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Solid-state hydration/dehydration of erythromycin A investigated by ab initio powder X-ray diffraction analysis – stoichiometric and non-stoichiometric dehydrated hydrate Kotaro Fujii, Masahide Aoki, and Hidehiro Uekusa Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg400121u • Publication Date (Web): 19 Mar 2013 Downloaded from http://pubs.acs.org on March 22, 2013

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Crystal Growth & Design

Solid-state hydration/dehydration of erythromycin A investigated by ab initio powder X-ray diffraction analysis – stoichiometric and non-stoichiometric dehydrated hydrate

Author Kotaro Fujii, Masahide Aoki and Hidehiro Uekusa* Department of Chemistry and Materials Science, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro-ku, Tokyo 152-8551, Japan

Author for correspondence: Hidehiro Uekusa. E-mail: [email protected]; Tel/ Fax: +81-3-5734-3529

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Abstract The stable dihydrate crystalline phase (DH) of erythromycin A loses water upon heating to give the anhydrous phase I (AI). Further heating then results in a polymorphic transformation via the amorphous state (melt) to give another anhydrous phase II (AII). The anhydrous phases of AI and AII undergo hydration when increasing the humidity. The crystals of AI showed stoichiometric hydration to give DH, whereas the crystals of AII showed non-stoichiometric hydration to give the humidity dependent non-stoichiometric hydrate phase (NSH). The crystal structures of AI and AII were directly determined from powder X-ray diffraction data using the direct space strategy for the structure solution followed by Rietveld refinement. From the structural properties of AI and AII, aspects of the mechanism of the solid-state transformations of DH and the hydration behavior of AI and AII have been determined, and the importance of the hydrophilicity of the voids has been revealed.

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1.

Introduction Organic molecules often exist in several different crystalline forms, such as polymorphs, hydrates

and solvates. These different crystalline forms are important research targets, particularly in the pharmaceutical industry, because they commonly have different physical or chemical solid-state properties, such as solubility, dissolution rate, stability, and bioavailability.1,2 Another important aspect of polymorphs and hydrates of pharmaceutical molecules are solid-state transformations between different solid forms induced by environmental changes, e.g., temperature, humidity and pressure, because they may occur during drug manufacturing processes or storage of the drug compound or dosage form. Erythromycin A (Figure 1) is a widely used macrolide antibiotic, and is known to crystallize as a dihydrate crystalline phase (DH) by recrystallization from aqueous solution. The crystals of DH undergo dehydration upon heating to give an anhydrous phase (anhydrous phase I; AI), which transforms to another anhydrous phase (anhydrous phase II; AII) via the amorphous state (melt) by further heating.4 The change in the powder X-ray diffraction pattern during the dehydration process of DH is small, and AI is considered a dehydrated hydrate, which is also referred to as an isomorphic desolvate.4,5 The terms “dehydrated hydrate” and “isomorphic desolvate” define the dehydrated (or desolvated) crystalline phase that retains the molecular packing of the parent hydrate (or solvate) after dehydration (or desolvation). Although AI is considered as a dehydrated hydrate, there is no structural evidence for this classification. Furthermore, the crystal structure and hydration properties of AII have not been reported. The main problem in obtaining structural information of the anhydrous phases is disintegration of the single crystalline form of the parent hydrate phase during the dehydration process, which makes it difficult to analyze the crystal structures by single crystal X-ray diffraction. In



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this context, many solid-state transformations of pharmaceutical materials are not well understood and the nature of dehydrated hydrates and isomorphic desolvates is unclear, which includes the case of erythromycin A.

Figure 1. Molecular structure of erythromycin A.

