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Do Charge-Remote Fragmentations Occur under Matrix-Assisted Laser Desorption Ionization Post-Source Decompositions and Matrix-Assisted Laser Desorption Ionization Collisionally Activated Decompositions?

M. Rosario M. Domingues, M. Grac¸a O. S.-Marques, Carla A. M. Vale, M. Grac¸a Neves, J. A. S. Cavaleiro, and A. J. Ferrer-Correia Department of Chemistry, University of Aveiro, Aveiro, Portugal

Olga V. Nemirovskiy and Michael L. Gross Department of Chemistry, Washington University, St Louis, Missouri, USA

The precursor ions of tetraphenylporphyrins that are substituted with fatty acids can be introduced into the gas phase by matrix-assisted laser desorption ionization (MALDI) and undergo post-source and collisionally activated decompositions (CAD) in a time-of-flight . The goal of the research is to obtain a better understanding of post-source decompositions (PSD); specifically, we asked the question of whether ions undergoing PSD have sufficient energy to give charge-remote fragmentations along an alkyl chain. We chose the porphyrin macrocycle because we expected it to act as an inert “support,” allowing the molecule to be desorbed by MALDI and to be amenable to charge-remote fragmentation. MALDI-PSD and MALDI-CAD spectra are similar to high-energy CAD spectra and consid- erably more informative than low-energy CAD spectra, showing that charge-remote fragmen- tations of the fatty acid moieties do occur upon MALDI-PSD and MALDI-CAD. (J Am Soc Mass Spectrom 1999, 10, 217–223) © 1999 American Society for

ost-source decompositions (PSD) of ions pro- ever, observed different fragmentation pathways in duced by matrix-assisted laser desorption (MALDI) MALDI-PSD spectra when compared with those ob- Phas become a valuable structural tool for peptides served in LE-CAD and HE-CAD spectra. In contrast, [1, 2], carotenoids [3], oligosaccharides [4], and other Gooden et al. [11] found similar MALDI-PSD and glycoconjugates [5]. Spengler and Kaufmann [6, 7] were HE-CAD spectra for various modified DNA bases. the first to introduce the term “post-source decay” (PSD) Porphyrins that are substituted with fatty-acid to cover the possibilities of metastable decompositions groups appear to be an appropriate system for investi- that occur after MALDI. In one view, PSD are unimo- gating whether charge-remote processes occur as PSD. lecular processes driven by the internal energy that ions Principally side-chain fragmentations are expected be- take up by interaction with the desorbing laser light [7]. cause of the high stability of the porphyrin macrocycle According to another view, collisional events during [12–14], and indeed charge-remote processes are ob- ion acceleration are responsible for the activation [8]. served when porphyrins with substituent aliphatic Comparisons of MALDI-PSD with the better under- chains are submitted to HE CA in tandem sector mass stood, low-energy (LE) and high-energy (HE) collision- [15]. If this expectation is realized under ally activated decompositions (CAD) of ions produced PSD conditions, the fragmentation observed along the by (FAB) and electrospray ion- fatty-acid acyl chains of suitably derivatized tetraphe- ization (ESI) are an appropriate approach to shed light on nylporphyrins will resemble the characteristic charge- the mechanism of PSD and their energetics. Several au- remote fragmentations of long-chain fatty acids [12]. thors have taken this tack. Rouse et al. [2] and Ospina et al. Although certain charge-remote fragmentation occur [9] found that the spectra from MALDI-PSD are similar to upon low-energy collisional activation (CA), or even as those from LE CAD of ions produced by FAB of peptides metastable-ion processes, those of molecules containing and antibiotics. Kaufmann and co-workers [10], how- long alkyl chains occur only upon HE CA [16, 17]. Furthermore, porphyrins are amenable to MALDI,

Address reprint requests to M. Grac¸a O. S.-Marques, Department of whereas simple fatty acids are not. Chemistry, University of Aveiro, Portugal. E-mail: [email protected] The ability to characterize structures of new fatty-

