The electrochemical properties of Shewanella oneidensis cytochrome c nitrite reductase (ccNiR), a homodimer that contains 5 hemes per protomer, were investigated by UV/Visible and EPR spectropotentiometries. Global analysis of the UV/Vis spectropotentiometric results yielded highly reproducible values for the heme midpoint potentials. These midpoint potential values were then assigned to specific hemes in each protomer (as defined in previous X-Ray diffraction studies) by comparing the EPR and UV/Vis spectropotentiometric results, taking advantage of the high sensitivity of EPR spectra to the structural microenvironment of paramagnetic centers. Addition of the strong-field ligand cyanide led to a 70 mV positive shift of the active site’s midpoint potential, as the cyanide bound to the initially 5-coordinate high-spin heme and triggered a high-spin to low-spin transition. With cyanide present three of the remaining hemes gave rise to distinctive and readily assignable EPR spectral changes upon reduction, while a fourth was EPR silent. At high applied potentials interpretation of the EPR spectra in the absence of cyanide was complicated by a magnetic interaction that appears to involve three out of five hemes in each protomer. At lower applied potentials the spectra obtained in the presence and absence of cyanide were similar, which aided global assignment of the signals. The midpoint potential of the EPR-silent heme could be assigned by default, but the assignment was also confirmed from UV/Vis spectropotentiometric analysis of the H268M mutant of ccNiR, in which one of the EPR-silent heme’s histidine axial ligands was replaced with a methionine.

Polarographic studies of sulfur linkages other than the disulfide group have been reported. Schwabe (1) studied the polarographic reduction of p-rhodaniline. Bonner and Kahn (2) investigated the polarography of several carbohydrate analogs of S-alkylthiuronium salts. The present report deals with the polarographic reduction of trimethylsulfonium and cresyldimethylsulfonium salts.Early reports on the polarographic reduction of substituted ammonium and quaternary ammonium salts were made by Podrauzek (3) and Pech (4). The poorly defined waves found by Pech for quaternary ammonium salts suggested (5) the possibility that these reductions correspond merely to the decomposition voltage of water. Van Rysselberghe and McGee (6) showed that specific halfwave potentials can be found for different ammonium and quaternary ammonium ions. This later work (6) seems to indicate a "true" polarographic reduction for quaternary ammonium salts at the dropping mercury electrode.A previous report (7) from this laboratory dealt with the mechanism of the polarographic reduction of iodonium salts. The present investigation of sulfonium salts extends the polarographic study of "onium" compounds.Symmetrically substituted sulfonium salts are easily prepared by the usual bimolecular SN2 "onium" reaction. If the sulfonium salt is not symmetrical, it will usually decompose partially into other more stable sulfonium salts that have smaller molecular weights and are symmetrically substituted. Disproportionation and recombination finally yields an equilibrium mixture if sufficient time is allowed. Ray and Levine (8) discuss the equilibrium problem in preparing "mixed" sulfonium salts. Alkyl and aralkyl halides or methosulfates formed in disproportionation reactions are capable of exchange type recombinations, thereby yielding "mixed" water-soluble sulfonium salts. By-products that are water-insoluble are often formed if aryl groups are involved in the reaction.In the present case, sulfate analyses indicate that the water-soluble layers correspond to two-thirds 0-and m-cresyldimethylsulfonium methosulfates and onethird trimethylsulfonium methosulfate. No attempt has been made to separate these similarly soluble salts from their mixtures. Polarographic results on these naturally formed mixtures and on pure trimethylsulfonium salts confirm the conclusions indicated by sulfate analyses. Typical composite recorded sulfonium salt polarograms are shown in Figure 1. Pertinent polarographic results are given in Tables I and II. Pure trimethyl-

New information on the nature of the nuclear charge distribution in thermal-neutron fission of 235 U has been obtained from measurements of fission yields of the principal constituents in each of three adjacent mass chains. A regular variation in the width of the charge-distribution curves with mass number has been shown, and values of the most probable nuclear charge (Z p ) have been determined. No effects attributable to the 50-proton or 82-neutron closed shells have been observed in the charge-distribution curves examined. Independent and cumulative fission yield values were determined for the principal constituents in each of the three mass chains 131, 132, and 133. Values measured are: 131 Sn, (1.28=1=0.21)% cumulative; 131 Sb, (1.66d=0.40)% independent; 132 Sn, (0.59=b0.17)% cumulative; 132 Sb, (2.76=1=0.35) % independent; 132 Te, (0.86=1=0.10)% independent; 133 Sn, <0.015% cumulative; 133 Sb, (3.05=1=0.39)% independent; 133w Te, (2.19d=0.33)% independent; 133 *Te, (2.07=1=0.31)% independent; and 133 I, (0.17=±=0.06)% independent. The charge distribution curve for each mass chain was established from the fission yield data. The data were fitted to the Gaussian relationship, P(Z) -A exp-(Z-Z p ) 2 /c. The widths of the curves as given by values of c for mass chains 131, 132, and 133 are, respectively: 1.10=1=0.05, 0.74=b0.08, and 0.57=1=0.05. The data show a monotonic decrease in the width of the charge distribution curves with increasing fission-product mass number. Implications of this observation are discussed in respect to previous assumptions of a single charge distribution curve for all fission-product masses in low-energy fission. Values of Z v as given by the Gaussian curves for mass chains 131, 132, and 133 are, respectively: 50.72=b0.07, 51.07=b0.06, and 51.59 d=0.04. These values are compared with published values derived from assumptions of a single charge distribution curve for all mass chains. Predicted effects of nuclear shells on charge distribution have been examined in the light of the present data. No effect was detected in mass chains 131, 132, and 133. New half-life values determined are: 131 Sn, 1.32=b0.23 min; 132 Sn, 1.00=1=0.17 min; 132 Sb, 3.13=1=0.33 min; and 133 Sb, 2.67=L0.33 min. The half-life of 133 Sn was estimated to be 55 sec.

No abstract

No abstract