Donald G. Davis

An electrochemical study of the oxidation of L-cysteine

Abstract The oxidation of cysteine has been studied at platinum electrodes by chronopotentiometry, polarography, and controlled-potential electrolysis. Although the major product was found to be cystine, the electrode reaction(s) is kinetically complex and is greatly suppressed by the formation of an oxide film or surface oxygen.

ELECTROCHEMICAL STUDIES OF METALLOPORPHYRINS

Several metalloporphyrins are well known participants in various types of biologically important electron exchange reactions. These include iron porphyrins which are components of cytochromes, catalase and peroxidase; a cobalt porphyrinlike derivative which is an essential component of vitamin BIZ; and chlorophyll, a dihydroporphyrin, containing magnesium. In order to better understand these biological electron transfer processes, studies which elucidate the mechanisms and factors governing the rate of metalloporphyrin redox reactions are important. h hemoglobin the rate at which the iron (11) porphyrin moiety is oxidized is extremely slow in contrast to the rapid oxidation of simple ferroheme. The difference in redox potentials is not nearly enough to account for the difference in rate. It has been suggested that the slowness of oxidation is due to such factors as the hydrophobic media which surround the iron porphyrin in or to the large change in spin quantum number associated with the oxidation of hemoglobin by ~ x y g e n . ~ On the other hand, the rate of oxidation of cytochrome c is very rapid.4 The application of electrochemical techniques to the study of the rates of electron transfer involving metalloporphyrins holds promise for elucidating the factors which govern the rate of electron transfer involving metalloporphyrins in biological systems. During the past few years a number of studies have appeared which applied electrochemical techniques to the investigation of metallop~rphyrins.~-~ It was found that in nonaqueous solvents it is possible to oxidize the ring of a typical metalloporphyrin in two discrete oneelectron steps yielding srcation radicals and dications and to also reduce the ring by two electrons yielding sr-anion radicals and d i a n i ~ n s . ~ ~ Ordinarily, if the central metal ion can change oxidation state (i.e., Fe(II)+ Fe(III)), this occurs between the first ring oxidation and the first ring reduction. The question of whether the electron transfer involves the porphyrin ring or the metal can be settled by ESR spectra or magnetic meas~remen t s . ~~*~ Also, the rates of axial ligand exchangelo and the rates of electron transfer in certain cases have been determined.12*14 Recently, using the technique of cyclic voltammetry, we have measured the rates and examined the mechanisms of electron transfer of a large number of metalloporphyrins in nonaqueous solvents such as butyronitrile and dimethylsulfoxide. s Examination of thermodynamic half-wave potentials (Et) indicates that the difference in potential between the first ring oxidation and first ring reduction is constant and has a value of 2.25 f 0.1 5 volts. Furthermore, the Ets of the ring oxidation and the ring reduction can be correlated with the net ring charge as calculated by Gouterman.16s17 In addition a constant difference between the first ring oxidation and the second (0.30 f 0.05 volts), as well as the first ring reduction and the second (0.42 f 0.05 volts), was consistent with such reports in the literature for similar

A New Highly Sensitive Preconcentrating Sampling Technique for Flameless Atomatic Absorption Spectroscopy

Abstract A new technique for sampling metal ions in solution for flameless atomic absorption has been developed. A tungsten alloy wire loop is soaked in the sample solution for a specified period of time. Metal ions are concentrated on the surface of the wire loop apparently by an ion exchange mechanism. The wire loop is then used as an electrically heated atomizer for atomic absorption spectroscopy. Detection limits of 4 × 10−14 grams for cadmium and 2 × 10−11 grams for lead have been achieved.