© 1999 American Society for Mass Spectrometry. Published by Elsevier Science Inc. Received May 19, 1998 1044-0305/99/$20.00 Revised October 19, 1998 PII S1044-0305(98)00144-5 Accepted October 19, 1998 218 DOMINGUES ET AL. J Am Soc Mass Spectrom 1999, 10, 217–223 acid derivatives of porphyrins is of value in other fields. nol/acetic acid (50/49.5/0.5%) was the infusion solvent Porphyrins with long alkyl chains have found applica- for porphyrin electrospray experiments [14, 24]. tion as liquid crystals and in Langmuir–Blodgett films CA of the ions was via multiple, low-energy colli- [18–21]. Liquid crystals represent one of the best known sions. Energetic collisions were promoted by subjecting classes of self-organizing materials, exhibiting suitable the precursor ion to excitation via a tickle voltage properties for application in electronics, chemistry, and applied to the end caps. The applied energy for LE CAD medicine [22]. Cregg et al. [18, 19] reported that por- was approximately 25 eV in the laboratory frame. phyrins (with and without metal ions) that are substi- The MALDI-TOF experiments were carried out with tuted with eight long alkyl chains show interesting a PerSeptive Biosystems, Voyager RP-DE (Cambridge, optical properties of the ordered porphyrin chro- MA). A nitrogen laser (337-nm, 3-ns pulse width) was mophore. The synthesis of tetraphenylporphyrin deriv- used to desorb and ionize the samples. The instrument atives with long alkyl chains is now feasible, as shown was operated in the reflectron mode with an accelerat- by Kugimiya and Takemura [20] and by Bonnett et al. ing potential of 25 kV and with delayed extraction: [21]. A mass spectrometric method for characterizing delay time of 100 ns, grid voltage of 18.75 kV, and these materials should facilitate developments in these guide-wire voltage of 25 V. PSD and CAD experiments fields. were carried out under the same conditions. CA of MALDI-produced ions was achieved by collisions with ϫ Ϫ7 Experimental Ar gas at the pressure of 9 10 torr (measured in the ion-source housing); this housing pressure is 1.7 times HE-CAD spectra were obtained by using a VG ZAB-T greater than the residual-gas pressure of the PSD exper- (Micromass, Manchester, UK) four-sector instrument, iments. Timed precursor ion selection (mass width of which consisted of two high-mass, double focusing Ϯ 7 u) and a low-mass gate were applied. mass spectrometers of BEBE design [23]. To obtain Samples were desorbed from a matrix of ␣-cyano-4- product-ion mass spectra, the precursor ions were se- hydroxycinnamic acid (Aldrich), which was recrystal- lected by the first mass spectrometer (MS-1) and acti- lized from methanol and stored in the dark as a vated with collisions with helium gas, which was saturated solution in acetonitrile/water/trifluoroacetic introduced to a pressure sufficient to give 50% main- acid 50/50/0.1%. Other matrices were 2,5-dihydroxy- beam suppression. The fragment ions formed in the benzoic acid and dithranol (Aldrich). The samples were third field-free region were detected by the single-point dissolved in chloroform and mixed with the matrix detector. The acceleration potential and the collision cell solution on the sample plate. were 8 and 4 kV, respectively, for FAB-produced ions. The oleic-acid derivative, 1 (Scheme 1), was pre- Data acquisition was carried out with a VG Opus 3.1 data system, which was interfaced to the mass spec- trometer by a VG SIOS I unit, and the results of approximately 20 scans were signal averaged to pro- duce a product-ion spectrum. For FAB, samples were dissolved in chloroform, and a 1-␮L aliquot of a 1 nmol/␮L was loaded on the probe tip with 1 ␮Lofa ϩ 3-nitrobenzyl alcohol matrix. The Cs gun was oper- ated at 25 kV. LE-CAD experiments were carried out with a Finni- gan LCQ (San Jose, Ca) ion-trap mass spectrometer that was equipped with the commercial Finnigan capillary ESI source. All samples, at a concentration of 10 pmol/ ␮L, were introduced into the electrospray source at a flow rate of 8 ␮L/min. The spray needle was held at 4.2 kV, and 70-psi coaxial flow of nitrogen was used to stabilize the spray. The heated (200 °C) stainless steel capillary, held at 13.5 V, served as a counterelectrode. In all experiments, helium was introduced into the to a pressure of 1 mtorr (measured on a remote ionization gauge) to improve the trapping efficiency of the ions and to serve as the collision gas during the CAD event. Typical experimental parameters were ion-injection times of 100–300 ms, ion-isolation width of 1 u, ion detection using the mass-selective instability mode with resonance ejection at a scan rate of 5000 u/s, and ion detection by a 15-kV conversion dynode with a chan- neltron . Dichloromethane/metha- Scheme 1 J Am Soc Mass Spectrom 1999, 10, 217–223 CHARGE-REMOTE FRAGMENTATIONS BY MALDI-PSD 219

