MERCURY(II) THIOLATE AND SELENOLATE INTERACTIONS, AND CHELATION THERAPY BY ALAN PETER ARNOLD B.Sc. Melb.), B.Sc. (Hons.), ARACI A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy Chemistry Department University of Tasmania Hobart Tasmania Australia 1982 This thesis contains no material which has been accepted for the award of any other degree or diploma in any University, and to the best cf my knowledge, contains no copy or paraphrase of material previously presented by another person, except where due reference is made in the text. Alan P. Arnold. FOR MY PARENTS ACKNOWLEDGEMENTS It is a pleasure to respectfully acknowledge the continuous patience and guidance given by Dr. A.J. Canty throughout this study. I am grateful to Dr. G.B. Deacon and Mr. M. Hughes (Mbnash University) for their assistance with the measurement of far infrared spectra, to Dr. A.H. White and Dr. B.W. Skelton (University of Western Australia) for the X-ray crystallographic studies, and to Dr. R.N. Sylva (Australian Atomic Energy Commission) for supplying a listing of his new version of the program MINIQUAD. My sincere thanks are due to Mr. J.C. Bignall of the Central Science Laboratory (University of Tasmania) for his expert tuition and assistance with the intricacies of Laser-Raman spectroscopy of intractable samples, his colleagues Mr. N.W. Davies and Mr. M. Power for the measurement of mass spectra and to Mr. R.R. Thomas for the 1 H nmr spectra. Mr. R. Ford of the Geology Department (University of Tasmania) is gratefully acknowledged for his assistance with the determination of the X-ray powder diffraction patterns of several malodorous mercury(II) selenolates. I wholeheartedly thank Mrs. B. Dix and Mrs. M. Stafford for their immaculate typing and Mrs. H. Hen for several of the diagrams in this thesis. Any virtues in presentation are due to these ladies, all deficiencies to me. To the academic and technical staff of this Department, I express my appreciation for their always cheerful assistance, and humbly apologise to them for the indiscretions of an amateur organoselenium chemist. Last, but by no means least, I thank my wife, Julie, for her understanding and encouragement during this work. Her patient assistance with many tedious potentiometric titrations and transcription of my writing into readable text has been invaluable. Financial support from the National Health and Medical Research Council and the Commonwealth of Australia for a Postgraduate Research Award is gratefully acknowledged. ABSTRACT This thesis is an account of a study of some aspects of the biological chemistry of mercury. The interactions of mercury compounds with both simple and naturally occurring thiols, some selenols, and several antidotes for mercury poisoning have been investigated. The dithiol antidotes 2,3-dimercaptosuccinic acid (DISH 4 ) and the sodium salt of 2,3-dimercaptopropane-l-sulfonate (Unithiol, Na[UH 2 ]) form isolable complexes of stoichiometry (MeHg) 2 ENSH 2 and Na[(MeHg) 2U]. The mercury(II) complexes Hg(DMSH 2 ).2H 20 and Na[HgU] have polymeric structures, (-Hg-S'S-), similar to that reported for the Hg(II) complex of the classic heavy metal antidote, British Anti- Lewisite (BALH 2 ). The stability constants for the interaction of MeHg(II) with several monothiols in aqueous solution have been determined potentio- metrically. High stability of complexes with a-mercaptocarboxylic acids and a-mercaptoamines (log 1`15-17) necessitated the use of a 3 110 ( titration method involving iodide competition. 2,3-Dimercaptosuccinic acid forms a MeHg(II) complex with log = similar to that expected for interaction of MeHg(II) with a monothiol; but BALH 2 and Unithiol form complexes with log fi lo 2-3 orders of magnitude higher, suggesting the presence of chelation. Implications of the results obtained from both synthetic and solution studies for the use of dithiols as antidotes for MeHg(II) poisoning are discussed. A computer program for the potentiometric evaluation of ligand hydrolysis constants has been written. The algorithm uses a rigorous least-squares procedure and can be applied to mixtures of multiprotic acids or bases. Any titration parameter can be refined. Complexes of 400 with selenols have been prepared and the first structural studies of Hg(II) selenolates obtained. Comparison of vibrational spectra and X-ray powder diffraction patterns for these complexes and their thiol analogs allows assignment of structures for some complexes and suggests that, for Hg(SeR) 2 , polymeric structures may be more common than for Hg(SR) 2 . Thus., Hg(SeR) 2 (R.Me, Et, Bu t) are polymeric and Hg(SeCH 2CO2H) 2 is linear,'but for a large number Of analogous thiolate complexes, only Hg(SBU ) is known to be polymeric. 