Spectroscopic Buffers

The mechanism of decarboxylation of acids containing an amino substituent is further complicated by the possibility of protonation of the substituent and the fact that the species NH2ArCOOH is kinetically equivalent to the zwitterion NHj ArCOO. Both of these species, as well as the anion NH2 ArCOO" and even NH3 ArCOOH must be considered. Willi and Stocker644 investigated by the spectroscopic method the kinetics of the acid-catalysed decarboxylation of 4-aminosalicyclic acid in dilute hydrochloric acid, (ionic strength 0.1, addition of potassium chloride) and also in acetate buffers at 20 °C. The ionisation constants K0 = [HA][H+][H2A+] 1 (for protonation of nitrogen) and Kx = [A"][H+] [HA]-1, were determined at /i = 0.1 and 20 °C. The kinetics followed equation (262)... 

Many pitfalls await the unwary. Here is a short list, compiled from more detailed considerations by Bunnett.8 One should properly identify the reactants. In particular, does each retain its integrity in the reaction medium A spectroscopic measurement may answer this. The identities of the products cannot be assumed, and both a qualitative identification and a quantitative assay are in order. Pure materials are a must—reagents, salts, buffers, and solvent must be of top quality. Careful purification is always worth one s time, since much more is lost if all the work needs repeating. The avoidance of trace impurities is not always easy. If data are irreproducible, this possibility must be considered. Reactions run in the absence of oxygen (air) may be in order, even if the reactants and products are air-stable. Doing a duplicate experiment, using a spent reaction solution from the first run as the reaction medium, may tell whether the products have an effect or if some trace impurity that altered the rate has been expended. 

Table 4. Chemical structure and spectroscopical and photophysical properties of riboflavin (RF) in aqueous phosphate buffer pH 7.4 (Valle et al, 2011).

Multiple hyphenation ( hypemation ) provides comprehensive spectroscopic information from a single separation. The first doubly hyphenated HPLC-NMR-MS appeared in 1995 [661], and its value is now accepted meanwhile fully integrated on-line LC-NMR-MS and MSn systems (QMS, QTTMS) are commercially available. On-line LC-NMR-MS coupling is by no means trivial. For example, the sensitivity of NMR is limited, while MS is incompatible with non-volatile buffers. The... 

Many recombination and spectroscopic studies have been carried out in decaying rare-gas plasmas. Invariably, the buffer gas was the parent gas of the ion under study. In addition to recording the emitted spectra, Frommhold and Biondi60 examined the line shapes of many afterglow lines in neon and in argon. The intent was to find spectroscopic evidence for Doppler broadening of the lines which would prove that... 

In the PDB 1 YEW structure, the zinc ion site in pmoC is not identified in pMMO by other spectroscopic techniques such as ICP-AES. The reference 190 authors believe that this zinc ion is either adventitious (perhaps depositing from the crystallization buffer) or in a location usually occupied by another metal ion, possibly another copper ion or an iron ion in vivo. 

Pitts, J. N., Jr., J. M. McAfee, W. D. Long, and A. M. Winer. Long-path infrared spectroscopic investigation at ambient concentrations of the 2% neutral buffered potassium iodide method for determination of ozone. Environ. Sci. Tedinol. 10 787-793, 1976. 

We have recently described another spectroscopic rnethod for observing IM reactions at atmospheric pressure that utilizes the photodetachment-modulated electron capture detector (PDM-ECD) as a means of monitoring the negative ions either consumed or produced in an IM reaction. The reaction of interest is made to occiu in a steady-state flow-through reactor in which ionization of the buffer gas is continuously caused by a Ni-on-Pt foil beta emitter. A chopped light beam of... 

The peptide [H-(Gly-Pro-Hyp)5-Gly-Pro-Gln-Gly-Ile-Ala-Gly-Gln-Arg-Gly-Val-Val-Gly-Pro-Hyp-Gly-Pro-Cys-Gly-OH] 30 was dissolved at 4 mM concentration in degassed and argon-sat. 50 mM NH4OAc buffer (pH 5.5). The resulting soln was added dropwise to a 10 mM soln of peptide 29 in 50 mM NHtOAc buffer (pH 5.5) with exclusion of air oxygen. The reaction was monitored spectroscopically at 430 nm. After 2-3 h the mixture was concentrated and the heterodimer 31 isolated by gel chromatography or HPLC yield 68% the product was characterized by HPLC and MALDI-TOF-MS. 

Spectroscopic techniques require calibration with standards of known analyte concentration. Atomic spectrometry is sufficiently specific for a simple solution of a salt of the analyte in dilute acid to be used, although it is a wise precaution to buffer the standards with any salt which occurs in large concentration in the sample solution, e.g. 500 pg ml-i or above. Calibration curves can be obtained by plotting absorbance (for AAS), emission signal (for AES), fluorescence signal (for AFS) or ion count rate (for MS) as the dependent variable against concentration as the independent variable. Often the calibration curve will bend towards the concentration axis at higher concentrations, as shown in Fig. 

In order to qualitatively and quantitatively evaluate the various deactivation channels of the electronically excited bilatriene chromophore in Pr, the photophysical and photochemical parameters of this chromophore were determined by stationary fluorescence [76] and time-resolved spectroscopic methods (for in-house developments and first applications of fluorescence and optoacoustics see [77-80] and [81,82], respectively) [2,9,10,83,84], Furthermore, a time-resolved fluorescence study has been carried out also with the selectively excited tryptophane residues of Pt and Pfr. The results are discussed in Sections II.A-C and III.A. Unless specified otherwise, the experiments were performed in vitro with 124-kDa phytochrome isolated from etiolated oat seedlings (Avena sativa L ), and the measurements were carried out with ethylene glycol-containing phosphate buffer solutions at 3s 275 K. 

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