Figure 1. Higher order (MS5) product-ion spectra of the ESI-produced [M ϩ H]ϩ ions of 2. The spectra were obtained with an ion-trap mass spectrometer. paredby esterification of the meso-(p-hydroxyphenyl) to eliminate a ketene. Charge-remote fragmentations triphenylporphyrin with oleic acid, and the two tetra- leading to cleavages along an alkyl chain are not substituted derivatives were obtained by esterification detectable. For example, the LE CAD spectrum of of meso-tetrakis (p-hydroxyphenyl) porphyrin with compound 1 shows one fragment ion, that resulting palmitic acid, to give compound 2, and with arachidic acid from the cleavage of the ester bond. Similarly, com- to give compound 3, according to a literature procedure pound 2, which is tetrasubstituted, also undergoes one [25]. Thin-layer chromatography showed no detectable major cleavage of the ester bond with elimination of a impurities, and the structures were in accord with ketene and a minor amount of elimination of a second NMR spectra, which were taken with a Bruker ketene. AMX 300 spectrometer (Wissenbourg, France) at 300 We also performed four stages of activation and five MHz; the samples for NMR were introduced in a dilute of mass analysis (MS5)of2 by using the ion trap (Figure deuterated chloroform solution (ϳ0.3%). 1). For each stage, we observed similar ion chemistry, giving predominantly one major fragment ion by Results and Discussion ketene elimination, which occurs at the ester bond, and minor fragments from consecutive eliminations of all The three fatty-acid substituted porphyrins, 1, 2, and 3, the remaining substituents (Figure 1). LE CAD gives were synthesized as described in the Experimental information about the number of the substituent groups section, and their structures are presented in Scheme 1. present on the porphyrin, their number of carbons, and We chose to study the new, fatty-acid substituted their number of unsaturations. Locating a functional porphyrins by using three different ionization tech- group along the chain by charge-remote fragmentations niques and three different tandem mass spectrometric is not possible under these experimental conditions. methods. HE CAD of FAB-Produced Ions LE CAD of ESI-Produced Ions We obtained full-scan FAB mass spectra of the fatty- ESI produced very simple mass spectra of these por- acid substituted porphyrins by using the first double phyrin derivatives, giving mainly the protonated mol- focusing analyzer of a four-sector instrument. The spec- ecule. We obtained structurally informative fragmenta- tra are very simple and show formation of primarily the tions through the use of in protonated molecules of m/z 895 for 1, m/z 1632 for 2, the ion trap. LE CA of the protonated porphyrins also and m/z 1856 for 3. Besides the protonated species, some produced very simple product-ion spectra, showing fragment ions arise upon FAB from the cleavage of the one predominant dissociation pathway by which an ester bond, followed by back transfer of a hydrogen and ester bond cleaves accompanied by hydrogen transfer elimination of acyl groups as ketenes. 220 DOMINGUES ET AL. J Am Soc Mass Spectrom 1999, 10, 217–223

protonated porphyrins. The porphyrin macrocycle has two pirrolenine nitrogen atoms that are capable of accepting and holding the charge in the center of the protonated molecule [27], where it is localized and remote from the site of fragmentation. Facile pro- tonation of porphyrins is in accord with their solubility in dilute mineral acids [28]. We attribute fragments at m/z values higher than that of the most abundant fragment ion in the HE-CAD spectra of compounds 1, 2, and 3 (Figures 2 and 3) to charge-remote fragmentation of an acyl chain. There are two members of the series: one formed by losses of

