the cell and an x-y positioning system.

re

Figure 2. General views of the HPHT cell module that includes a pressure capturer for pressure control

na

For further analysis, the obtained spectra are corrected by baseline subtraction and the analytical spectral domain is chosen according to the QCC results. For quantitative analysis of the experimental

Jo ur

spectra in the spectral range corresponding to the analytical domain we apply a standard procedure of spectral deconvolution into spectral components basing on the superposition principal. For this purpose, basing on the IR spectra calculated with QCC we estimate the necessary quantity of spectral contributions to be used in the analytical spectral model for good reproducibility of the experimental spectral band in the wavenumber range corresponding to the analytical spectral band. We assume that each of these spectral contributions is associated with a certain API conformer in scCO2. As a rule, we use Lorentzian profiles for approximation of experimental spectra within our model. It is connected with the fact that the API concentration in scCO2 varies from ~10-6 to 10-4 mole fractions and, as a result, its molecules exist in the scCO2 phase as monomers and the formation of hydrogen bonds between them can be neglected. Moreover, in order to improve the quality of approximation and to properly reproduce the experimental spectra, within this model, we also utilize additional spectral profiles necessary to fit the spectral bands that lie in the straight neighborhood to the analytical spectral domain (see, e.g., Refs. [4, 5]). For approximation of these spectral contributions we use the PseudoVoigt functions that are a linear combination of Lorentzian and Gaussian profiles. However, it is reasonable to take into account these additional contributions only when there is a strong overlapping

Journal Pre-proof of the analytical spectral bands with the neighboring one, when correct isolation of the analytical spectral domain is not trivial. For implementation of the fitting procedure we apply “Fityk” freeware that includes a spectra fitting tool [26]. As a result of applying the spectral deconvolution procedure we get a set of spectral parameters of each spectral component (peak positions, dispersions of peaks or its full width at half height and integral intensity (A)) constituting the analytical spectral domain. The final assignment of these spectral components to certain conformers is fulfilled based on the QCC results. Further, in order to calculate the mole fractions of the conformers in the scCO2 phase we use the integral intensity values, which are proportional to the molar concentration of API in the scCO2 phase according to the light absorbance universal law (A=εint∙l∙c, where εint is the integral extinction

of

coefficient, l is the optical path length, с is the API molar concentration). In its turn, the mole fraction

ro

of each conformer is calculated as: n

i 1

(3)

-p

X i  Ai /  Ai

Thus, calculated in such way, the mole fractions of the conformers as a function of state parameters

re

allow estimating the effect of the external perturbation factor (e.g., T, P) on the API conformational

lP

equilibrium in the scCO2 phase.

na

3. Approach validation. Conformational equilibria of carbamazepine dissolved in scCO2. To validate the accuracy of our approach and to show its applicability to the analysis of conformational equilibria of drugs, whose molecules have constrained conformational lability, we

Jo ur

studied the conformational equilibria of CBZ dissolved in scCO2 being in permanent contact with an excess of the CBZ solid form under isochoric heating conditions in the temperature range of 40 – 110°C and at the scCO2 density equal to 1.3 of its critical value. Using QCC in accordance with our approach, we determined a set of CBZ conformers. The optimization of their geometries and calculations of vibration frequencies were performed by the density functional method with the APFD functional and 6-311++g(2d,p) basis set. This hybrid functional already includes its own dispersion correction and allows avoiding the spurious long-range attractive or repulsive interactions that are found in most density functional theory models [27]. Moreover, in comparison with the other functionals (PBE and B3LYP) and basis sets (6-311, ccpvtz), which were also used for test calculations both with an additional dispersion correction (the GD3 method) and without it, only APFD provides better reproducibility of experimental IR spectra, in particular, spectral bands positions and interrelations of their intensities for both conformers. Thus, by QCC we determined two mirror conformers that are realized by rotating the carboxamide functional group along the central C−N bond. However, as these two conformers are mirror-symmetric, their calculated IR spectra are identical. An accurate potential energy surface scan