Techniques for carrying out structure determination directly from powder X-ray diffraction data are clearly essential for the structural analysis of polycrystalline product materials obtained directly by dehydration process. The techniques for carrying out complete structure determination of organic molecular materials from powder X-ray diffraction data have progressed considerably in recent years,6-18 particularly through the development of the direct-space strategy for structure solution.19 Because these techniques have the potential to provide structural understanding of polycrystalline products obtained from solid-state transformations, e.g. originating from single crystal materials of the type discussed above, they have a key role to play in understanding the structural properties of materials produced from solid-state grinding processes, solid-state reactions, desolvation processes, and polymorphic transformations.20-33 Some dehydration processes of pharmaceutical hydrates have already been revealed using these techniques.34-37 In the present study, the crystal structures of AI and AII have been determined from powder X-ray


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diffraction data and mechanisms of the solid-state transformations of erythromycin A have been proposed. In addition, the hydration behavior of the two different anhydrous phases were investigated using dynamic vapor sorption (DVS) and powder X-ray diffraction (PXRD) experiments with the aim of understanding the nature of the dehydrated hydrates (isomorphic desolvates). Erythromycin A is a relatively large molecule and it is challenging to determine its crystal structure from powder X-ray diffraction data, especially because the crystal structure of AII contains two independent erythromycin A molecules in the asymmetric unit. However, we have successfully determined its crystal structure from the high resolution synchrotron powder X-ray diffraction data.

2.

Experimental Section

2.1.

Preparation

Erythromicin A was purchased from Wako Pure Chemical Industries, Ltd. and was confirmed as the pure dihydrate crystalline phase (DH) from PXRD measurements. The purchased sample was used for thermal analysis (thermogravimetric analysis/differential thermal analysis (TGA/DTA) X-ray diffraction-differential scanning calorimetry (XRD-DSC)). The crystals of the anhydrous phases I (AI) and II (AII) of erythromycin A were prepared from DH by drying or heating.

2.2.

Thermal analysis

TGA and DTA were simultaneously measured on a Rigaku Thermo Plus 2 instrument. 18.11 mg of DH was heated at a rate of 3 °C/min. The TG curve was recorded from 20 °C to 250 °C in a dry nitrogen atmosphere with a flux of 20 mL/min. The simultaneous XRD-DSC measurements were carried out on a Rigaku D/MAX 2400 diffractometer and a Rigaku Thermo Plus 2 differential scanning calorimeter at a heating rate of 3°C/min. 5 ACS Paragon Plus Environment


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2.3.

Dynamic Vapor Sorption

The water sorption and desorption processes of AI and AII were measured on a DVS-Advantage (SMS Ltd, UK). Accurately weighed powder crystals (ca. 10 mg) were mounted on a balance and the relative humidity (RH) was increased from 0% to 95% in 5% steps, then decreased to 0% in 5% steps. During each step, the RH was maintained until the mass change was less than 0.001 w.t.%/min. The measurements were carried out at 25 °C.

2.4.

Synchrotron X-ray powder diffraction measurement

Synchrotron PXRD data of AI were recorded at 100 °C on beamline 02B2 at SPring-8 using a Debye-Scherer camera equipped with a curved imaging plate detector with a wavelength of 0.998649(5) Å. The crystals of AI readily transform to DH at ambient temperature in the presence of water vapor, and it was difficult to avoid hydration even when the sample was sealed in a glass capillary after preparation. Therefore, the sample of AI was directly prepared by heating DH on a goniometer and the measurement was carried out at high temperature. The sample was introduced into a 0.3 mm diameter unsealed borosilicate glass capillary and was slowly heated (ca. 5 °C/min) to 100°C. The data collection time was 30 minutes. Synchrotron PXRD data of AII were recorded at ambient temperature on beamline 4B2 (Multiple Detector System; parallel beam with analyzer monochromators) at Photon Factory with wavelength 1.196175(6) Å. The sample was prepared by heating DH and was placed in a 2.0 mm diameter borosilicate glass capillary with saturated aqueous solutions to control the humidity conditions. The measurements were carried out at low humidity (RH 3%) and high humidity (98%) conditions using the saturated aqueous solutions of cesium fluoride (CsF) and barium sulfate (BaSO4), respectively. The data collection time was ca. 10 hours for each humidity condition.


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2.5.

