ESIMS Experiments

The calculated mass for these tautomers is 393.1801 (monoisotopic mass) or 393.45 (average mass), which shows excellent agreement with the experimental mass of 393.1830 (monoisotopic mass from accurate mass measurement, sample 2) or 393.45 (average mass from LC/ESIMS experiment, sample 1) for the derivatized active site cofactor. The underivatized cofactor itself has a structure of the type shown as 3. This is consistent with the strong evidence for a quinone-like structure in LO (6, 8, 9, 23), and with the conclusion that the cofactor is comprised of a crosslink between a tyrosine derivative and a lysine residue. This structure could arise from an initial hydroxylation of Tyr to form dopa, followed by the oxidation of dopa to dopa quinone, and subsequent nucleophilic attack by the e-amino group of a lysine side chain to generate an aminoquinol (13). 

Extract a 5 mm long sample of A as well as a 1 mm samples of both A and B , and dissolve the extracts in 500,100, and 100 pL, respectively, of the appropriate solvent mixture. Using a 10 pL loop injector, sequentially infuse samples of the 1 mm extracts and obtain the ESI-MS. If the ESIMS are not essentially identical the proposition is refuted and the experiment is complete. If the ESI-MS spectra are essentially identical in m/z content and relative intensity, proceed to 2. 

Grimm and Beauchamp have shown that the gas-phase ions ultimately resulting from FIDI can be detected with a mass spectrometer and that the method might have some analytical potential. These experiments bear some resemblance to earher work by Hager et al., who also produced neutral droplets with a vibrating orifice and then exposed them to a nonhomogeneous electric field near a rod electrode and detected the ions with a mass spectrometer. These authors named the method droplet electrospray. As could be expected, due to the small number of droplets, the sensitivity of the droplet method was much lower than that obtained with ESIMS. 

In ESIMS the analytes experience both solution and gas-phase conditions, and therefore the mass spectrum of the analyte will reflect both gas- and solution-phase conditions. Many... 

One can repeat the experiment at several, gradually increasing concentrations of S and examine if the association constant, evaluated with Eq. (22b), remains constant. This procedure is called the Titration Method. ESIMS determinations of Ips/Ip x Is with this method have been often in agreement with the requirements of Eq. (22b) and have also provided Kas values in agreement with data in the literature obtained by conventional methods When the molecular mass of S is much smaller than those of P and PS, as is often the case, erroneous results may be obtained due to m and m/z factors including the transmission of the MS analyzer. Therefore, it is advantageous to use only the ratio of 7ps/7p, because P and PS have a similar mass when S is smaller than P, which is often the case. Zenobi and co-workers have provided an equation for the determination of Kas with the titration method in which 7s is eliminated. 

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