Journal Pre-proof also allowed us to find two more two stable CBZ conformers. Their molecular structures are presented in Fig. 3. The main difference between these two conformers is in the N2 atom position in the carboxamide group relative to the C-H1-H2 plane. For Conf. I the N2 atom is out-of-plane and shifted in the direction out of the aromatic system, for Conf. II the N2 atom is out-of-plane and shifted in the opposite direction. The differences between these two conformers can be determined by the dihedral angles τ1, τ2 and τ3 (see Fig. 3). The values of these angles are presented in Table 1. It has also been found that the first conformer has lower energy, and the energy difference between these two conformers is 1.49 kJ·mol-1. Moreover, the height of the potential barrier of transition from Conf. I to Conf. II does not exceed the RT value within the studied range of thermodynamic state parameters and equals 1.71 kJ·mol-1.

of

We have also calculated the values of these dihedral angles (see above) for these two conformers

ro

for the case of their interaction with one CO2 molecule involving the carboxamide group as shown in Fig. 4. These values of the dihedral angles are also presented in Table 1. The IR spectra obtained by

-p

QCC for the two CBZ conformers are presented in Fig. 5. One pair of the spectra corresponds to the CBZ–CO2 dimer and the other pair of the spectra corresponds to the CBZ monomer. The spectral

re

region that is related to the C=O group stretching vibrations in the carboxamide fragment was chosen

lP

as the analytical one. In spite of the small difference in the position of this spectral band for the two conformers (about 1 cm -1 for both cases), the ability of its usage as the analytical one is supported by

na

a number of factors. In particular, in this spectral domain there are only spectral contributions related to the C=O stretching vibrations, and in the high frequency and low frequency neighborhood within ~90 cm-1 there are no other spectral bands. This means that new spectral contributions appearing in the

Jo ur

experimental IR spectra can be considered, in principle, as a consequence of specific intermolecular interactions between CBZ molecules or the new conformer formation. As far as the specific interactions between CBZ molecules are negligible due to their low concentration, the only possible reason for the appearance of a new band is the change in the conformers’ distribution. Indeed, our additional study of CBZ solubility in scCO2 in the studied temperature range for the chosen isochore based on the IR spectra analysis has shown that the CBZ concentration is within 1.71·10-5 – 1.84·10-4 molar fractions. Table 1. Values of dihedral angles τ1, τ2 and τ3 (see Fig. 3) for two CBZ conformers as obtained by QCC. Conf. I

Conf. II

Conf. I – CO2

Conf. II – CO2

τ1

-165.809°

166.656°

-166.106°

167.458°

τ2

-24.006°

15.739°

-21.630°

12.944°

τ3

-6.986°

-10.438°

-8.052°

-10.519°

Journal Pre-proof For measuring the mid-IR spectra of CBZ in the CO2 phase, we used our HPHT optical cell with an optical path length of 0.970±0.005 mm. The thermal evolution of the experimental spectra of CBZ dissolved in the scCO2 phase in the wavenumber range of 1200–1900 cm-1 is shown in Fig. 6a. The analytical spectral domain related to the C=O group stretching vibrations which was used for conformational analysis is presented in Fig. 6b. As this figure shows, the temperature growth leads to an increase in the total spectral intensity. This is related to the increase of equilibrium concentration of CBZ in scCO2 phase (as was mentioned above). Along with this, the redistribution of the spectral contributions constituting the analytical spectral domain takes place. Namely, the intensity of the low

re

-p

ro

of

frequency component grows faster compared to the high frequency one.

Figure 3. Molecular structures of two CBZ conformers. The main difference between these two

lP

conformers is in the N2 atom position in the carboxamide group in relation to the C-H1-H2 plane (as shown with arrows). For Conf. I the N2 atom is out-of-plane and shifted in the direction out of the

na

aromatic system, for Conf. II the N2 atom is out-of-plane and shifted in the opposite direction. The differences between these two conformers can be determined by the values of the dihedral angles τ1

Jo ur

(N1-C-N2-H1), τ2 (N1-C-N2-H2) and τ3 (CC-N1-C-O).

Figure 4. Molecular structure of the CBZ–CO2 dimer formed by the Lewis Acid–Base mechanism involving the carboxamide group.