Structure determination from powder X-ray diffraction data

The PXRD pattern of AI was indexed using the program DICVOL0438 giving the following unit cell (M20 = 26.3,39 F20 = 120.7 40) with orthorhombic metric symmetry: a = 46.22 Å, b = 9.54 Å, c = 9.52 Å, and V = 4200.7 Å3. Given the volume of the unit cell and considering the density, the number of formula units in the unit cell was assigned as Z = 4. From the estimated Z value and systematic absences, the space group was determined as P212121. Profile fitting using the Pawley method41 incorporated in the program DASH42 gave Rwp = 9.97% and χ2 = 5.88 (calculated on the background subtracted intensity). The structure solution was carried out using the simulated annealing method incorporated in the program DASH. The structural fragment is comprised of one erythromycin A molecule, which has a total of 13 structural variables. Twenty runs with 108 simulated annealing moves per run were performed for the structure solution using a molecular model imported from the crystal structure of DH5. The erythromycin A molecule contains a 14-membered lactone ring that may take several different conformations. However, the conformations of the 14-membered lactone rings are the same in all the different crystal structures, such as the DH,5 the zinc complex solvate of erythromycin A43, erythromycin B5 and clarithromycin5,44,45. Thus, the conformation of the 14-membered lactone ring was fixed during the structure solution calculations. The best solution had profile χ2 41.0 and intensity χ2 93.9. Following structure solution, Rietveld refinement46 was carried out using the GSAS program.47 Standard restraints were applied to the bond lengths and bond angles, and a global isotropic displacement parameter was used. The hydrogen atoms were located geometrically. The final Rietveld refinement gave the following parameters: a = 9.5166(5) Å, b = 9.5454(4) Å, c = 46.256(2) Å, V = 4201.8(4) Å3, Rwp = 0.0426, Rp = 0.0292, and RF2 = 0.1007 (corresponding Le Bail profile fitting gave Rwp = 0.0262, Rp = 0.0179, Figure 2a). The crystallographic data and the final Rietveld plot are shown in Table 1 and Figure 2b, respectively. The PXRD pattern of AII (RH 3%) was indexed using the program DICVOL04 giving the 7 ACS Paragon Plus Environment


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following unit cell (M20 = 30.8, F20 = 173.9) with orthorhombic metric symmetry: a = 32.44 Å, b = 23.82 Å, c = 10.75 Å, and V = 8305.4 Å3. Given the volume of the unit cell and considering the density, the number of formula units in the unit cell was assigned as Z = 8. From the systematic absences, the space group was determined as P21212. Profile fitting using the Pawley method incorporated in the program DASH gave Rwp = 11.06% and χ2 = 11.91 (calculated on the background subtracted intensity). The structure solution was carried out using the simulated annealing method incorporated in the program DASH. The structural fragment is comprised of two erythromycin A molecules, which have a total of 26 structural variables. Twenty runs with 108 simulated annealing moves per run were performed for the structure solution using a molecular model imported from the crystal structure of DH. The best solution had profile χ2 37.4 and intensity χ2 50.1. Following structure solution, Rietveld refinement46 was carried out using the GSAS program47. Standard restraints were applied to the bond lengths and bond angles, and a global isotropic displacement parameter was used. The hydrogen atoms were located geometrically. The final Rietveld refinement gave the following parameters: a = 32.517(1) Å, b = 23.8535(9) Å, c = 10.7658(3) Å, V = 8350.3(6) Å3, Rwp = 0.0561, Rp = 0.0422, and RF2 = 0.0867 (corresponding Le Bail profile fitting gave Rwp = 0.0326, Rp = 0.0253, Figure 2c). The crystallographic data and the final Rietveld plot are shown in Table 1 and Figure 2d, respectively.


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Figure 2. Le Bail refinement and final Rietveld refinement of (a), (b) erythromycin A anhydrous phase I (AI), and (c), (d) anhydrous phase II (AII). The experimental powder X-ray diffraction pattern (red + marks), calculated powder X-ray diffraction pattern (green solid line) and difference profile (black line) are shown. The tick marks indicate the peak positions.



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Table 1. Crystallographic data of erythromycin A anhydrous phase I (AI), and anhydrous phase II (AII).

