STUDIES ON ELECTROCHEMICAL BEHAVIOUR OF SOME ORGANIC MOLECULES OF BIOLOGICAL IMPORTANCE

Abstract

Investigations of the redox chemistry of compounds of biologically significant and naturally occuring biomolecules by electroanalytical techniques provide major challenges to the electrochemists. Presently electroanlytical techniques are usefully employed to elucidate significant information on biophysico-chemical and the biocolloidal behaviour of compounds of physiological importance. Early approaches, based upon resemblance between the electrode surface and the cell membrane to electrochemical oxidation-reduction have had their limitations in elucidating a forthright relationship regarding redox pathways. Nevertheless, modern researches provide reasonably significant body of evidences that redox molecules, using purely electroanalytical techniques, do give unique insights into biological redox processes. Hopefully, the results obtained from such studies may have profound and far reaching effects in further resolving the intercacies of invivo redox mechanisms of biological systems. The importance of sulfa drugs, adenines and heterocyclic amino compounds has been recognised in numerous biological and pharmaceutical fields and the aim of the present work is directed to investigate the electrochemical redox behaviour of some organic compounds containing these groups or moities. Typical compounds selected for the study are sulfathiazole, sulfapyridine, adenine, 2-hydroxyadenine, 8-hydroxyadenine, 2,8-dihydroxyadenine and 2-amino-5-methyl-l,3,4-thiadiazole. CO The first chapter of the thesis is the General Introduction which highlights "the importance of electrochemical studies in biological systems along with an overc.view of experimental approach to understand the principle of the basic techniques used in the present studies. A brief review and significant results relevant to the present study on the electrochemical investigations of the different class of compounds selected for the present studies is also presented in this chapter. The second chapter of the thesis is devoted to the oxidation chemistry of two sulfonamides, sulfathiazole and sulfapyridine. Sulfathiazole exhibited two well defined oxidation peaks (Ia and II ) in linear sweep cyclic voltammetry at a sweep rate of 10 cl mVs-1 over the entire pH range of 1.2-10.2. The Ep versus pH relation for peak Ia exhibited one break at around pH 6.7, whereas, peak IIa exhibited two breaks at around pH 2.9 and 7.4 and corresponded to . pKa of the species. The ip versus concentration plot for peak Ia was found to be almost linear at concentration < 2.0 mM indicating thereby the involvement of adsorption at the electrode surface during the process. This behaviour was further confirmed by the increase in the peak current function ip/AcV1/2 with increase in sweep rate. The coulommetric studies suggested the oxidation of sulfathiazole as an overall 2e, 2H+ process. On the basis of linear and cyclic sweep voltammetry, product separation and identification by TLC, melting point, IR, NMR and mass spectra., a mechanism for the electrochemical oxidation was proposed to account for the formation of p-hydroxylaminobenzene-N- (2-thiazolyl)sulfonamide and azobenzene-4,4'-(N-2-thiazolyl) (ii) disulfonamide as products. In contrast to the sulfathiazole, the electrochemical oxidation of sulfapyridine exhibited 2e, 2H+ oxidation to give only one product, viz, azobenzene-4,4'-(N-2-pyridinyl) disulfonamide The mechanism of chemical oxidation of these drugs has also been compared with that of periodate and persulfate oxidation. The results for chemical oxidation of sulfathiazole and sulfapyridine with periodate and persulfate indicated that the oxidation product(s) formed are identical to electrochemical oxidation. Nevertheless, the mechanism for the oxidation may be different, the electrochemical oxidation of sulfa drugs undergoes essentially by a radical process while chemical oxidation depends upon the nature of oxidant used. It is through radical process in case of persulfate and through ionic process in case of periodate. The chemical oxidation of sulfathiazole and sulfapyridine was carried out by sodium periodate (NaI04) and potassium persulfate (K2S208). In both the sulfa drugs, the reaction followed an overall second order kinetics for periodate and persulfate oxidation. For both the sulfa drugs the rate of reaction increase with increase in pH upto a certain pH and then becomes independent of pH, indicating thereby the involvement of ionic form of reacting species. These reactions also obeyed Arrhenius equation and a plot of logK versus 1/T was found to be linear. On the basis of thermal studies energy of activation (E ) and entropy of reaction (£»S) was also calculated. The main oxidation products were separated and charecterized by TLC, (iii) lting point, IR, NMR and mass spectra and found to be identical with that obtained in the electrochemical oxidation of both the sulfa drugs. The third chapter is devoted to the electrochemical oxidation of adenine and its hydroxy derivatives to establish the redox mechanism of this naturally occuring purine. Linear sweep voltammetry, coulommetry and spectral studies indicated that adenine undergoes in a single 6e, 6H+ step oxidation to give several products depending upon the pH of the solution. The first 2e, 2H+ oxidation was found to give 2-hydroxyadenine which is easily oxidisable than 8-hydroxyadenine and hence converted into 2,8-dihydroxyadenine which on further oxidation converts to diimine and leads to the formation of alloxan, parabdjiic acid and urea as major products. To separate and confirm the products HPLC has also been used in addition to column chromatography. HPLC pjovided strong evidence for the formation of urea which was never observed in column chromatography due to elution with phosphate because of its low molecular weight. The probable redox mechanisms for the formation of the products are described in detail. The fourth and the final chapter of the thesis describes the rsults obtained during the oxidation of an important antibacterial drug 2-amino-5-methyl-l,3,4-thi^iazole. During the electrochemical oxidation 2-amino-5-methyl-l,3,4-thi^iazole undegoes a 2e, 2H+ oxidation at PGE to give an electroactive product 2,2'-azo-bis-5-methyl-l,3,4-thiadiazole by following a radical-radical coupling mechanism. This product was separated by column chromatography, identified by using melting point, IR, NMR (iv) me and mass spectra and a mechanism for the formation of azo product has been suggested. The mechanism of electrooxidation has also been compared with the chemical oxidation of 2-amino-5-methyl-l,3,4-thiadiazole by periodate and persulfate. The kinetics were carried out in the same way as in case of sulfonamides and the order of reaction was found to be twoT However, it was found that reaction was faster in case of persulfate oxidation than in case of periodate. In both the cases the reaction obeyed Arrhenius equation and the energy of activation Ea for persulfate was 42.0 KJ;while in case of periodate was 60.0 KJ^ The pH effect in both the cases suggested that the reacting species was unprotonated 2-amino-5-methyl-l,3,4-thiadiazole and persulfate dianion (SoOo") and periodate monoanion (I04"). However a radical * 2 8 mechanism is operated for persulfate and ionic for periodate oxidation to lead the formation of the same product as obtained during electrooxidation in both the cases.