For quantitative analysis of the analytical spectral domain we applied a model that contains two spectral contributions related to conformers I and II (according to QCC). Taking into account the low CBZ concentration in the fluid solution and, consequently, the negligible probability of specific interactions between its molecules, in this model we used Lorentzian profiles for these spectral contributions. Fig. 7 shows the results of approximation of the experimental mid-IR spectra of CBZ in

Journal Pre-proof the scCO2 phase in the wavenumber range of 1660–1800 cm-1. In order to improve the experimental spectral curve reproducibility, in addition to two Lorentzian spectral profiles we used a linear component, which allows compensating for the probable inaccuracy of the baseline correction that was done at the first stage of the spectra processing (see above). Thus, we reached good agreement of the experimental curve and the modelled one for each of the studied temperatures. For these spectral profiles the maximum positions, dispersions and integral intensities as functions of temperatures were

lP

re

-p

ro

of

obtained (see Fig. 8).

Figure 5. IR spectra of two CBZ conformers in the wavenumber range of 1000–2000 cm-1 obtained by

Jo ur

monomer.

na

QCC. The dash lines correspond to the CBZ–CO2 dimer, the solid lines correspond to the CBZ

Figure 6. (a) Thermal evolution of experimental mid-IR spectra of CBZ dissolved in scCO2 in the range of 1200–1900 cm-1 (these spectra were obtained by a direct subtraction of the neat CO2 spectra from the spectra of the binary mixture of CBZ–CO2 measured at the same thermodynamic parameters, no baseline correction was done); the area limited by a pink rectangular corresponds to the analytical spectral band related to the C=O group stretching vibrations (the wavenumber range of 1660–1800 cm1

). (b) Analytical spectral domain corrected by base line subtraction.

Jo ur

na

lP

re

-p

ro

of

Journal Pre-proof

Figure 7. Approximation of experimental mid-IR spectra of CBZ in the scCO2 phase in the wavenumber range of 1660–1800 cm-1 corresponding to the analytical spectral band related to the C=O group stretching vibrations. Two Lorentzian spectral profiles were used for the analytical spectral curve.

re

-p

ro

of

Journal Pre-proof

lP

Figure 8. Temperature dependences of spectral profile characteristics related to two conformers of CBZ: (a) peak maximum positions of ν(T), (b) peak dispersions of γ(T) and (c) integral intensities of

na

A(T).

Relying on the QCC results and taking into account the low CBZ concentration in scCO2, we can

Jo ur

attribute these spectral components to the different CBZ conformers. Thus, the high frequency component is related to the most stable conformer (Conf. I) and the low frequency one is related to Conf. II. In spite of the larger difference between the positions of their peaks that was found to be about 20 cm-1 (see Fig. 8a) against 1 cm-1 according to the QCC, the reasonability of this assignment can be supported by a number of factors. First, these two bands linearly shift to the higher frequency region under heating, and this shift does not exceed 4 cm-1 (from 1728 to 1730 cm-1 for Conf. I and from 1706 to 1710 cm-1 for Conf. II). Second, the dispersions of both of these bands have the same order and their increase does not exceed 5 cm-1 (see Fig. 8b). Moreover, the linear behavior of the temperature dependences of the peak dispersions that reflects the homogeneous broadening of both spectral components is not typical of hydrogen bonded species. Indeed, for H-bonded species one can expect an inhomogeneous broadening of the spectral component with temperature, and this component must be better reproduced by the Gaussian profile. This is also in good agreement with the statements made in our works published earlier (see Refs. [4, 5, 24]). And finally, as it has been shown above, when the interaction of the CBZ molecule with one molecule of CO2 is taken into account, it is seen that the difference between the frequencies of the C=O group stretching vibrations for the two

Journal Pre-proof conformers tends to increase. It means that CO2 plays a certain role in the formation of the CBZ geometry and it can affect the vibration parameters of the carboxamide group fragments. However, additional MD calculations of the binary CBZ–CO2 system have to be made in order to study in detail the environment effect on the CBZ molecule dissolved in CO2. The authors of Ref. [28], when analyzing a large number of theoretical and experimental works published within the last two decades, also demonstrated the important role of CO2 which can interact with a CO2-phile group in the processes of solvation via the Lewis Acid–Lewis Base mechanism. The temperature dependences of the integral intensities of the spectral profiles related to Conf. I and Conf. II are presented in Fig. 8c. These values were used to calculate the molar fractions of the conformers by formula (3). The obtained values of the molar fractions as functions of temperature are

of

presented in Fig. 9. These temperature dependences show a redistribution of the mole fractions of

na

lP

re

-p

ro

these conformers under heating.

