The reaction of nitric oxide with oxy-myoglobin (oxyMb) to form ferric myoglobin (metMb) and nitrate, and the metMb-catalyzed isomerization of peroxynitrite to nitrate, have long been assumed to proceed via the same iron-bound peroxynitrite intermediate (metMb(OONO)). More recent research showed that the metMb-catalyzed isomerization of peroxynitrite to nitrate produces detectable amounts of nitrogen dioxide and ferryl myoglobin (ferrylMb). This suggests a mechanism in which the peroxynitrite binds to the metMb, ferrylMb is transiently generated by dissociation of NO2, and nitrate is formed when the NO2 nitrogen attacks the ferrylMb oxo ligand. The presence of free NO2 and ferrylMb products reveals that small amounts of NO2 escape from myoglobin's interior before recombination can occur. Free NO2 and ferrylMb should also be generated in the reaction of oxyMb with NO, if the common intermediate metMb(OONO) is formed. However, this report presents a series of time-resolved UV/vis spectroscopy experiments in which no ferrylMb was detected when oxyMb and NO reacted. The sensitivity of the methodology is such that as little as 10% of the ferrylMb predicted from the experiments with metMb and peroxynitrite should have been detectable. These results lead to the conclusion that the oxyMb + NO and metMb + ONOO(-) reactions do not proceed via a common intermediate as previously thought. The conclusion has significant implications for researchers that propose a possible role of oxyMb in intracellular NO regulation, because it means that toxic NO2 and ferrylMb are not generated during NO oxidation by this species.
We present for the first time analytic solutions for the nonlinear dynamics of a Wien bridge oscillator stabilised by three common methods: an incandescent lamp, signal diodes, and the field effect transistor. The results can be used to optimise oscillator design, and agree well with measurements. The effect of operational amplifier marginal nonlinearity on oscillator performance at high frequencies is clarified. The oscillator circuits and their analysis can be used to demonstrate nonlinear dynamics in the undergraduate laboratory.
Simple apparatus is developed, providing undergraduate students with a solid understanding of Fourier transform (FT) infrared (IR) spectroscopy in a hands on experiment. Apart from its application to measuring the mid-IR spectra of organic molecules, the experiment introduces several techniques with wide applicability in physics, including interferometry, the FT, digital data analysis, and control theory.
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