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N A Stombaugh

Publications and source records attributed to N A Stombaugh.

4 recordsLinked to original sources
PubMed1976

Oxidation-reduction properties of several low potential iron-sulfur proteins and of methylviologen.

Apparent oxidation-reduction potentials at pH 7.0 and 25 degrees C were determined using the H2-hydrogenase system with ferredoxins from the following sources: Clostridium pasteurianum, -403 mV; C tartarovorum, -424 mV; C. acidi-urici, -434 mV; Peptococcus aerogenes, -427 mV; Chromatium D, -482 mV (pH 8.0); B. polymyxa, Fd I, -377 mV, and Fd II, -422 mV; and spinach, -428 mV. The pH dependence of these values was variable, ranging from -2 to -24 mV/pH unit increase for different ferredoxins. Over the range of buffer concentrations between 0.05 and 0.2 M, the potentials did not vary significantly. The number of electrons transferred during reduction (as determined by integrations of EPR spectra and by dithionite titration) is 2 for the first five proteins, while potentiometric data for all the cases fit a Nernst equation for which n = 1. The E degrees' value for the redox indicator methylviologen at pH 7.4 was found to be -460 mV, according to both the H2-hydrogenase system and cyclic voltammetry, significantly different from the value previously reported at higher pH's. Additionally, the presence of C. pasteuranum ferredoxin appears to shift the E degrees value of methylviologen to even more negative values. An analysis of sources of error inherent with potential determinations with H2 and hydrogenase is presented. The electronic and EPR spectra of P. aerogenes ferredoxin, for which the x-ray structure has been published, are given here. It appears that the determination of potentials of ferredoxin and other low-potential porteins with the H2-hydrogenase system affords certain experimental advantages over alternative methods currently employed with these and similar substances.

Binding Sites↗
PubMed1975

Determination of the iron-sulfur distances in rubredoxin by x-ray absorption spectroscopy.

The high intensity x-ray flux from the synchrotron radiation at the Stanford Synchroton Radiation Project has been used to study the extended x-ray absorption fine structure (EXAFS) of the iron-sulfur protein Peptococcus aerogenes rubredoxin. Absorption measurements were made from 7080 eV, which is below the K-edge of iron, to about 650 eV above the edge and structure was obtained over the entire region. By means of a model iron-sulfur compound for evaluating the phase shifts, the variation of the absorption above the edge of lyophilized, oxidized rubredoxin was converted to iron-sulfur distances. The data were fitted with a least squares program to a model in which three distances R3 were kept equal and the fourth R1 was allowed to differ. The mean square error was constant over a region of this parameter space, becoming twice as large at R3 = 2.217, R1 = 2.389 and R3 = 2.268, R1 = 2.108 A. These values, which are the extreme differences allowed by the present data, are definitely closer to being equal than those found by the determination of the x-ray diffraction crystal structure of the similar protein from Clostridium pasteurianum. However, the average distance from our experiment is in excellent agreement with the average distance from the crystal structure determination. Preliminary EXAFS measurements were also made on the oxidized rubredoxin in solution at pH 7.0. The spectra were unchanged, indicating that the average iron-sulfur distance change is less than 0.02 A. Upon reduction the average iron-sulfur bond length increased by about 0.05 A. Since the EXAFS measurements can give accurate determinations of distances in proteins both in crystals and solution, the technique should be widely applicable.

Binding Sites↗
PubMed1974

Proton magnetic resonance and magnetic susceptibility characterization of ferredoxin I from Bacillus polymyxa.

Magnetic susceptibility and proton magnetic resonance spectra are reported for the oxidized and reduced forms of the iron-sulfur protein Bacillus polymyxa ferredoxin I. The magnetic susceptibility of the oxidized form indicates antiferromagnetic exchange coupling between component iron atoms that is quantitatively similar to that observed for the clostridial ferredoxins and for the [(C(2)H(5))(4)N](2) [Fe(4)S(4)(SCH(2)C(6)H(5))(4)] analog. Contact-shifted resonances observed in the proton-magnetic-resonance spectra of oxidized and reduced forms of the B. polymyxa protein can be correlated with the contact-shifted resonances of corresponding redox forms of the clostridial ferredoxins. Characteristics of the contact-shifted resonances observed in partially reduced B. polymyxa ferredoxin I are compatible with a "slow" rate of electron exchange between redox forms, which suggests that the "fast" electron exchange earlier observed in the eight-iron clostridial ferredoxins may derive from an intramolecular component.

Bacillus↗