Jo ur

Figure 9. Molar fractions of CBZ conformers as functions of temperature as obtained from IR spectroscopy.

We also calculated the populations of these conformers via Boltzmann's factor for the temperature of 40°C by applying QCC. The obtained fractions were found to be 0.64 and 0.36 for Conf. I and Conf. II, respectively. Moreover, these populations were calculated by the metadynamic approach within MD simulations of CBZ conformers in scCO2 at the same temperature and pressure that corresponded to the selected isochore. For these two conformers we obtained values of their populations that were calculated through integration of the free energy map (see Fig. 10) in the coordinates of the dihedral angles τ1 and τ2 (see Fig. 3) following expression 2. The Two energy minima located in the ranges of 175° – -175° (for τ1) and -90° – 90° (for τ2) correspond to two conformers and are in good agreement with the QCC results. It has also been found that there are two additional conformational states that are related to the two minima, located in the range of -4.8° – 4.8° for τ1 and in the ranges of -141° ÷ -139° and 136° ÷ 141° for τ2 (see Fig. 10). However, because their population did not exceed 5–6% in total and these conformers were not found

Journal Pre-proof by the QCC, we excluded them from further consideration. Finally, the obtained and normalized values of populations of the two conformers that were taken into account were found to be 0.69 and 0.31. Although, these values were found to be closer to the experiment compared to the ones obtained by the QCC (see Fig. 11), there was still some discrepancy between the results obtained by MD and

-p

ro

of

from IR spectroscopy.

re

Figure 10. Gibbs free energy maps in the coordinates of two dihedral angles τ1 and τ2 obtained within

lP

MD simulation for the CBZ molecule in the scCO2 phase at 40°C and at the CO2 density equal to 1.3 of its critical value. The energy minima (blue areas) correspond to the CBZ stable conformational

Jo ur

na

states.

Figure 11. Comparison of the populations of two conformers calculated by the QCC and MD with the ones obtained from the IR spectroscopy data at 40°C

Conclusions In this work we gave a detailed description of the self-consistent approach to analysis of the conformational equilibria of active pharmaceutical ingredient molecules dissolved in scCO2. This approach is based on using a combination of computational and spectroscopic methods. Namely, QCC

Journal Pre-proof allow performing a preliminary conformational search and determining the energy characteristics of conformers of active pharmaceutical ingredient molecules and the energy barriers of transitions between them. Analysis of in situ IR spectroscopy data based on the QCC results gives the information on the equilibrium of conformers of the pharmaceuticals dissolved in scCO2 in the range of thermodynamic parameters of interest. By the example of CBZ that is a representative of the packing type of polymorphism, whereas its molecules have constrained conformational lability, we demonstrated the applicability of the approach presented. First of all, we have found that there are two conformers of CBZ molecules and the transition between them is related to the N2 atom position inversion relative to the C-H1-H2 plane in the carboxamide functional group (see Fig. 3). We have established the dependency of population of

of

these conformers on the SCF solvent thermodynamic parameters. We have also found the

ro

conformational crossover appearing under heating, which allows us to assume that in the CBZ solid phase, being in equilibrium with the fluid solution phase, a polymorphic transformation is observed.

-p

Thus, taking into account the results of our previous publications aimed at studying the conformational equilibria of drug molecules dissolved in scCO2, we can conclude that irrespective of

re

the drug compound polymorphism type or the conformational lability of its molecules, the self-

lP

consistent approach developed by us can be considered as an effective technique for monitoring the conformational equilibria in a wide range of thermodynamic parameters of state. This, in turn, allows

na

monitoring and controlling the polymorphic transformations of drugs within the application of CO2-

polymorphism.

