POLAROGRAPHIC STUDY OF THE REDUCTION OF SOME COMPOUNDS OF BIOLOGICAL INTEREST

Abstract

The importance of applying polarographlc technique in elucidating problems of basic and applied chemistry is well established. Although this technique started as a tool fbr the analysis of inorganic ions,its potentiality for solving problems of organic compounds••• was realised by Prof. Heyrovsky,its inventor, during the early stages of its development. Recent developments have shown that polarograp hy cm be advantageously employed in ascertaining the redox behaviour of organic compounds as well as In solving quite a few complicated structural problems. Moreover polaro 3rap hy has proved to be a very convenient analytical tool fbr the estimation of compounds of biological a,medlcinal4*a and agricultural Importance. lb quote a few cases, the current voltage curves have been recorded fbr the estimation of vitamins*"8 in fruits and vegetables,and in the determination of sex hormones (testesterone,r rogestrone etc.) In urine9 and blood1® and for testing the purity of medleines,particular!y antibiotics. Even its diagnostic utility in clinical medicine, e. g.,in diagnosis of cancer'% has been claimed. From the large array of organic compounds,which have been subjected to polarographlc analysis at the d.m. e. and other suitable electrodes, the nitrogen het&ocycles deserve 2 special mention. They fbrm the constituents of co-pounds of physiological,biological and medicinal Importance. These compounds are now being extensively studied from the physicochemical point of view in order to have a better appreciation of the biocheeicai and metsbolie | rocesses taking place In the living system, since polarography provides useful Information regarding the existence of oxidation-reduction couple In these compounds and can throw lighten their possible structure, basic studies on them using this technique are highly desirable. The choice of the method fbr dissolving the sample and of the supporting electrolyte cannot be generalized In organic lolarogrsphy. Moreover thepHof the solution also plays an lmrortant role. The buffer,sel acted fbr th«* study should also have sufficient capacity to take care of any add or alkali that may be present in the sample solution. The nature of ths buffer constituent Is equally important. In larje number of reduction cases where reduction is achieved at very negative potentials special surperting electrolytes like totram ethyl ammonium chlori do, tetri** ethyl ammonium bromide etc are used,which ext^d the rente of polarosrs? hie study. The reduction or oxidation of organic compounds at d.m. e» does not follow a simple mechsnisw. The limiting current and other wave characteristics are influenced by largo maber of factors and the limiting currnt may not be governed by diffusion alone,but eomeoth^r currents lime catdyttc^dsor. tion and klnetki 3 currents may also govern the electrode reaction. A survey of the liter©ture would reveal that amongst the heferocycles the pyrlraidines and the pyrazoles have not been sy® terrati call y investigated polaro graphically insplte of the fact that both these categories of compounds are of great physiological and medicinal importance. Pyrdmidines and their condensation derivatives are the main constituents of nucleic acids,e.g. ,R.N. a. and Q.N.A. Comprehensive studies on a variety of these compounds were,therefbre,considered worth undertaking and these fbrm the main theme of this thesis. Anumber of papers,**,• on the polarography of pyrlml dines have appeared from time to time since 1940. The relevant information available from these publications Is as follows! 1. Since most of these are water soluble,their polarographic reduction at d.m. e. can be conveniently carried out in the aqueous medium with and without the us© of buffers, almost all of them give diffusion controlled reversible w&ves over a wide pH range and the various tests fbr verifying thei reversible character, vi z., slope of log plots,validity of Ibme's method,independence of \/2 to concentration of the depolarizer,shape of the oscillographic curve etchold good for these waves. 2. These compounds generally undergo reduction with two electron transfer but In a few cases when sterochemical effects are operative,reduction takes place with 4 to 6 electrons,depending on the nature and position of the substituents. 4 3. Substituents bring about a shift in E^ /g which ranges from small values to appreciable values depending on the nature of these substituents. 4. The most obvious site of reduction In pyrlml dines is the carbon nitrogen double bond(C • N ) either st position 1,6 or at 3,4, Reduction at sites 1,2 or 2,3 Is not observed due to the presence of a stable cyclic ami dine structure In ryrimidines. In contrast to pyrlml dines,pyrazoles have attracted little attention from the polaro^rrtj: hie point of view. Systematic studies on them were taken up for the first time In this laboratory. Preliminary results hnve revealed that these comrounds undergo Irreversible reduction at d.m. ©., and the possible site of reduction is C * N. Pyrazoles and Its substituted products are generally water insoluble. Their polarogrsphic behaviour cm, therefore, be studied in non-aqueous solvents, viz., dimethyl formaffiide, acetonitrile,dioxsne etc. This limits their study to only one acidity scale and the effect of pH on the shape and ^2 of the polarogramp cannot be studied. Irreversible electrode trocess Fbr en irreversible electrode processi 0 ♦ ne% R The expression for the cathodic current is given by the excression. -i i xnFACQcK?exp |"-*nF(E-ff ) /RTj where «t Is the transfer coefficient, A is the area of the electrode, K% Is the values of the rate constant and CQ° is the concentration of the depolarizer. Other symbols have the! r usual si gni ft cmce. \/2 *s related to E^ by the expression. EJ 0.0642 . _ 1 d. e. T./2 ocn log y jt d In case where t ( the drop time) varies with thechange of applied potential, the following modified aquation holds goodt v n oaio* 0.05915 ,__ 1.349 Wfk Ed.e. * °'8412 * —=S log i/2 - Do 0.0542, Hog ^JL^. -0.646 log t] *n ^ d •"* The above equation makes it possible to evaluate * from the plots of E^^ vs [log [#~2 - 0.546 log tjor from the d" relationship, dflog t ) «n Since most organic compounds get reduced at the expense of H* ions of the medium, their reduction is shown by the equationi 0-°^ * pH From which p,the number of protons involved In the rate determining step can be evaluated. The behaviour of these Irreversible wevez can therefore, be summer!sed as follows! 1. The \/o value changes with concentration as well as with pH. 2. The &y„ value dso varies with drop time(t). 3. The value of K»ftf although not pertinent in multL step processes (eg.reduction of pyrazoles in the present etudes) ,1s useful in establishing the nature of the waves. If it exceeds 2 xlO'•cm/sec the reduction Is considered to be reversible whereas if it is of the order of 3 xlO"9 cm/sec it makes it partially irreverdble and values lower than the latter indcate completely irreversible nature. Chemistry of pyrlml dnes and t>yrazolcg pyrlmldnest- The compounds belonging to pyrlmldne group ere represented by (I). Since long they were considered as the breakdown products of uric add. Systematic studes on them were started by Pinner•',who first gave the name pyrlmldne to the unsubstituted parent body. Its derivatives play a key role In many biological systems. Thdr presence in nudelc 6 7 adds,vitamins,coenzymes and uric add and other purines is wdl recognized. Quite a few of the pyrlmldne derivatives are used as drugs,eg., barbituric add derivatives, sulfadazine etc. pyrimidine can be regarded as a cyclic amidne,and the chemical behaviour of Its derivatives is dominated by this fact. Its structured) is shown bdowi 4 3 N CH HC1 'fCH •6 1 * (D In this structure,positions 2,4 and 6 are similar regardng reactivity position 5 stands somewhat apart,bdng more aromatic in character. Pyrimidnes are synthesised by the technique known as ring synthesis.