Methods and Mechanisms for Producing Ions From Large Molecules

Abstract

9691 efficiency with mass up to at least m / z 12 360. For molecular weight determination by matrix-assisted laser desorption, the results give some hints for future instrumental design of linear or reflectron time-of-flight instruments, regarding vacuum conditions , instrumental geometry and others. Furthermore, assuming that ion decay predominantly is characterized by fragment ion formation rather than by charge stripping, the obtained reaction cross sections can be taken as being valid for the efficiency of daughter ion formation as well. If this assumption can be confirmed in future investigations, then re-flectron-time-of-flight mass spectrometry of peptides or proteins employing postsource decay6v2' will turn out to be a valuable alternative to collision-induced or photoinduced decomposition for structure analysis. CID experiments might then have to be reevaluated with respect to the question whether the observed loss of fragment yield above m / z = 1000 is really due to physical properties of the collision process or predominantly due to instrumental transmission effects. Acknowledgment. Support by the Bennigsen-Foerder-Program of the Ministry of Science and Research, NRW, Germany, is gratefully acknowledged. References and Notes (1) Karas, M.; Hillenkamp, F. Anal. Chem. 1988, 60, 2288. (2) Karas, M.; Bachmann, D.; Bahr, U.; Hillenkamp, F. Inr. J. Mass (3) Egge, H.; Katalinic, J. P.; Karas, M.; Stahl, B. Pure Appl. Chem. 1991, (4) Schar, M.; Bornsen, K. 0.; Gassmann, E. Rapid Commun. Mass (5) Spengler, B.; Kaufmann, R.; Kirsch, D. Rapid Commun. Mass Spec-(6) Spengler, B.; Kirsch, D.; Kaufmann, R.; Jaeger, E. Rapid Commun. The coordination properties of square-planar complexes of Cu(I1) with N4 or N202 ligation using imidazole, pyridine, glycine, ethylenediamine, diethylenetriamineimidazole, and ammonia as ligands were investigated by electron spin resonance (ESR) and electron nuclear double resonance (ENDOR) spectroscopies at 77 K and about 5 K, respectively. Combining isotopic substitution in metal, ligand, and solvent with ESR measurements at X-(9.5 GHz) and Q-band (34 GHz) and second-order spectra simulation, a comprehensive set of ESR parameters was obtained. In ENDOR, emphasis was given to extracting the nitrogen ligand couplings for all compounds and the proton interactions for imidazole and pyridine. The assignments were substantiated by powder-type ENDOR simulations. The data are discussed in terms of nitrogen anisotropy parameters as a function of ligand binding modes. In relation to the 'type 2" copper protein superoxide dismutase (SOD), the tetraimidazole compound was found to have model character for the proton but not the nitrogen couplings, whereas tetrapyridine could not model either of the interactions.