Phase

dihydrate (DH) Ref. 2

anhydrous phase I (AI)

anhydrous phase II (AII) at RH =3%

Formula

C37H67NO13 2H2O

C37H67NO13

C37H67NO13

Formula weight

769.97

733.94

733.94

Crystal system

Orthorhombic

Orthorhombic

Orthorhombic

Space group

P212121

P212121

P21212

a/Å

9.1829

9.5166(5)

32.517(1)

b/Å

9.6316

9.5454(4)

23.8535(9)

c/Å

47.151

46.256(2)

10.7658(3)

V / Å3

4170.3

4201.8(4)

8350.3(6)

Temperature / °C

20

100

20

Z

4

4

8

Z’

1

1

2

ρcalc / g cm-3

1.226

1.160

1.168

2θ range / °

-

1.51 – 39.99

2.01 – 39.99

Rwp

-

0.0426

0.0561

Rp

-

0.0292

0.0422

RF2

-

0.1007

0.0867

Goodness-of-fit

-

2.27

1.22


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3.

Results and Discussion

3.1.

Solid-state transformations of erythromycin A dihydrate

Simultaneous TGA and DTA measurements (Figure 3), and simultaneous PXRD and DSC measurements (Figure 4) were carried out to observe the thermal behavior of DH. The first endothermic peak observed in the temperature range 40-100 °C is assigned as a dehydration process with an accompanied mass decrease of 4.66%, corresponding to the calculated mass of two water molecules (4.70%). The second endothermic peak observed at 135 °C is a melt of AI, which was confirmed visually and from the PXRD pattern change (dark-purple pattern in Figure 4), and this melt state formed an amorphous state after cooling. The melt (amorphous state) crystallized at 155 °C, as observed by the next exothermic peak in the TGA plot, giving crystalline AII, which melted again at 195 °C. There are similarities in the PXRD patterns of DH (blue pattern in Figure 4) and AI (red pattern in Figure 4), whereas AII (green pattern in Figure 4) has a different PXRD pattern than the other crystalline phases. This indicates the structural similarity of DH and AI, and suggests that AI is a dehydrated hydrate, as has been previously reported.4,5

Figure 3. TGA/DTA plot of the erythromycin A dihydrate phase (DH).

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Figure 4. PXRD-DSC plot of the erythromycin A dihydrate phase (DH). The left figure shows the change in the PXRD pattern with temperature, and the right figure shows the DSC plot. Blue: dihydrate phase (DH), red: anhydrous phase I (AI), dark purple: amorphous state, and green: anhydrous phase II (AII).

3.2.

Hydration and dehydration behavior of anhydrous polymorphs

The hydration and dehydration behavior of AI and AII was investigated by DVS measurements (Figure 5). As shown in Figure 5a, AI underwent hydration at RH = 5% with a mass increase corresponding to two water molecules, and no significant mass change was observed with further humidification. The hydrated phase was confirmed as DH by PXRD. The dehydration of DH only occurred at RH < 5% (see desorption process in Figure 5a), suggesting the stability of DH. Because the number of incorporated water molecules is stoichiometric for both DH (two) and AI (zero), the dehydration and hydration processes between DH and AI are stoichiometric dehydration and hydration processes. On the other hand, AII showed non-stoichiometric hydration and dehydration (Figure 5b). In both processes, the mass gradually changed depending on the humidity, and the number of incorporated water 12


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molecules was not stoichiometric. Here, we call this hydrated phase of AII the non-stoichiometric hydrate phase (NSH). The maximum number of incorporated water molecules in the NSH was estimated to be about 1.3 water molecules per erythromycin A molecule from the DVS plot. Even though NSH incorporates some water, the PXRD patterns of AII and NSH are similar (green PXRD pattern in Figure 4 and Figure 6). The similarity of the PXRD data of AII and NSH clearly indicates that the crystal structure of AII is also a dehydrated hydrate of NSH (or NSH is an isomorphic hydrate of AII). Interestingly, one polymorph shows stoichiometric hydration/dehydration behavior and the other shows non-stoichiometric hydration/dehydration behavior. Because these two polymorphs consist of the same molecule, the different hydration/dehydration behavior is due to differences in the crystal structures. Therefore, the crystal structures of these two anhydrous phases should provide important information about the mechanism of the stoichiometric or non-stoichiometric hydration/dehydration processes.

