ELUCIDATION OF THE STRUCTURE AND RECOGNITION PROPERTIES OF ANTIMONY(III)-TARTRATE USING ELECTROSPRAY IONIZATION MASS SPECTROMETRY by ARUNA B. WIJERATNE Presented to the Faculty of the Graduate School of The University of Texas at Arlington in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY THE UNIVERSITY OF TEXAS AT ARLINGTON August 2010 Copyright © by Aruna B. Wijeratne 2010 All Rights Reserved ACKNOWLEDGEMENTS It is with my greatest pleasure I convey my sincere appreciation to all who made my doctoral degree a success. First and foremost I express my deepest gratitude to my co-supervisors; Dr. Kevin A. Schug and Dr. Daniel W. Armstrong, professors of the Department of Chemistry and Biochemistry, at the UT-Arlington for the enthusiasm, encouragement, guidance and supervision they have granted me to carry out effective and efficient research. Throughout my graduate-study period they provided sound advice, ideas and corrected various drafts, spending their valuable time on me. They made my research experience memorable and I would have been lost without them. I am indebted to Dr. Jose Gracia and Dr. Peter Kroll, post-doctoral associate and professor, respectively, of the Department of Chemistry and Biochemistry, at the UT-Arlington, for their invaluable support in carrying out molecular calculations and advise on structural elucidations. I am also indebted to Dr. Richard B. Timmons, from whom I acquired plasma- Chemistry knowledge related to my research and Dr. Carl J. Lovely, from whom I took continuous consultation in NMR-spectroscopy. I am thankful for my dissertation committee members Dr. Carl J. Lovely and Dr. Richard X. Guan for their support towards my dissertation work. Special thanks are also conveyed to Dr. Brian Edwards and Mr. Charles Savage, technical staff members of the Department of chemistry and Biochemistry, at the University of Texas at Arlington, for their utmost technical support provided throughout my research. I gratefully thank our present and past group members from both groups, Pritesh S. Sharma, Samuel H. Yang, Bilal H. Bazzi, Sandra E. Spencer and others for their support in carrying out research, friendship and anonymous help throughout my stay at UT-Arlington. iii Last but not least, I am deeply thankful for my beloved wife, Bhagya K. Wijayawardena, my mother S. Wijeratne, my parents in law, Mr. and Mrs. Wijayawardena, my uncle W. M. J. Kumarasinghe and my very best friend D. N. Wijesundera, Physics-PhD graduate student at the University of Houston, TX, for their warm support, love and company throughout my graduate- study period, which made me accomplish my Ph.D. July 19, 2010 iv ABSTRACT ELUCIDATION OF THE STRUCTURE AND RECOGNITION PROPERTIES OF ANTIMONY(III)-TARTRATE USING ELECTROSPRAY IONIZATION MASS SPECTROMETRY Aruna B. Wijeratne, PhD The University of Texas at Arlington, 2010 Supervising Professors: Kevin A. Schug and Daniel W. Armstrong Chirality of molecules is a major topic in the design, discovery and development of new drugs. Currently, analytical chemistry research divisions are challenged to find new methods to screen enantiospecific molecular recognition properties of potential chiral drug candidates with targeted biological molecules in a time-sensitive environment. Recently, mass spectrometry (MS) has received significant attention for its use in studying molecular recognition systems. Electrospray ionization mass spectrometry (ESI-MS) has received the most attention for the investigation of stereochemical association phenomena, due to its capacity to transfer solution phase chiral interactions into the gas phase for analysis in single-stage mass spectra. ESI-MS based chiral molecular recognition studies on cinchona alkaloid-carbamate model systems had been initiated by our group in order to test the viability of mass spectrometric techniques to trace solution phase enantiomeric interactions in their respective mass spectra. Importantly, these studies have returned results in good correlation with those found previously by in liquid phase separation techniques. It was realized that mass spectrometry is an invaluable tool to pre-screen such recognition systems. In order to further corroborate this argument, our group v had been in search of novel, less well understood chiral molecular recognition systems to be analyzed using ESI-MS, where other frequently used analytical techniques have failed to clarify their underlying mechanistic details. Antimony(III) tartrate also known as “tartar emetic”, the bis-potassium salt of dianionic antimony(III)-L-tartarate, has a long history which traces back to medieval times and is filled with intrigue and peril. It has also been used as a therapeutic indication for diseases like typhoid, bronchitis, pneumonia and schistosomiasis. To