Characterization techniques proton transfer

The identification of different carbonate binding modes in copper(II) and in zinc(II)/2,2 -bipyridine or tris(2-aminoethyl)amine/(bi)carbonate systems, specifically the characterization by X-ray diffraction techniques of both r)1 and r 2 isomers of [Cu(phen)2(HC03)]+ in their respective perchlorate salts, supports theories of the mechanism of action of carbonic anhydrase which invoke intramolecular proton transfer and thus participation by r)1 and by r 2 bicarbonate (55,318). 

Another characterization procedure of the excited clusters can be obtained by ionization by a second photon and detected by mass spectrometry (processes VI and VII). Tuning this second photon, the first one being fixed on the - S0 transition of a given cluster, allows one to determine the ionization threshold of this cluster. The ionization potentials of AH and A- being significantly different, the ionization process A - HB + - A HB + + e will occur at lower energies than the AH B - AH+ - - B + e process. The two-photon ionization techniques can provide a mass selective way of detecting proton transfer in clusters. 

Vibrational spectroscopy measures atomic oscillations practically on the scale as the scale of proton dynamics, 10-15 to 10 12 s. Fillaux et al. [110] note that optical spectroscopies, infrared and Raman, have disadvantages for the study of proton transfer that preclude a complete characterization of the potential. (However, the infrared and Raman techniques are useful to observe temperature effects inelastic neutron spectra are best observed at low temperature.) As mentioned in Ref. 110, the main difficulties arise from the nonspecific sensitivity for proton vibrations and the lack of a rigorous theoretical framework for the interpretation of the observed intensities. 

FAB and LSIMS are matrix-mediated desorption techniques that use energetic particle bombardment to simultaneously ionize samples like carotenoids and transfer them to the gas phase for mass spectrometric analysis. Molecular ions and/or protonated molecules are usually abundant and fragmentation is minimal. Tandem mass spectrometry with collision-induced dissociation (CID) may be used to produce abundant structurally significant fragment ions from molecular ion precursors (formed using FAB or any suitable ionization technique) for additional characterization and identification of chlorophylls and their derivatives. Continuous-flow FAB/LSIMS may be interfaced to an HPLC system for high-throughput flow-injection analysis or on-line LC/MS. 

The reaction mechanism of cytochrome bc complex is known as the proton motive Qcycle originally proposed by Peter Mitchell (Mitchell, 1976). This mechanism is the basis of his chemiosmotic theory for which he was awarded the Nobel prize in 1978. Since then, the enzyme has been characterized extensively using various techniques. Redox centers have been characterized spectroscopically (for review, see Trumpower and Gennis, 1994), electron transfer pathways have been determined using kinetic experiments with specific inhibitors (De Vries 1986 Zhu et al., 1984), and the positions of quinone binding sites and redox centers have been determined using biochemical and mutational analysis (for review, see Esposti et al, 1993 Brasseur et al, 1996). As a result of these efforts, the latest modified Qcycle has been widely accepted by researchers in the field (for reviews, see Crofts et al, 1983 Trumpower, 1990 Berry et al, 2000). 

An important application of combined electrochemistry and ESR spectroscopy is the characterization and identification of intermediates and products of electrode reactions [334,336,379-391]. For instance, the ESR technique is particularly useful to measure the degree of protonation under conditions where the radical ions take part in acid-base equilibria [380,381]. Such information may be obtained only with difficulty by other methods, but the coupling pattern of the ESR spectrum may often give the answer directly. An illustrative example is found in the anodic oxidation of 2,4,6-tri-rert-butylaniline, which, as expected, gives the radical cation as the initial electrode product [380]. In an aprotic solvent like MeCN or CH3NO2 the radical cation is stable and the ESR spectrum observed is in accordance with the reversible one-electron transfer indicated by CV. However, when the electrolysis is carried out in the presence of diphenylguanidine as a base, the ESR spectrum changes drastically and can be attributed to the presence of the neutral free radical formed by deprotonation of the radical cation. 

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