Figure 5. DVS plot of erythromycin A anhydrous phases (a) I and (b) II. 13



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Figure 6. Powder X-ray diffraction patterns of the erythromycin A non-stoichiometric hydrate phase (NSH, hydrated AII) under different relative humidity conditions.

3.3.

Crystal

structure

analysis

of

anhydrous

polymorphs

and

mechanisms

of

transformation To understand the dehydration and solid-state transformation of DH and the mechanism of the stoichiometric or non-stoichiometric hydration/dehydration, the crystal structures of AI and AII were analyzed. Because the dehydration process is accompanied by the disintegration of the single crystalline form of DH, the samples of AI and AII were only obtainable as powders. For this reason, the crystal structures of AI and AII were determined directly from the PXRD data measured using synchrotron radiation (see Sections 2.4 and 2.5 for details of the measurements and structural analysis). 3.3.1.

Structural comparison of the dihydrate and anhydrous phase I and the mechanism of

the stoichiometric and isomorphic dehydration/hydration processes The crystal structures of DH5 and AI are shown in panels a, c and b, d of Figure 7, respectively. In the crystal structure of DH, two independent water molecules are arranged along the a-axis forming a water channel. These water molecules bridge erythromycin A molecules by hydrogen bonds (Figure 7c). 14


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The crystal structure of AI (Figure 7b) has similar molecular packing to DH (Figure 7a) and thus these two crystalline phases are isostructural. This indicates that AI is a dehydrated hydrate of DH, as expected from the change in the PXRD pattern. By loss of the water molecules, the water mediated hydrogen bonds in DH switch to inter- and intramolecular hydrogen bonds between erythromycin A molecules. Another important aspect of the crystal structure of AI is a presence of voids, as shown by yellow translucent sphere in Figure 7b. There are two independent voids in AI with volumes of about 10 and 15 Å3.48 The positions of the voids in AI and the positions of the water molecules in DH are almost the same, and, therefore, the voids are created by the loss of the water molecules. The sum of the volumes of the two voids observed in AI (25 Å3) is less than the volume the two water molecules occupy in DH (54 Å3). Therefore, the voids created by the removal of water do not remain and structural relaxation occurs to reduce the void volume. Given that incorporation of water molecules in the voids could give the stable crystalline state DH without large structural reconstruction, it is natural that the crystals of AI readily transform to DH. Both the water molecules in DH and the voids in AI form channels along the a-axis. Therefore, the loss and incorporation of water molecules in the dehydration and hydration processes could proceed though this channel with only small structural change.

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Figure 7. Crystal structures of (a) and (c) erythromycin A dihydrate, and (b) and (d) anhydrous phase I. Hydrogen atoms are omitted for clarity. The red spheres in (a) and (c) represent the water molecules. The yellow translucent spheres in (b) show the voids. The blue dash lines in (c) and (d) indicate the hydrogen bonds.

3.3.2.

Crystal Structure of Anhydrous Phase II and the mechanism of non-stoichiometric

hydration The crystal structure of AII has completely different molecular packing to DH and AI. The erythromycin A molecules aggregate to form a tetramer via intermolecular hydrogen bonds (Figure 8a), and the tetramer units are arranged in a herringbone pattern as shown in Figure 8b (tetramer units are shown in different colors). There are no hydrogen bonds (D…A distance less than 3.2 Å) between the tetramer units and all hydroxyl 16


