We studied by tandem mass spectrometry two isomers of nitro meso-tetraphenylporphyrin, one with a nitro group in the para position of a phenyl ring and the other with the same group in a beta-pyrrolic position, and their copper complexes. Collisional activation of the molecular ions of both free-base porphyrins and of their copper complexes produces an array of product ions that permit ready differentiation of the two positional isomers. The diagnostic ions, when the nitro group is in a beta-pyrrolic position, may be produced through intramolecular and double cyclization processes, triggered by the interaction of the nitro substituent with the neighboring meso-phenyl ring. These diagnostic ions do not form when the nitro group is in the para position. The gas-phase processes have precedents in solution chemistry.

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 mass spectrometer. 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 considerably more informative than low-energy CAD spectra, showing that charge-remote fragmentations of the fatty acid moieties do occur upon MALDI-PSD and MALDI-CAD.

The protonated molecules and radical cations of meso-tetraphenylporphyrins with ␤-pyrrolic substituents, when formed by fast atom bombardment (FAB) and subjected to high-energy collisions, give rise to unexpected fragment ions. The reaction involves hydrogen migration from the ortho position of the phenyl ring to the ␣ atom of the substituent, with formation of an intramolecular, six-membered ring. The process is analogous to condensed-phase cyclizations described for the same type of compounds. The fragmentation requires the presence of a double bond in the substituent group attached to the pyrrolic ring. A rearrangement process involving anchimeric assistance by the phenyl group (analogous to an ortho effect) is proposed for the formation of these ions. T he porphyrin macrocycle is present in a variety of systems found in nature and participates in numerous important biochemical functions. New porphyrins are being synthesized to mimic the natural systems. Research on porphyrins has accelerated recently because they are applicable in a variety of fields of chemistry, physics, medicine, and biology [1]. Most promising are applications as agents in photodynamic therapy in cancer treatment [2], receptor models in molecular recognition [3], and use in materials chemistry [4] owing to their optical properties and their ability to form microporous solids, conductive polymers, and sensors. The functionalization of the ␤-pyrrolic positions of meso-tetraphenylporphyrins is particularly of interest because the properties of the porphyrin macrocycle can be modified by small changes in the ␤-pyrrolic substituents [5]. The ability of these substituents to interact with the adjacent phenyl groups may lead to deviations of planarity of the porphyrin rings, affecting the chemical, spectroscopic, and other properties of the substituted porphyrins [6,7]. An example is the role of non-planar conformations of chlorophyll, vitamin B12, and cytochrome P450 in biological processes [8].Despite the increasing interest in this class of compounds, the mass spectrometric analysis of porphyrins is still relatively unexplored [9], particularly that of substituted meso-tetraphenylporphyrins [10 -16]. The few studies that describe the fragmentation of tetraphenylporphyrins in the MS/MS mode [12][13][14][15][16] show that this approach gives valuable structural information about the peripheral substituents. MS/MS is expected to be useful for the chemical characterization of porphyrins because it allows a rapid identification of porphyrins in mixtures, especially those resulting from synthetic procedures. Further, the assembly of spectral databases will make widely available the valuable structural data from MS [17,18]. Nevertheless, mass spectral data of tetraphenylporphyrins substituted in ␤-pyrrolic positions comprise only two recent reports: One on ␤-bromotetraphenylporphyrins [19,20] and another on the fragmentation of ␤-nitrotetraphenylporphyrin [16], which is the precedent for this research.In this report, we discuss the unexpected fragm...

High-energy (HE) and low-energy (LE) collisionally activated decompositions of octaethylporphyrin (OEP) and its metal complexes (ZnOEP and CuOEP) depend on whether the precursor is produced by electrospray ionization as protonated molecules or by fast atom bombardment as radical cations or protonated molecules. LE activation leads to such simple product-ion spectra that a complete picture of fragmentation emerges only after nine stages of tandem mass spectrometry (MS). HE activation, on the other hand, gives product-ion spectra that afford an integrated view of all the decomposition channels in a single MS/MS experiment. These results are the basis of a recommendation that OEP is an appropriate model compound for investigating energy effects in the collisional activation of organic and bioorganic molecule ions.

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