Acknowledgments

Jo ur

based SCF technologies for obtaining pharmaceuticals with a high purity degree and specified

This work was financially supported by the Ministry of Science and Higher Education of the Russian Federation, grant No. RFMEFI61618X0097. The IR spectroscopy experiment was performed using the molecular fluid spectroscopy facility (http://www.ckp-rf.ru/usu/503933/) of G.A. Krestov Institute of Solution Chemistry of the Russian Academy of Sciences (ISC RAS) (Russia)

References [1] A. Nangia, Conformational Polymorphism in Organic Crystals, Accounts of Chemical Research, 41 (2008) 595-604. [2] A.J. Cruz-Cabeza, J. Bernstein, Conformational Polymorphism, Chemical Reviews, 114 (2014) 2170-2191. [3] R. Purohit, P. Venugopalan, Polymorphism: An Overview, Resonance – Journal of Science Education, 14 (2009) 882-893.

Journal Pre-proof [4] R.D. Oparin, M. Moreau, I. De Waele, M. Paolantoni, A. Idrissi, M.G. Kiselev, The interplay between the paracetamol polymorphism and its molecular structures dissolved in supercritical CO2 in contact with the solid phase: In situ vibration spectroscopy and molecular dynamics simulation analysis, European Journal of Pharmaceutical Sciences, 77 (2015) 48-59. [5] R.D. Oparin, D.V. Ivlev, A.M. Vorobei, A. Idrissi, M.G. Kiselev, Screening of conformational polymorphism of ibuprofen in supercritical CO2, Journal of Molecular Liquids, 239 (2017) 49-60. [6] R.D. Oparin, Y.A. Vaksler, M.A. Krestyaninov, A. Idrissi, S.V. Shishkina, M.G. Kiselev, Polymorphism and conformations of mefenamic acid in supercritical carbon dioxide, J. Supercrit. Fluids, 152 (2019) 104547. [7] S. Sala, Y. Danten, N. Ventosa, T. Tassaing, M. Besnard, J. Veciana, Solute–solvent interactions governing preferential solvation phenomena of acetaminophen in CO2-expanded organic solutions: A spectroscopic and theoretical study, J. Supercrit. Fluids, 38 (2006) 295-305.

of

[8] P. Vitoux, T. Tassaing, F. Cansell, S. Marre, C. Aymonier, In situ IR spectroscopy and ab initio calculations

ro

to study polymer swelling by supercritical CO2, The Journal of Physical Chemistry B, 113 (2009) 897-905. [9] B. Farbos, T. Tassaing, Substituent effect on the interaction of aromatic primary amines and diamines with

-p

supercritical CO 2 from infrared spectroscopy and quantum calculations, Physical Chemistry Chemical Physics, 11 (2009) 5052-5061.

re

[10] S. Foltran, R. Méreau, T. Tassaing, On the interaction between supercritical CO 2 and epoxides combining

(2011) 9209-9215.

lP

infrared absorption spectroscopy and quantum chemistry calculations, Physical Chemistry Chemical Physics, 13

[11] A.L. Grzesiak, M. Lang, K. Kim, A.J. Matzger, Comparison of the four anhydrous polymorphs of

na

carbamazepine and the crystal structure of form I, Journal of Pharmaceutical Sciences, 92 (2003) 2260-2271. [12] R.K. Harris, P.Y. Ghi, H. Puschmann, D.C. Apperley, U.J. Griesser, R.B. Hammond, C. Ma, K.J. Roberts, G.J. Pearce, J.R. Yates, C.J. Pickard, Structural Studies of the Polymorphs of Carbamazepine, Its Dihydrate, and

Jo ur

Two Solvates, Organic Process Research & Development, 9 (2005) 902-910. [13] G.M. Day, J. Zeitler, W. Jones, T. Rades, P. Taday, Understanding the influence of polymorphism on phonon spectra: Lattice dynamics calculations and terahertz spectroscopy of carbamazepine, The Journal of Physical Chemistry B, 110 (2006) 447-456. [14] J.-B. Arlin, L.S. Price, S.L. Price, A.J. Florence, A strategy for producing predicted polymorphs: catemeric carbamazepine form V, Chemical Communications, 47 (2011) 7074-7076. [15] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery, Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09, Revision D.01, in, Gaussian, Inc., Wallingford CT, 2013.