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groups point towards the center of the tetramer unit, apart for one hydroxyl group that forms an intramolecular hydrogen bond. Interestingly, as observed in AI, the crystal structure of AII also has voids around the tetramer units, shown as yellow translucent sphere in Figure 8b. There are four independent void spaces, which have volumes of 48, 24, 22 and 17 Å3. Because there are no hydrophilic groups around these void spaces, they can be considered hydrophobic voids, whereas the voids observed in AI are hydrophilic. The presence of the hydrophobic voids may be a reason for the non-stoichiometric hydration behavior of AII. The voids incorporate water molecules, although no energetic stabilization by hydrogen bonds is possible because the water molecule cannot form hydrogen bonds. However, the volumes of the voids are large enough to incorporate water molecules, and it enables non-stoichiometric incorporation of water molecules without major structural change. Structural analysis of NSH at RH=98% was also carried out, although the positions of the water molecules could not be determined. Considering that the PXRD patterns measured at the low and high humidity conditions are quite similar, the water molecules would be highly disordered in the voids in the crystal structure of NSH, and incorporation of the water molecules would not greatly contribute to the diffraction intensities. Although the change in the PXRD pattern when varying the relative humidity was small, a small increase in unit cell volume was observed. Whole profile fittings using the Le Bail method were carried out for the PXRD data measured at several different humidity conditions, and the results show that the unit cell volume slightly increased with increasing relative humidity (Figure 9). The unit cell volume increased by about 45 Å3 when the relative humidity was increased from 3% to 98%. The unit cell contains eight erythromycin A molecules and the increase in volume per erythromycin A molecule is quite small (ca. 5 Å3). The increase in weight from RH=0% to RH=95% calculated from the DVS plot is about 3%, which corresponds to 1.3 water molecules per erythromycin A molecule. Therefore, the increase in the unit cell volume was significantly smaller than the expected increase in volume by the incorporation of 1.3 water molecules. The crystal structure of AII has large voids and these voids are hydrophobic, which causes the non-stoichiometric and isostructural hydration 17



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and dehydration behavior of AII and NSH.

Figure 8. Crystal structures of erythromycin A anhydrous phase II. (a) Hydrogen bond interactions and (b) packing viewed along the c-axis. The blue dash lines in (a) indicate the hydrogen bonds. Each hydrogen bond tetramer unit is drawn in a different color. The yellow translucent spheres in (b) indicate the voids.

Figure 9. Changes in the unit cell parameters of AII/NSH when increasing the relative humidity from RH=3% 18


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to RH=98%.

4.

Concluding Remarks Crystals of erythromycin A dihydrate (DH) undergo dehydration upon heating to give an anhydrous phase

(AI). Further heating gives a melt, which then crystallizes to give another anhydrous phase (AII). In the present work, the hydration/dehydration behavior of AI and AII were investigated by DVS and PXRD measurements. The crystal structures of AI and AII were determined from the PXRD data to establish the mechanistic aspects of the transformations of DH and the stoichiometric and non-stoichiometric hydration/dehydration behavior of AI and AII. The dehydration/hydration between DH and AI proceeds stoichiometrically without major changes in the crystal structures, and, therefore, the processes are stoichiometric and isomorphic. The crystal structure of AI contains hydrophilic voids that are created in the dehydration process, and the presence of these hydrophilic voids would cause instability of AI. Considering that the incorporation of water molecules in the voids could give the stable crystalline state DH without large structural reconstruction, it is natural that the crystals of AI readily transform to DH. Because there is a stable and structurally similar dihydrate phase (DH), the crystals of AI readily undergo stoichiometric and isomorphic hydration. The hydration/dehydration of AII is non-stoichiometric and was found to be an isomorphic process. The crystal structure of AII is quite different from the crystal structures of DH and AI, but it also has void spaces in the crystal packing. The voids are hydrophobic and energy stabilization is not expected by the incorporation of the water molecules. Therefore, it shows non-stoichiometric and isomorphic hydration/dehydration behavior. The present study shows two cases of the dehydrated hydrate (isomorphic desolvate). In the first case, it is stoichiometric, and in the second case it is non-stoichiometric. The dehydration and hydration behavior were rationalized from the crystal structures, and it was found that the voids and their hydrophilicity are very important for understanding the dehydrated hydrate and isomorphic desolvates. It is interesting that the same 19



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compound crystallizes in two different crystal structures that are related by thermal solid-state transformation, and both have different types of the voids resulting in different dehydration/hydration behavior.