CnH2nϩ2 and the other by losses of CnH2nϩ1 species (see Figure 2). The CnH2nϩ1 elimination occurs by homolytic C–C cleavages, giving rise, at least initially, to distonic Figure 2. HE CAD of FAB-produced [M ϩ H]ϩ ions of 1. The radical cations. Elimination of CnH2nϩ2 produces more spectra were obtained with a four-sector tandem mass spectrom- abundant closed-shell product ions presumably by eter. The inset is shown for the mass range from m/z 680 to m/z 880; the symbol “¢” indicates the ion that would be produced by elimination of a neutral alkene and H2. Because the cleavage of the double bond. precursor ion is a protonated molecule, it should pref- erentially lose neutral molecules and give stable, closed- shell fragment ions. A disruption of the fragmentation The tetraphenylporphyrin substituted with one oleic pattern is caused by the presence of the double bond of acid (1) undergoes elimination of the acyl chain as a the oleic acid (inset in Figure 2). This gap in the ketene upon HE CA (Figure 2). For compounds 2 and 3, fragmentation pattern is made explicit by the expected which have fatty-acid chains in each para position of the two abundant product ions that are formed by allylic four phenyl rings, we were able to promote the elimi- cleavages and three low-abundance ions of intervening nation of not only one but also two, three, and four acyl masses; the latter are of low abundance because the groups (Figure 3). The fragment ion from elimination of cleavages of double and vinylic bonds require relatively one acyl chain is the most abundant product ion in the high energy. HE-CAD spectra. There is one dominant charge-remote fragmentation, Charge-remote fragmentations are expected to take in addition to breaking of the ester bond, of these place for these compounds when activated by high- compounds. The ions of m/z 685 for 1, m/z 1448 for 2 energy collisions, as was shown recently in a study of (Figure 2B), and m/z 1616 for 3 (Figure 2A), which are similar porphyrins [15]. A requirement for charge- denoted in the figures as ion a, presumably result from remote fragmentation is that the site of the charge is 1,4-elimination of H and loss of an alkene, as exempli- stable, does not migrate, and does not affect in any 2 fied in Scheme 2 for compound 2. These ions are of major way the fragmentation of the ion [26]. The requirement for charge stabilization is fulfilled by these

Scheme 2

higher relative abundances than those of other fragment ions that are produced by charge-remote processes, because the product ions possess a double bond that is conjugated with that of the carbonyl group. Fragment ions of m/z 641 for 1, m/z 1404 for 2, and m/z 1572 for 3, which are designated in the HE-CAD spectra as b, are also abundant. The reaction may be

ϩ ϩ elimination of CO2 and CnH2nϩ2 to give a fragment ion Figure 3. HE CAD of FAB-produced [M H] ions of 3 (A) and A ϩ 2 (B). The spectra were obtained with a four-sector tandem mass of the structure TPP–CH CH2 . We do not yet under- spectrometer. stand the reason for their high abundance, and it will be J Am Soc Mass Spectrom 1999, 10, 217–223 CHARGE-REMOTE FRAGMENTATIONS BY MALDI-PSD 221

Figure 4. MALDI-PSD (A) and MALDI-CAD (B) spectra of 1. Figure 5. MALDI-PSD (A) and MALDI-CAD (B) spectra of 2. The symbol “¢” indicates the ion that would be produced by The spectra were obtained with a time-of-flight mass spectrome- cleavage of the double bond. The spectra were obtained with a ter. time-of-flight mass spectrometer.

occurring in the reflectron. Although such decomposi- the subject of future studies. Conjugation of the newly tions would be expected to add ion current throughout formed double bond with those of the aromatic rings the m/z range between that of the precursor and the may be the reason for their high abundance. product, fragment ions that are formed during the relatively long time the precursors turn around in the MALDI-PSD and MALDI-CAD reflectron will concentrate ion current in a narrow mass range. We obtained MALDI mass spectra of the three com- The MALDI-PSD and MALDI-CAD spectra of 2 pounds by using a TOF instrument; these spectra are (Figure 5A, B) and 3 (Figure 6A, B) are very similar. We also simple, showing only the protonated molecule, see abundant fragment ions resulting from elimination ϩ [M ϩ H] . The PSD and CAD spectra of the protonated of one, two, three, and four acyl groups, as observed in molecules are similar although CA causes more lower the HE-CAD spectra of these compounds. The elimina- mass ions to appear. Given that the latter spectra arise tions of the second, third, and fourth acyl groups give from HE CA, we conclude that PSD is more similar to fragment ions with higher relative abundance than HE CAD than to LE CAD for these compounds. The those in the HE-CAD spectra. Most importantly, we see monosubstituted porphyrin, 1, with one fatty-acid chain, shows in both spectra (Figure 4A, B) the expected major fragment ion of m/z 631, from ester-bond-directed cleavage. In the mass range above this peak, we see low-abundance products resulting from charge-remote fragmentations along the fatty-acid chain. The disrup- tion of the pattern because of the presence of the double bond is barely detectable and much less obvious than in the HE CAD. The ion of m/z 795/796 is of dramatically higher abundance than seen in the sector tandem mass spectra, suggesting the ion is of m/z 796 and the precursor is in an internal-energy regime where charge- remote homolytic cleavages are preferred [29]. In the MALDI-CAD spectrum of 1, we also observe low- abundance fragment ions of m/z smaller than 631; these may be the result of fragmentation of the phenyl ring of the tetraphenylporphyrin and are also seen in the HE-CAD spectra of the FAB-produced protonated mol- ecule, b. We have made no detailed study of these fragments. Figure 6. MALDI-PSD (A) and MALDI-CAD (B) spectra of 3. There is also an unusual and broad peak centered at The spectra were obtained with a time-of-flight mass spectrome- m/z 871, which may be the result of decompositions ter. 222 DOMINGUES ET AL. J Am Soc Mass Spectrom 1999, 10, 217–223