2 Single crystal X-ray diffraction studies show a polymeric structure for Hg(SeMe) 2 based on .distorted tetrahedral geometry for mercury with bridging selenolate groups . The structure of [Bu tSeHgCl(py) m]4 :is similar to those previously reported for the sulfur analogs with pyridine.and 47methylpyridine, and is isomorphous with the latter. The structure is based on an eight- t Membered ring of alternating Hg and Se atoms (-Hg-SeBu 7 )4 having a centre of symmetry and two mercury environments, 'Hg(u 7SeBu t) 2 (u-C1) 2.1 and 'Hg(u-SeBu ) 2Cl(py)', with a dichloro bridge linking the former mercury atoms. The complex [EtSeHgC1(py)] 4 has a similar structure but a dichloro bridge is absent and all mercury atoms have the environment 'Hg(U.-SeEt) 2C1(py) . The first valid comparison between Hg-S and Hg-Se bond lengths for analogous thiolate and selenolate complexes of Hg(II) indicates that the Hg-Se bond lengths are slightly shorter than expected from comparisons of sulfur and selenium covalent radii. Possible synthetic routes to selenium analogs of antidotal dithiois are discussed. Although selenium analogs of BALH 2 and 1 .,3-dimercapto-2- propanol (DMPH ) could not be isolated the new compounds selenetan-3-ol, 1a-diselenan-4-ol and 1-bromo-3-selenocyanato-2-propanol were obtained from their attempted syntheses. The'Hg(II) derivatives of the selenium analogs, Hg(SeBAL) and Hg(SeDMP), were isolated as intractable polymers. These polymers have tetrahedral geometry for Hg(II) in contrast to linear geometry for their thiol analogs, consistent with structural differences between simple complexes Hg(XR) (X=S,Se). The compounds 27 (benzylseleno)- fumaric.acid and 2-(benzylseleno)succinic acid have been prepared as inter- mediates towards a projected synthesis of SeDMSH4. INDEX Pate CHAPTER ONE : CHEMICAL ASPECTS OF MERCURY TOXICITY 1.1 Introduction 1 1.2 Metabolism and toxicity of mercury(II) compounds 3 elemental mercury, Hg° 3 mercurous mercury, Hg 22+ 4 mercuric mercury, Hg 2+ 4 alkylmercury(II) compounds 5 alkoxyalkyl and arylmercury compounds 7 1.3 Antidotes for mercury toxicity 8 1.3.1 complexing agents 8 2,3-dimercapto-l-propanol (British Anti- Lewisite, BALH2) 10 (ii) D-penicillamine (PenH 2 ) 12 (iii)N-acetyl-DL-penicillamine (NAPH 2 ) 13 (iv) 2,3-dimercapto-1-propanesulfonate, sodium salt [Unithiol, Na(UTH2)] 14 (v) meso-2,3-dimercaptosuccinic acid (DMSH ) 15 (vi) other thiols 16 (vii)synergistic and mixed complexing agents 17 1.3.2 Extracorporeal hemodialysis and hemoperfusion 17 1.3.3 Enterohepatic complexing agents 18 1.4 Mercury-selenium interactions 19 1.4.1 Selenium interactions with inorganic mercury 23 1.4.2 Selenium interactions with organic mercury compounds 27 CHAPTER TWO : STRUCTURAL CHEMISTRY OF MERCURY(II) THIOLATES 2.1 Introduction 30 2.2 Structural features of MeHg(II) thiolates 32 2.3 Structural features of complexes of the type, Hg(SR) 2 43 2.4 Structural features of Hg(II) dithiolate complexes 53 2.5 Structural features of RSHgX species 55 2.6 Conclusions 59 CHAPTER THREE SOLUTION CHEMISTRY OF METHYLMERCURY(II) THIOLATES AND SELENOLATES 3.1 Introduction 61 3.2 The aqueous solution chemistry of.methylmercury(II) 3.2.1 Coordination of MeHg(II) in aqueous solution 62 3,2.2 NMR investigations and MeHg(II) interactions in solution 75 3.2.3 NMR evaluation of MeHg(II)-thiolate formation constants 77 3:3 Methylmercury(II) complexation.with thiolate ligands 80 3:3:1 MethylmercurAII) monothiolate-equilibria 80 3.3.2- Potentiometric determination of formation constants of MeHg(II) thiolates 87 analysis of HI- content.. 93. analysis of MeHg content 94 (i) 2-mercaptoethanol 97 (ii)mercaptoacetic acid 103 (iii)mercaptoacetic acid, 0-methylester 105 (iv)mercaptosuccinic acid 105 (v) L-cysteine 108 (vi) DL-homocysteine 116 (vii)DL-penicillamine 117 (viii)N-acetyl-DL-penicillamine 120 (ix) glutathione 123 (x) thiocholine perchloate 126 (xi) 4-mercapto-N-methylpiperidine 129 3.3.3 Methylmercury(II) formation constants with vicinal dithiols 136 (i) 2,3-dimercapto-1-propanol, BALH 2 136 (ii) 2,3-dimercapto-l-propanesulfonate(sodium salt), Unithiol 144 (iii)meso-2,3-dimercaptosuccinic acid, DMSH 4 149 3.4 Interactions of antidotal thiols with MeHg(II) in vivo 155 3.4.1 Physiological pH 156 3.4.2 Competition of antidotal thiols with endogenous ligands in vivo 157 3.5 Methylmercury(II) interactions with selenium donors in aqueous solution 162 3.5.1 MeHg(II)-selenolate equilibria 162 3.5.2 MeHg(II)-diselenide interactions 169 3.6 Conclusions 171 CHAPTER FOUR : SYNTHESIS, VIBRATIONAL SPECTROSCOPY AND STRUCTURE OF MERCURY(II) SELENOLATES 4.1 Preparation of Mercury(II) selenolates 173 4.1.1 Preparation of Hg(SeR) 2 complexes 173 4.1.2 Preparation of RSeHgX complexes 176 4.2 X-ray diffraction characterisation of mercury(II)-