Journal Pre-proof [16] N. Schmid, A.P. Eichenberger, A. Choutko, S. Riniker, M. Winger, A.E. Mark, W.F. van Gunsteren, Definition and testing of the GROMOS force-field versions 54A7 and 54B7, European Biophysics Journal, 40 (2011) 843. [17] Z. Zhang, Z. Duan, An optimized molecular potential for carbon dioxide, The Journal of Chemical Physics, 122 (2005) 214507. [18] S. Nosé, A molecular dynamics method for simulations in the canonical ensemble, Molecular Physics, 52 (1984) 255-268. [19] W.G. Hoover, Canonical dynamics: Equilibrium phase-space distributions, Physical Review A, 31 (1985) 1695-1697. [20] M. Parrinello, A. Rahman, Polymorphic transitions in single crystals: A new molecular dynamics method, Journal of Applied Physics, 52 (1981) 7182-7190.

of

[21] M. Abraham, E. Apol, R. Apostolov, H.J.C. Berendsen, GROMACS USER MANUAL Version 4.6.7 in.

ro

[22] U. Essmann, L. Perera, M.L. Berkowitz, T. Darden, H. Lee, L.G. Pedersen, A smooth particle mesh Ewald method, The Journal of Chemical Physics, 103 (1995) 8577-8593.

-p

[23] G.A. Tribello, M. Bonomi, D. Branduardi, C. Camilloni, G. Bussi, PLUMED 2: New feathers for an old bird, Computer Physics Communications, 185 (2014) 604-613.

re

[24] R.D. Oparin, A. Idrissi, M.V. Fedorov, M.G. Kiselev, Dynamic and Static Characteristics of Drug Dissolution in Supercritical CO2 by Infrared Spectroscopy: Measurements of Acetaminophen Solubility in a

lP

Wide Range of State Parameters, J. Chem. Eng. Data, 59 (2014) 3517-3523. [25] R.D. Oparin, M.A. Krestyaninov, E.A. Vorobyev, O.I. Pokrovskiy, O.O. Parenago, M.G. Kiselev, An

Liquids, 239 (2017) 83-91.

na

insight into possibility of chemical reaction between dense carbon dioxide and methanol, Journal of Molecular

1126-1128.

Jo ur

[26] M. Wojdyr, Fityk: a general-purpose peak fitting program, Journal of Applied Crystallography, 43 (2010)

[27] A. Austin, G.A. Petersson, M.J. Frisch, F.J. Dobek, G. Scalmani, K. Throssell, A Density Functional with Spherical Atom Dispersion Terms, Journal of Chemical Theory and Computation, 8 (2012) 4989-5007. [28] F. Ingrosso, M.F. Ruiz‐López, Modeling solvation in supercritical CO2, ChemPhysChem, 18 (2017) 25602572.

Journal Pre-proof

Declaration of interests

Manuscript under title: Conformational Equilibriums of Pharmaceuticals in Supercritical CO2. IR spectroscopy and Quantum Chemical Calculations.

Authors:

of

Roman D. Oparina, Michael G. Kiseleva a

re

-p

ro

G.A. Krestov Institute of Solution Chemistry of the Russian Academy of Sciences (RAS), Akademicheskaya str. 1, Ivanovo, 153045, Russia.

na

lP

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Jo ur

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Jo ur

na

lP

re

-p

ro

of

Journal Pre-proof

Journal Pre-proof

Highlights: • Approach for analysis of the conformations of drugs molecules in scCO2 is shown • It can be used when drugs production by CO2-based supercritical fluid technologies • Method was applied for conformational analysis of carbamazepine dissolved in scCO2

Jo ur

na

lP

re

-p

ro

of

• Conformational crossover was found under isochoric heating conditions