Acknowledgements We thank Prof. Katsuhide Terada for the DVS measurements. The synchrotron radiation experiments were carried out on beam line 4B2 at Photon Factory with the approval of the Photon Factory Program Advisory Committee (Proposal NO. 2011G551) and BL19B2 at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal NO. 2011A1935). This work was supported by CREST from the JST and a Grant-in-Aid for Scientific Research (KAKENHI) from MEXT.

Supporting Information Crystal information files (CIFs) of AI and AII

Reference 1.

Haleblian, J. K. J. Pharm. Sci. 1975, 64, 1269.

2.

Data, S.; Grant, D. J. W. Nature Rev. 2004, 3, 42.

3.

Stephenson, G. A.; Stowell, J. G.; Toma, P. H.; Pfeiffer, R. R.; Byrn, S. R. J. Pharm. Sci., 1997, 86, 1239.

4.

Miroshnyk, I.; Khriachtchev, L.; Mirza, S.; Rantanen, J.; Heinämäli, J.; Yliruusi, J. Cryst. Growth & Des. 2006, 6, 369.

5.

Stephenson, G. A.; Groleau, E. G.; Kleemann, R. L.; Xu, W.; Rigsbee, D. R. J. Pharm. Sci., 1998, 87, 536.

6.

Harris, K. D. M.; Tremayne, M. Chem. Mater. 1996, 8, 2554.

7.

Harris, K. D. M.; Tremayne, M.; Kariuki, B. M. Angew. Chem. Int. Ed. 2001, 40, 1626.

8.

Chernyshev, V.V. Russian Chem. Bull. 2001, 50, 2273.

9.

David, W. I. F.; Shankland, K.; McCusker, L. B.; Baerlocher, C. Eds.; Structure Determination from 20


Page 20 of 23

Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Powder Diffraction Data, OUP/IUCr, 2002. 10. Huq, A.; Stephens, P. W. J. Pharm. Sci. 2003, 92, 244. 11. Harris, K. D. M.; Cheung, E. Y. Chem. Soc. Rev. 2004, 33, 526. 12. Tremayne, M. Phil. Trans. R. Soc. 2004, 362, 2691. 13. Favre-Nicolin, V.; Cerny, R. Z. Kristallogr. 2004, 219, 847. 14. Shankland, K.; Markvardsen, A. J.; David, W. I. F. Z. Kristallogr. 2004, 219, 857. 15. Brodski, V.; Peschar, R.; Schenk, H. J. Appl. Crystallogr. 2005, 38, 688. 16. Karki, S.; Fabian, L.; Friscic, T.; Jones, W. Org. Lett. 2007, 9, 3133. 17. Tsue, H.; Horiguchi, M. ; Tamura, R.; Fujii, K.; Uekusa, H. J. Synth. Org. Chem. Japan 2007, 65, 1203. 18. David, W.I.F.; Shankland, K. Acta Crystallogr. Sect. A 2008, 64, 52. 19. Harris, K. D. M.; Tremayne, M. ; Lightfoot, P.; Bruce, P. G. J. Am. Chem. Soc. 1994, 116, 3543. 20. Guo, F.; Harris, K. D. M. J. Am. Chem. Soc. 2005, 127, 7314. 21. Mora, A. J.; Avila, E. E.; Delgado, G. E.; Fitch, A. N.; Brunelli, M. Acta Cryst. B, 2005, 61, 96. 22. Platteau, C.; Lefebvre, J.; Affouard, F.; Willart, J. F.; Derollez, P.; Mallet, F. Acta Cryst. B, 2005, 61, 185. 23. Hirano, S.; Toyota, S.; Toda, F.; Fujii, K.; Uekusa, H. Angew. Chem. Int. Ed. 2006, 45, 6013. 24. Guguta, C.; Meekes, H.; Gelder, R. Cryst. Growth & Des. 2006, 6, 2686. 25. Guo, F.; Cheung, E. Y.; Harris, K. D. M.; Pedireddi, V. R. Cryst. Growth & Des. 2006, 6, 846. 26. Albesa-Jové, D.; Pan, Z.; Harris, K. D. M.; Uekusa, H. Cryst. Growth & Des.2008, 8, 3641. 27. Fujii, K.; Ashida, Y.; Uekusa, H.; Hirano, S.; Toyota, S.; Toda, F.; Pan, Z.; Harris, K. D. M. Cryst. Growth & Des. 2009, 9, 1201. 28. Fujii, K.; Ashida, Y.; Uekusa, H.; Guo, F.; Harris, K. D. M. Chem. Comm., 2010, 46, 4264. 29. Martí-Rujas, J.; Morte-Ródenas, A.; Guo, F.; Thomas, N.; Fujii, K.; Kariuki, B. M.; Harris, K. D.M. Cryst. Growth & Des. 2010, 10, 3176. 30. Fujii, K.; Garay, A. L.; Hill, J.; Sbircea, E.; Pan, Z.; Xu, M.; Apperley, D. C.; James, S. L.; Harris, K. D.M. Chem. Commun., 2010, 46, 7572. 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

