Apparatus and Procedures. The ultramicroelectrodes were prepared bv sealing gold or Dlatinum wires (Goodfellow Metal Ltd.) according to -0.5 -1.0 23 24 Figure 9. Cyclic voltammetry of 0.2 mM 1,4-naphthoquinone in acetonitrile +0.1 M NEt4BF, at 20 OC at a platinum 2 mm diameter electrode in the absence of acid (a) and in the presence of a 8.8 mM phenol/l.2 mM phenate buffer (b). (c) Potential pH diagram of the 1,4-naphthoquinone/ 1.4-dihydroxynaphthalene couple.spite of a plateau value being 3-4 orders of magnitude below the diffusion limit. The pK, was then determined as the p H corresponding to the intersection of these two linear portions of the plot. In fact, the low p H rising linear section of the plot has a slope (0.39) much smaller than 1, clearly indicating that the forward and backward reactions are under activation control. The intersection between the two linear portions of the plot thus does not correspond to the pK, of the AH'+/A' couple, but is located, as are all the experimental data points, in a substantially exergonic region of the deprotonation reaction. Experimental SectionReagents. All pyridines used were commercially available. 10-Methylacridan and IO-methylacridinium iodide were prepared according to a previously published procedure.22 The solution were prepared with acetonitrile (Merck Uvasol) and deoxygenated with argon before use. Tetraethylammonium tetrafluoroborate (Fluka, puriss) was used as supporting electrolyte. (22) Roberts, R. M. G.; Ostovic, D.; Kreevoy, M. M. Faraday Discuss. Chem. SOC. 1982, 74, 251.a' previo;siy desched procedure:sa The reference electrode was-an aqueous SCE and the counterelectrode a platinum grid. The cell was placed in a Faraday cage. The potentiostat with a three-electron configuration was the same as previously described?z Chronoamperograms and voltammograms were recorded with a Nicolet 4094C/4180 (8 bits, 5-11s sampling time) digital oscilloscope.. The function generator was an Enertec 4431. In double potential step experiments, the current-time curves were repeated 10 times and accumulated before transfer into an IBM PC-AT microcomputer for treatment?b The whole set of measurements was repeated again 10 times. Under these conditions, the dispersion in the measurement of R was found to be less than 0.05. A 17-pm gold electrode was used for times 0 between 24 ps and 1 ms, and a 5-pm gold electrode for time 0 between 3 to 50 p~?~ In linear sweep voltammetry experiments, each curve was repeated 10 times and accumulated in the digital oscilloscope. The values of ip (peak in presence of bases) and i, (peak current is absence of bases) were measured directly at the same electrode with the same solution, by subtracting the base lineJ4 The dispersion error on the value of the ratio ip/im was found to be less than 0.15. Experimental absorption spectra were obtained with a Varian Superscan 3 UV-visible spectrophotometer. The reactions were carried out at 20 OC under nitrogen atmosphere and the spectra were recorded from aliquots diluted 20 times. E O Q , HZ was d...

The bacitracin antibiotic complex consists principally of bacitracin A, a peptide antibiotic containing seven amino acid residues in a ring and five amino acid residues in a blocked side chain, together with a mixture of minor components presumably related but of unknown structures. A preparative high-performance liquid chromatographic method was developed for isolating the minor components A2, B1 and B2 which were then characterized by amino acid analysis, exact mass fast atom bombardment (FAB) mass spectrometry, FAB tandem mass spectrometry (MS/MS) and electrospray ionization (ESI) mass spectrometry. For bacitracins A (MW 1421), A2 (MW 1421), B1a (MW 1407), B1b (MW 1407), B2 (MW 1407) and F (MW 1419), the side chain sequences were determined by ESI MS/MS and ESI nozzle-skimmer collision-induced dissociation (CID) mass spectrometry and the ring sequences elucidated by ESI nozzle-skimmer CID MS/MS. Relative to bacitracin A, bacitracin A2a has the modified isoleucine residue at position 1 replaced by a modified allo-isoleucine residue, bacitracin B1a has the isoleucine residue at position 8 replaced by a valine residue, bacitracin B1b has the isoleucine residue at position 5 replaced by a valine residue and bacitracin B2 has the modified isoleucine residue at position 1 replaced by a modified valine residue. FAB tandem mass spectra were shown to be consistent with the above structural assignments for the isolated bacitracin components. Structures were also proposed for the trace bacitracin components C1 (MW 1393) and D1 (MW 1379) using ESI MS/MS data obtained from the analysis of the bacitracin complex without isolation.

The effects of assigning inaccurate reference lifetimes in lifetime determinations are predicted theoretically by using standard equations. This theory leads to a method to remove reference error effects using common least-squares software. This method cannot, however, be used to deconvolute data collected with isochronal references. Uncorrected data can always be exactly solved with models containing one more degree of freedom than the true model. Monoexponential decays are fit by double-exponential decays or excited-state processes. Unimodal distributed decays often appear as discrete, double-exponential decays.

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