FU Berlin Digitale Dissertation
Thammarat Aree : Untersuchungen zur Hydratisierungsdynamik und Löslichkeit methylierter Cyclodextrine mittels Röntgenkristallographic und Neutronenstreuung X-Ray Crystallographic and Neutron Scattering Studies on Hydration Dynamics and Solubility of Methylated Cyclodextrins
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Abstract In this study, X-ray crystallographic and neutron scattering experiments have been performed in order to comprehend the negative solubility coefficient in water of methylated cyclodextrins (CDs). X-ray analyses have been carried out for two crystal forms of both heptakis(2,6-di-O-methyl)-beta-CD (DIMEB) and octakis(2,3,6-tri-O-methyl)-gamma-CD (TRIMEG) which were grown from cold water at 291 K, and neutron scattering measurements for aqueous solutions of DIMEB, TRIMEG, and gamma-CD at 287-323 K. In DIMEB.2H2O and DIMEB.15H2O, the DIMEB molecules adopt ``round'' conformations stabilized by interglucose O3(n)-H...O2(n + 1) hydrogen bonds. While the former has 2 water molecules, one being included in the cavity and one in the intermolecular space, the latter has 15 water molecules which are all located outside the cavity and form a channel clathrate hydrate host structure enclosing the guest DIMEB. The abundance of 15 host-guest hydrogen bond interactions give rise to high thermal stability of the DIMEB.15H2O crystal. (4TRIMEG).19.3H2O and TRIMEG.4.5H2O which have no interglucose O3(n)-H...O2(n + 1) hydrogen bonds (because all O-H groups are methylated) are more flexible and the molecular structure of TRIMEG is notably different. In (4TRIMEG).19.3H2O, all four TRIMEG molecules adopt ``elliptical'' conformations with two diametrically opposed glucose units 1 and 5 flipped by ca. 180o (anti orientation). The 19.3 water molecules are distributed over 27 positions both inside and outside the TRIMEG cavities and are hydrogen bonded in different patterns to the four TRIMEG molecules. This contrasts TRIMEG.4.5H2O in which the molecular structure is ``round'' with all glucoses orientated syn and the 4.5 water molecules are accommodated in its cavity. The highly hydrated crystal forms of DIMEB.15H2O, (4TRIMEG).19.3H2O, and TRIMEG.4.5H2O grown from cold water indicate that the hydration may be associated with the high solubility of these methylated CDs in cold water. This is evidenced by neutron scattering results showing that in aqueous solution at 287 K, DIMEB and TRIMEG are hydrated by a large number of water molecules and diffuse slowly as indicated by a broad quasielastic peak which is characteristic of diffusive motion in the liquid state. As the temperature rises to 305 K, the hydration number decreases, DIMEB and TRIMEG diffuse faster. When the temperarure reaches the crystallization point at 323 K, the hydration number decreases rapidly, DIMEB aggregates and crystallizes as shown by a sharp elastic peak which indicates very slow motion of larger, solid state particles. For comparison, for gamma-CD (with normal solubility behavior), the diffusion mobility increases with increasing temperature and the hydration number decreases and converges to an asymptotic value at higher temperature.

Table of Contents Download the whole PhDthesis as a zip-tar file or as zip-File For download in PDF format click the chapter title Cover I Abstract in German V Abstract in English VI Acknowledgements VII Contents IX List of figures XIII List of tables XV Acronyms XVII 1. Introduction 1 1.1 Cyclodextrins 1 1.2 Methylated cyclodextrins 20 2. Materials and methods 33 2.1 Materials 33 2.2 X-ray crystallographic method 33 2.3 Neutron scattering method 42 3. Results and discussion 51 3.1 X-ray crystallographic study 51 3.2 Neutron scattering study 78 4. Conclusions 89 Bibliography 93 A. Crystallographic supporting information 107 B. Neutron scattering supporting information 121 Curriculum vitae 127

More Information:
Online available:	http://www.diss.fu-berlin.de/2000/133/indexe.html
Language of PhDThesis:	english
Keywords:	cyclodextrin, methylated cyclodextrin, X-ray analysis, neutron scattering, solubility, hydration
DNB-Sachgruppe:	30 Chemie
Date of disputation:	29-Nov-2000
PhDThesis from:	Fachbereich Biologie, Chemie, Pharmazie, Freie Universität Berlin
First Referee:	Prof. Dr. Wolfram Saenger
Second Referee:	Prof. Dr. Hans Hartl
Contact (Author):	aree@chemie.fu-berlin.de
Contact (Advisor):	saenger@chemie.fu-berlin.de
Date created:	29-Nov-2000
Date available:	30-Nov-2000

 

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