31. Suzuki, M.; Imai, K.; Wakabayashi, H.; Arita, A.; Johmoto, K.; Uekusa, H.; Kobayashi, K. Tetrahedron, 2011, 67, 5500. 32. Cheng, E. Y.; Fujii, K.; Guo, F.; Harris, K. D. M.; Hasebe, S.; Kuroda, R. Cryst. Growth & Des. 2011, 11, 3313. 33. Kasugai, K.; Hashimoto, S.; Imai, K.; Sakon, A.; Fujii, K.; Uekusa, H.; Hayashi, N.; Kobayashi, K. Cryst. Growth & Des. 2011, 11, 4044. 34. Sugimoto, K.; Dinnebier, R. E.; Zakrzewski, M. J. Pharm. Sci. 2007, 96, 3316. 35. Fujii, K. ; Uekusa, H.; Itoda, N.; Hasegawa, G.; Yonemochi, E.; Terada, K.; Pan, Z.; Harris, K. D. M. J. Phys. Chem. C, 2010, 114, 580. 36. Halasz, I.; Dinnebier, R. E. J. Pharm. Sci. 2010, 99, 871. 37. Fujii, K.; Uekusa, H.; Itoda, N.; Yonemochi, E.; Terada, K. Cryst. Growth & Des. 2012, 12, 6165. 38. Boultif, A.; Louer, D. J. Appl. Cryst. 2004, 37, 724. 39. Wolff, P. M. J. Appl. Cryst. 1972, 5, 243. 40. Smith, G. S.; Snyder, R. L. J. Appl. Cryst. 1979, 12, 60. 41. Pawley, G. S. J. Appl. Cryst. 1981, 14, 357. 42. David, W. I. F.; Shankland, K. ; Cole, J.; Maginn, S.; Motherwell, W. D. H.; Taylor, R. DASH User Manual, Cambridge Crystallographic Data Centre, Cambridge, UK. (2001). 43. Oliver, J. D.; Strickland, L. C. Acta Cryst. C 1986, 42, 952. 44. Tian, J.; Thallapally, P. K.; Dalgarno, S. J.; Atwood, J. L. J. Am. Chem. Soc. 2009, 131, 13216. 45. Tian, J.; Dalgarno, S. J.; Atwood, J. L. J. Am. Chem. Soc. 2011, 133, 1399. 46. Rietveld, H. M. 1969, J. Appl. Cryst. 1969, 2, 65. 47. Larson, A. C.; Von Dreele, R. B. GSAS; Los Alamos Laboratory Report No. LA-UR-86-748; Los Alamos National Laboratory: Los Alamos, NM, 1987. 48. Speck, A. L. J. Appl. Cryst. 2003, 36, 7.

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Table of Content TOC Graphic

Synopsis The crystal structures of two anhydrous phases of the pharmaceutically relevant material erythromycin A have been directly determined by ab inito powder X-ray diffraction analysis and the mechanism of the dehydration, hydration and transformation processes have been established.

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Dehydration of Erythromycin ... - ACS Publications

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