In 1950 Zuman [1] showed the possibility, in principle, of determining amino acids by a polarographic method based on their reaction with carbonyl-containing compounds such as pyruvic acid and acetone. Later, Zhantalai and Tur'yan [2] developed a method for the quantitative determination of various (including indispensable) amino acids by poo larographic analysis of their azomethine derivatives formed in weakly acidic formaldehyde solutions. However, no data were reported on the polarographic behavior of formaldehyde derivatives of amino acids belonging to the indole seriesIn some cases, in particular, during the synthesis oftryptophan, tryptamine, and serotonin derivatives, an indirect polarographic analysis of the products can be preferred to other methods of determination. Moreover, investigation into electrochemical properties of the azomethine derivatives of these compounds may present an independent interest. In this connection, we have studied the electrochemical behavior of tryptophan, tryptamine, and serotonin at various pH in aqueous, aqueous-alcohol, and aqueous-dimethylformamide solutions with formaldehyde additives at a constant concentration of 0.6 M (a 100-fold excess relative to the substances analyzed).
EXPERIMENTAL PARTThe polarographic measurements were performed in a controlled-temperature potarographic cell equipped with mercury-dropping working electrodes and a saturated calomel reference electrode. In the differential-pulsed mode, the working electrodes had the parameters m = 0.75 mg/sec, z = 2.2 sec. The dc polarograms were measured using the working electrodes with pads, having the parameters m= 2.4 mg/sec, z = 2.18 sec. The measurements were made on L Chemical Drug Center--All-Russia Research Institute of Pharmaceutical Chemistry, Moscow, Russia. 2 Sechenov Medical Academy, Moscow, Russia.
557the Radelkis OH-105 (Hungary) and PU-1 (Russia) polarographs. All reagents were of the chemical-purity grade.
RESULTS AND DISCUSSIONUpon adding to a buffer formaldehyde solution (0.5 M, pH 6-11), all the compounds studied gave rise to the polarographic waves with a half-wave potential of about-1.0 V, reflecting reduction of the azomethine derivatives formed via the reaction R-NH 2 + CH20 -~ RNH=CH 2 where R-NH2 is tryptopban, tryptamine, or serotonin. However, in all cases the polarograms were poorly reproducible and showed no linear relationship between the limiting current and the depolarizer concentration. This behavior is apparently related to certain features of the adsorption of compounds of the indole series on mercury in aqueous solutions. Indeed, the presence of double conjugated bonds in the system and a nitrogen atom in the cycle may significantly increase the adsorption rate. This is indicated by a strong decrease in the charging current relative to the background charging curve, observed for the potentials more positive than -1.1 V in solutions with additives of the substances studied. This drop complicates calculation of the compound concentration.Reproducibility of the polarograms som...