we repeated the experiments by using dithranol (Figure 7A) and 2,5-dihydroxybenzoic acid (2,5-DHB) (Figure 7B). The PSD spectra are nearly identical when using these latter two matrices as when using the ␣-cyano-4- hydroxycinnamic acid matrix. A second experimental consideration is the use of delayed extraction, which should decrease the internal energy of decomposing ions and cause the charge-remote processes to be atten- uated. We found that under the conditions of delayed extraction, the PSD spectra of 1, 2, and 3 are nearly identical to those obtained without delayed extraction. The porphyrins themselves have high molar absorp- tivities, and direct light absorption from the laser may affect the internal energy in ways that are not available to peptides, for example. Further, the porphyrins have an unusually stable charge site and few reactions chan- nels besides the charge-remote processes. Therefore, there are no lower energy channels to preempt the Figure 7. PSD spectra of MALDI-produced ions of 3 from the matrix of dithranol (A) and 2,5-dihydroxybenzoic acid (B). charge-remote fragmentations as there are with the peptides that were previously studied [2, 9]. in the high-mass region fragment ions from charge- remote fragmentations occurring along the alkyl chains. Conclusions Although the same product ions produced by PSD and It is now clear that charge-remote fragmentations occur CAD of MALDI-produced ions are seen in the product- upon PSD of MALDI-produced ions. This fact is at odds ion spectrum taken on the four-sector instrument, the with conclusions that PSD spectra are similar to LE- relative abundances are different. The charge-remote CAD spectra, at least for these compounds desorbed fragments formed by PSD are relatively more abundant from the matrices we used. The MALDI-PSD process with respect to that produced by acyl-group loss (com- can impart sufficient internal energy to overcome the pare the m/z 631 ions in Figures 2 and 4). If MALDI- required activation energy and kinetic shift associated produced ions have higher internal energies than those with cleavage/rearrangement along an alkyl chain to produced by FAB, charge-remote, simple-cleavage pro- induce processes that are characteristic of only HE CA. cesses may dominate charge-remote rearrangements These results have important consequences not only because they are kinetically favored if the internal for understanding MALDI-PSD but also for developing energy is sufficient [30]. The remarkably facile cleavage a structural tool for porphyrin systems. Both HE CAD or rearrangement at the allylic bond distal to the por- and MALDI-PSD of tetraphenylporphyrins with sub- phyrin ring for 1 is consistent with this picture (Figure stituent fatty-acid moieties include charge-remote frag- 4—unfortunately this TOF instrument has insufficient mentations that provide detailed information about the resolving power to distinguish between simple cleav- fatty-acid side chains, their number, size, degree of age and rearrangement to lose H2 and an olefin). For the unsaturation, and structure. MALDI-PSD and MALDI- multiply substituted porphyrins, the low-mass frag- CAD spectra are very similar to each other in appear- ments are more abundant for MALDI-produced than ance and information content although the CAD spectra for FAB-produced ions. This difference includes those are more easily interpreted. In contrast, LE CA of ions ions produced by losses of acyl groups and by second produced by ESI in an ion trap induces no charge- and higher rounds of charge-remote fragmentation to remote fragmentations and only provides information give ions in the range of m/z 1156–1394, indicating on the number, size, and extent of unsaturation of the greater energy deposition during the lifetime of the fatty-acid chains. The advantages of MALDI-PSD with MALDI-produced ions. respect to sensitivity, simplicity, and cost make it a Figure 5A shows again a broad peak centered at powerful tool that yields, for these compounds, struc- approximately m/z 1590. This signal may be due to tural information that was formerly accessible only with decompositions occurring in the reflectron, as was HE CAD on sector instruments. One shortcoming of the discussed for the analogous broad peaks in the PSD MALDI data reported in this study is the inadequate spectra shown in Figures 4A, B. mass resolving power, but this limitation pertains less Specific experimental conditions and properties of to contemporary MALDI TOF research instruments. the porphyrins may preclude generalization of these results. For example, the ␣-cyano-4-hydroxycinnamic acid matrix is considered to be “hot” [29] and may give Acknowledgments rise to higher energy fragmentations than would be The research conducted at Washington University was supported seen with a “cooler” matrix. To explore this possibility, by the National Institutes of Health, National Center for Research J Am Soc Mass Spectrom 1999, 10, 217–223 CHARGE-REMOTE FRAGMENTATIONS BY MALDI-PSD 223