The determination of lead and nickel by atomic-absorption spectrometry with a flameless wire loop atomizer

Abstract In an extension of studies of flameless atomizers for atomic-absorption spectrometry, an electrically heated tungsten-rhenium alloy wire loop was examined. Reduction of metallic salts to ground-state metal atoms was accomplished with the high temperature produced by the loop. Lead and nickel were investigated. Experimental parameters such as wavelength, slit width, atomization temperature and sheathing gas flow rate were optimized. Absolute detection limits of 6.6·10 −10 and 1.2·10 −10 , and absolute sensitivities of 7·10 −10 and 8·10 −11 g of lead, were established for unenclosed and enclosed cells, respectively. The interferences of twenty cations and sixteen anions were studied; foreign cations generally enhanced the lead absorption by retarding its vaporization, allowing the slow detection system to respond more efficiently. Nickel was investigated as a representative less volatile metal; an absolute detection limit of 1.6·10 −9 and an absolute sensitivity of 9·10 −10 g of nickel were established.

Triangle-Wave Generator for Cyclic Voltammetry†

SUMMARY In inexpensive triangle-wave generator is presented and discussed which incorporates the capabilities most needed for cyclic voltammetry: anodic and cathodic scan rates continuously and independently programable with an accuracy of 2% over the range 10-4 10-4 V/sec; a scan amplitude control which is independent of scan rate and is resettable to within 0.1%; less than 0.4% deviation from linearity and symmetry (in symmetric mode); drift in initial potential of less than 0.1 mV/day; negligible overshoot and ringing up to 103 V/sec; and single-scan or continuous operation modes with provision for synchronization with external equipment. The generator requires two operational amplifiers and utilizes a silicon-controlled rectifier as the switching element. The triangle-wave output is compatible with any electronic potentiostat.

Nickel(IV) dimethylglyoxime

Abstract The red solutions of nickel dimethylglyoxmie in strongly basic solution have been studied by electrochemical, spectrophotometric and magnetic methods. It is concluded that the red complex is a species of Ni(IV). A controlled potential coulometric method for the analysis of nickel is also presented.

Chronopotentiometry of uranium(iv)

Abstract The chronopotentiometry of uranium at platinum electrodes has been investigated. A determination of uranium has been devised based on the conversion of the uranium to uranium(IV) and the oxidation of the uranium (IV). The effect of several possible interfering substances has been checked.

Complexes of Manganese Hematoporphyrin IX With Picoline and other Nitrogenous Bases

Abstract The results of polarography and cyclic voltammetry studies of hematoporphyrin IX with eight different added ligands (other than H2O and OH−) in aqueous solvent are reported. Two nitrogenous ligand molecules were found to coordinate with both manganese (III) and manganese(II) hematoporphyrin IX. Oxygenated ligands such as acetate and pyrophosphate showed no tendency to coordinate with either manganese (III) or manganese (II) porphyrin. Porphyrin adsorption on the mercury electrode is quite evident. The adsorption mechanism appears to be complicated.

Electrochemistry of cobalt-porphyrin complexes in aqueous solution

Abstract The polarography and cyclic voltammetry of cobalt(III) hematoporphyrin was investigated in buffered aqueous solutions. In the absence of nitrogenous bases, which can act as axial ligands, the cobalt porphyrin is not electroactive. Polarographic waves are found in solutions containing pyridine and similar compounds. Both cobalt(III) and cobalt(II) hematoporphyrin can add two pyridine molecules, although only one ligand may react at lower concentrations. Adsorption problems were encountered.

EPR studies of amino and other quinones

Abstract The semiquinone anion radicals, amino quinones and quinone thioethers formed from benzoquinones and various amines and mercaptans have been studied by EPR. The influence of thioether substitution is close to that of alkoxy substitution on quinones on the spin distribution in semiquinone radical anions. Participation of lone pair electrons is slight. Aminoquinones are greatly stabilized by electron release from the nitrogen. Anomalous behavior of aminoquinones and quinone thioethers having methyl or phenyl substituents ortho to the heteroatom substituents are interpreted as evidence of steric hindrance. The labile proton of aminoquinones and aminosemiquinones is identified as an amine proton. Strong intramolecular hydrogen bonding of the amine proton causes a large down field NMR chemical shift, slow deuterium exchange and an anomalously small hyperfine splitting constant.