ELECTROCHEMICAL INVESTIGATIONS OF SOME PURINE NUCLEOSIDES AND NUCLEOTIDES

Abstract

Electrochemical investigations of biologically important compounds provide major challenges both from theoretical and mechanistic viewpoint. In recent years, it has been clearly demonstrated that the electron transfer processes that occur in biological systems are appreciably more complex than in vitro systems. The electrochemical studies of biologically significant molecules represent one of the most useful applications of electrochemistry as it provides deep insights into the invivo redox mechanism of these compounds. Literature survey revealed the applications of electrochemistry in various fields of human system including diagnosis and treatment of large number of diseases. Electrochemical biosensors find their applications in DNA damage study, environmental pollutant monitoring, pathogenic gene detection inherited disease diagnosis and pharmacological analysis. Electrochemotherapy is used in the treatment of liver malignancies and tumors in the brain. A combination of electrochemistry with other sophisticated analytical techniques like HPLC, HNMR and GC-MS has further been claimed as a powerful approach for elucidation of redox behavior of biomolecules. In view of the importance of electrochemical techniques, our laboratory has been investigating the redox behavior of various biologically important compounds such as guanosine, deoxyguanosine monophosphate etc. Purines are constituents of well-known nucleic acids viz., deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), which play an important role in physiology of human system. Recently, electrooxidation of some purine 11 nucleosides and nucleotides were carried out in our laboratory due to their biological and clinical significance. The work presented in this thesis is asmall part of the big proposal submitted by Prof. R.N. Goyal in connection with redox chemistry of biomolecules. The present thesis deals with the electrochemical oxidation studies of nucleosides viz., inosine, Z-deoxyinosine and nucleotides viz., guanosine-5'- triphosphate and adenosine-3',5'-cyclic monophosphate. For simplicity the results of the investigation are organized in the thesis as follows: The first chapter of the thesis is "General Introduction" and presents a brief review of the pertinent work and significant results of various compounds studied. The salient features of the various techniques used in the investigation have also been highlighted. Guanine nucleotides are high-energy sources. They play an important role in regulatory systems like protein synthesis, signal transduction and in intracellular protein transport. In view of the importance of guanosine-5'-triphosphate (GTP) in human physiology, the second chapter of the thesis has been devoted to electrochemical investigations carried out on GTP(I). 0 CH2-0-(?)-®-® HO OH (I) ill Cyclic voltammetry of GTP at pyrolytic graphite electrode (PGE) at a sweep rate of 100 mVs"1 exhibited single well - defined oxidation peak (la) in the entire pH range 1.54 - 10.88, when the sweep was initiated in the positive direction. In the reverse sweep no cathodic peak was obtained. On further reversal of sweep towards positive potentials a new well-defined oxidation peak (lla) was observed in the acidic pH range 1.54 - 6.93. The peak potentials of both la and lla anodic peaks were dependent on pH and shifted to less positive potential with an increase in pH. The UV spectral changes and kinetics of the decay of the UV absorbing intermediate generated during the reaction were also studied. The ultimate products of electrooxidation were characterized by GC-MS studies. Based on the experimental results, tentative mechanism for the oxidation of GTP has been suggested. The third chapter of the thesis presents electrochemical oxidation of inosine (II), a well-known metabolite of ATP degradation at PGE. Inosine and its derivatives have been claimed as anticonvulsant, antianxiety, anticancer and antiviral agents. Cyclic sweep voltammetry of inosine at a sweep rate of 100 mVs"1 exhibited one well-defined oxidation peak la in the entire pH range (3.37 - 10.90) studied, when the sweep was initiated in the positive direction. In the reverse sweep no cathodic peak was obtained. Further reversal of the sweep direction towards positive potential did not exhibit any new peak. The peak potential of anodic peak la was dependent on pH and shifted to less positive potential with increase in pH. EC nature of the reaction was established in which electrode reaction is coupled with an irreversible follow up chemical step. Spectral changes during electrolysis were monitored to detect formation of an UV-absorbing intermediate. The decay of the IV UV-absorbing intermediate occurred in a pseudo-first-order reaction. The products of electrooxidation were separated by HPLC and characterized by m.p., TLC, FT-IR, A HNMR and GC-MS studies. Aplausible mechanism has been suggested for the formation of the products. .....................