Resources (grant no. P41-RR-00954). Rosario M. Domingues is of Mn(III)-Tetraarylporphyrin Complexes by FAB MS. Org. grateful to Calouste Gulbenkian Foundation and FLAD for a Mass Spectrom. 1991, 26, 161–166. research fellowship. We thank the reviewers for helpful sugges- 14. Van Berkel, G. J.; McLuckey, S. A.; Glish, G. L. ESI of tions. Porphyrins Using a for Mass Analysis. Anal. Chem. 1991, 63, 1098–1109. 15. Shibue, T.; Kambe, H.; Nishide, H.; Tsuchida, E.; Akashi, S.; References Ohashi, Y. Structural Characterization of Picket-fence Porphy- ␻ 1. Spengler, B.; Kirsch, D.; Kaufmann, R.; Jaeger, E. Peptide rinatoiron with -Hydroxy-2,2-dimethylicosanoyl Side- Sequencing by Matrix-assisted laser-desorption Mass Spec- Chains by FAB High-Energy CID MS/MS. J. Mass Spectrom. trometry. Rapid Commun. Mass Spectrom. 1992, 6, 105–108. Soc. Jpn. 1998, 46, 250–253. 2. Rouse, J. C.; Yu, W.; Martin, S. A. A Comparison of the Peptide 16. Wysocki, V. H.; Bier, M. E.; Cooks, R. G. Internal Energy Fragmentation Obtained from a Reflector MALDI TOF and a Requirements for Remote Site Fragmentation. Org. Mass Spec- Tandem Four Sector Mass Spectrometer. J. Am. Soc. Mass trom. 1988, 23, 627–633. Spectrom. 1995, 6, 822–835. 17. Hayes, R. N.; Gross, M. L. In Methods in Enzymology, Vol. 193, 3. Kaufmann, R.; Wingerath, T.; Kirsch, D.; Stahl, W.; Sies, H. Mass Spectrometry; McCloskey, J. A., Ed.; Academic: San Analysis of Carotenoids and Carotenol Fatty Acid Esters by Diego, 1990, pp 237–262. Matrix-assisted Laser Desorption Ionization (MALDI) and 18. Cregg, B. A.; Fox, M. A.; Bard, A. J. 2,3,7,8,12,13,17,18- ␤ MALDI-post-source-decay Mass Spectrometry. Anal. Biochem. Octakis( -hydroxyethyl)porphyrin and its Liquid Crystalline 1996, 238, 117–128. Derivatives: Synthesis and Characterization. J. Am. Chem. Soc. 4. Spengler, B.; Kirsch, D.; Kaufmann, R.; Lemoine, J. Struc- 1989, 111, 3024–3029. tural Analysis of Branched Oligosaccarides Using PSD in 19. Cregg, B. A.; Fox, M. A.; Bard, A. J. Effects of Order on the ␤ MALDI Mass Spectrometry. Org. Mass Spectrom. 1994, 29, Photophysical Properties of the Liquid Crystal Zinc Octakis( - 782–787. octoxyethyl)porphyrin. J. Phys. Chem. 1989, 93, 4227–4234. 5. Harvey, D. J. Matrix-assisted Laser Desorption/ionisation 20. Kugimiya, S. I.; Takemura, M. Novel Liquid Crystals Consist- Mass Spectrometry of Oligosaccharides and Glycoconjugates. ing of Tetraphenylporphyrin Derivatives. Tetrahedron Lett. J. Chromatogr. 1996, 720, 429–446. 1990, 31, 3157–3160. 6. Spengler, B.; Kaufmann, R. Gentle Probe for Tough Molecules: 21. Bonnett, R.; Ioannou, S.; James, A. G.; Pitt, C. W.; Soe, M. M. Z. MALD Mass Spectrometry. Analysis 1992, 20, 91–101. Octadecyl Ethers of 5,10,15,20-tetra(4-hydroxyphenyl)porphy- 7. Spengler, B.; Kirsch, D.; Kaufmann, R. Fundamental Aspects rin: Synthesis and Film-Forming Properties. J. Mater. Chem. of PSD in MALDI Mass Spectrometry 1. Residual Gas Effects. 