date, mechanistic details of its actions, especially its biomolecular recognition properties, are not very well understood. While trying to indicate ESI-MS as an invaluable tool to study molecular recognition phenomena, we also hypothesized that ESI-MS based noncovalent binding studies of antimony(III)-D/L-tartrate complexes may provide useful information about its molecular recognition capacity. Since tartar emetic has also shown antibacterial properties, ESI-MS studies were carried out to visualize tartar emetic’s selective binding towards biologically relevant amino acid enantiomers. Consequently, a previously unprecedented proton-assisted enantioselective character of “tartar emetic” towards neutral D-amino acid enantiomers was revealed. A serendipitous observation resulted from these studies was its ability to capture unusual solvent reaction products generated during negative ionization mode ESI, which in fact provided a new understanding for the overall electrospray ionization mechanism. Additionally, theoretical studies, accompanied by multinuclear magnetic resonance experiments revealed that tartar emetic can assume a previously unknown structural isomer which is proven to co-exist with its crystallographically- determined structure. These findings and many ongoing research efforts suggest that this phenomenon may be responsible for many of tartar emetic’s inexplicable observations. This dissertation discusses our ongoing efforts to indicate ESI-MS for studying interesting molecular recognition phenomena of antimony(III)-tartrate binding to amino acids as well as providing useful molecular information of an inexpensive organo-metallic complex which could potentially be structurally altered to be used in many applications. vi TABLE OF CONTENTS ACKNOWLEDGEMENTS ................................................................................................................iii ABSTRACT ...................................................................................................................................... v LIST OF ILLUSTRATIONS............................................................................................................... x LIST OF TABLES ........................................................................................................................... xv Chapter Page 1. INTRODUCTION……………………………………..………..….. ..................................... 1 1.1 Chirality ............................................................................................................ 1 1.1.1 Significance of Chirality in Drug Development or Therapeutics ................................................................................... 1 1.1.2 Measurement of Enantioselectivity .................................................. 3 1.1.3 Mass Spectrometry (MS) for Screening Enantioselectivity .............. 5 1.2 Electrospray Ionization Mass Spectrometry (ESI-MS) ..................................... 6 1.2.1 Electrospray Ionization Mechanism ................................................. 7 1.2.2 Electrospray Ionization Mass Spectrometry (ESI-MS) for Enantioselective Molecular Recognition ......................................... 8 1.2.3 Cinchona Alakaloid-Carbamate: Model Chiral Molecular Recognition System Studied using ESI-MS ................................... 9 1.3 Metal-tartrates or Binuclear Tartrato(4-) Metal Bridged Complexes .............. 11 1.4 Antimony(III)-tartrates .................................................................................... 12 1.5 Dissertation Overview .................................................................................... 13 2. MOLECULAR RECOGNITION PROPERTIES OF TARTRATES AND METAL-TARTRATES IN SOLUTION MEDIA AND GAS PHASE .............................. 17 2.1 Introduction..................................................................................................... 17 2.2 Solution Phase Molecular Recognition Properties of Tartrates ..................... 21 vii 2.3 Gas Phase Molecular Recognition Properties of Tartrates ............................ 24 2.4 Solution Phase Molecular Recognition Properties of Metal-Tartrates ........... 31 2.5 Gas Phase Molecular Recognition Properties of Metal-Tartrates .................. 37 2.6 Summary and Concluding Remarks .............................................................. 39 3. ANTIMONY(III)-D,L-TARTRATES EXHIBIT PROTON-ASSISTED ENANTIOSELECTIVE BINDING IN SOLUTION AND IN THE GAS PHASE ............. 42 3.1 Introduction..................................................................................................... 42 3.2 Proton-assisted enantioselectivity of antimony(III)-D,L-tartrate in solution phase targeted single-stage ESI-MS .............................................