Steady state spectral scans

Figure 6. Steady-state spectral scans for dehydration of isopropanol at reaction conditions...

The steady-state spectral scans when recorded on the IBM/1000 may be processed. 

The final cell design, as in semi-infinite electrochemistry, depends very much on the goals of the experimenter. For example, when one is interested in single-electrode coulometry for n value, concentration, or spectral measurements, the cell requirements are minimal. Potential scan experiments (voltammetry, potential scan coulometry, steady-state voltammetry) or experiments... 

The chromophoric pyridoxal phosphate coenzyme provides a useful spectrophotometric probe of catalytic events and of conformational changes that occur at the pyridoxal phosphate site of the P subunit and of the aiPi complex. Tryptophan synthase belongs to a class of pyridoxal phosphate enzymes that catalyze /3-replacement and / -elimination reactions.3 The reactions proceed through a series of pyridoxal phosphate-substrate intermediates (Fig. 7.6) that have characteristic spectral properties. Steady-state and rapid kinetic studies of the P subunit and of the aiPi complex in solution have demonstrated the formation and disappearance of these intermediates.73-90 Fig. 7.7 illustrates the use of rapid-scanning stopped-flow UV-visible spectroscopy to investigate the effects of single amino acid substitutions in the a subunit on the rate of reactions of L-serine at the active site of the P subunit.89 Formation of enzyme-substrate intermediates has also been observed with the 012P2 complex in the crystalline state.91 ... 

Steady-state, or dummy, scans are used to allow a sample to come to equilibrium before data collection begins. As in a regular experiment, a number of scans are taken, but data are not collected during what would be the normal acquisition time. Steady-state scans are usually performed before the start of an experiment, but, for certain experiments on older instruments, may be acquired before the start of each incremented time value. This technique is not necessary in typical one-dimensional NMR experiments, but is employed in onedimensional methods that involve spectral subtraction (e.g., DEPT Section 7-2b) and virtually all two-dimensional experiments. 

Steady-state scans (Section 2-4i) are used before the start of essentially all 2D experiments. They are particularly important in a number of pulse sequences in order to compensate for spin-lock (Sections 7-7b and 7-10b) and decoupler (Section 7-8) heating effects. Larger numbers of steady-state scans are employed in experiments that have either particularly long spin-lock times or X-nucleus decoupling over especially wide spectral widths. 

Fig. 8. Rapid-scanning visible spectra showing the changes in the Co(II)E spectral bands during the pre-steady-state and steady-state phases of benzyl alcohol oxidation by NAD at pH 9 (A), pH 4.8 (B, D), and pH 5.6 (C) and 25°. Scanning was carried out as described in the caption to Fig. 5. The pre-steady-state reaction at pH 9.0 (A) occurs in two kinetic relaxations the spectra in A-1 show the changes in both relaxations as the steady state is approached the changes which occur in each relaxation are shown offset in A-2 (the fast relaxation) and A-3 (the slow relaxation), respectively. The time-resolved spectra in (B) and...

The steady state fluorescence measurements were performed at 665 nm. The emission from our sample was then scanned from 670 to 750 nm. This resulted in an emission spectrum with a maximum around 684 nm.If one takes the small Stokes shift into account, clearly this must be the emission from the PS 2 core chlorophylls. It is thus reasonable to assume that we have indeed achieved an effective energy transfer from the LHC-II to the PS 2 core, i.e. the system seems to be intact.The experiment was then continued by adding small amounts of Triton X-100. As a result of this titration we could see a spectral shift of the emission maximum towards shorter wavelengths. The total fluorescence yield seemed to increase dramatically compared to the yield measured before the addition of Triton X-100. Furthermore, the fluorescence anisotropy increased from a value of below 0.1 to... 

Figure 6 reveals absorption bands in selected regions of the spectrum for several concentration levels of isopropanol vapour. Each curve. A, B, or C, represents a spectral scan at steady-state reaction conditions with all reaction parameters except feed composition of isopropanol being kept constant. If different curves (A, B, and C) result, the adsorbed species associated with the spectra are considered to be germane to the reaction mechanism. In the event that the spectral bands do not change the adsorbed species are considered to be spurious. Subsequently, the reactor may be operated in a batch mode and the questionable band monitored continuously. The failure of this band to change with the extent of reaction would provide extra support to the view that the band is associated with a by product species not involved in the dehydration mechanism. 

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