1992, 2, 823–828. J. Phys. Chem. 1992, 96, 9678. 22. Meier, G.; Sackmann, E.; Grabmaier, J. G. Applications of Liquid 8. Kaufmann, R.; Chaurand, P.; Kirsch, D.; Spengler, B. Post- Crystals; Springer: Berlin, 1975; p 36. source Decay and Delayed Extraction in Matrix-assisted Laser 23. Gross, M. L. In Methods in Enzymology, Vol. 193, Mass Spec- Desorption/Ionization-Reflectron Time-of-flight Mass Spec- trometry; McCloskey, J. A., Ed.; Academic: San Diego, 1990; pp trometry. Are there trade-offs? Rapid Commun. Mass Spectrom. 131–153. 1996, 10, 1199–1208. 24. Van Berkel, G. J.; McLuckey, S. A.; Glish, G. L. Electrochemical 9. Ospina, M.; Powell, D. H.; Sapp, L.; Denslow, N.; Yost, R. A. Origin of Radical Cations Observed in ESI Mass Spectra. Anal. Comparison of Low and High Energy CID to MALDI PSD. Chem. 1992, 64, 1586–11593. Proceedings of the 44th ASMS Conference on Mass Spectrometry 25. March, J. Advanced Organic Chemistry; Wiley: New York, 1992; and Allied Topics; Palm Springs, 1997; p 90. pp 392–396. 10. Kaufmann, R.; Spengler, B.; Lu¨ tzenkirchen, F. Mass Spectro- 26. Adams, J. Charge-Remote Fragmentation: Analytical Applica- metric Sequencing of Linear Peptides by Product-ion Analysis tions and Fundamental Studies. Mass Spectrom. Rev. 1990, 9, in a Reflectron Time-of-flight Mass Spectrometer Using Ma- 141–186. trix-assisted Laser Desorption Ionization. Rapid Commun. Mass 27. Berezin, B. D. Coordination compounds of Porphyrins and Phtalo- Spectrom. 1993, 7, 902–910. cyanines, translation by Vopian, V. G.; Wiley: New York, 1981; 11. Gooden, J. K.; Byun, J.; Chen, L.; Rogan, E. G.; Cavalieri, E. L.; pp 59–67. Gross, M. L. Application of MALDI and PSD to the Structure 28. Smith, K. M. In Porphyrins and Metalloporphyrins; Elsevier: Determination of Adducts between DNA Bases and the Car- Amsterdam, 1975; pp 11–14. cinogen 7H-Dibenzo[c, g]carbazole. Int. J. Mass Spectrom. Ion 29. Cheng, C.; Pittenauer, E.; Gross, M. L. Charge-Remote Frag- Processes 1997, 169/170, 241–249. mentations are Energy-Dependent Processes. J. Am. Soc. Mass 12. Dale, M. J.; Costello, K. F.; Jones, A. C.; Langridge-Smith, Spectrom. 1998, 9, 840–844. P. R. R. Investigation of Porphyrins and Metalloporphyrins 30. Karas, M.; Bahr, U.; Strupat, K.; Hillenkamp, F.; Tsarbopoulos, using Two-step Laser Mass Spectrometry. J. Mass Spectrom. A.; Pramanik, B. N. Matrix Dependence of Metastable Frag- 1996, 31, 590–601. mentations of Glycoproteins in MALDI TOF Mass Spectrom- 13. Rubino, F. M.; Mascaro, P.; Banfi, S.; Quici, S. Structural Study etry. Anal. Chem. 1995, 67, 675–679.