Investigation of MALDI-TOF Mass Spectrometry of Diverse Synthetic Metalloporphyrins, Phthalocyanines and Multiporphyrin Arrays

We investigated the utility of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) for analyzing porphyrinic compounds using a variety of different synthetic porphyrins, azaporphyrins, phthalocyanines and multiporphyrin arrays. Comparisons of spectra obtained from these analytes deposited either as neat samples or codeposited with neutral or acidic matrices have been made with the goal of identifying conditions that yield minimal demetalation, transmetalation, adduct formation and fragmentation. It was found that the molecular masses of many porphyrins can be successfully measured from neat sample preparations and do not require a matrix to facilitate desorption and ionization, although the measurement of large multiporphyrin arrays was facilitated by the use of matrices. Demetalation of magnesium porphyrins occurred in the presence of acidic matrices, but not with neutral matrices such as 1,4-benzoquinone. Positive ion spectra were obtained for each compound and negative ion spectra were also collected for the azaporphyrins and phthalocyanines. Examination of selected samples (prepared neat, with 1,4-benzoquinone, 2,3,5,6-tetrachloro-1,4-benzoquinone or α-cyano-4-hydroxycinnamic acid) showed that the dominant process of ionization involved oxidation yielding the radical cation M+· rather than the protonated molecule [M+H]+. MALDI-TOF-MS is shown to be a powerful analytical tool for the characterization of diverse synthetic porphyrinic compounds.

Identification of hemes and related cyclic tetrapyrroles by matrix-assisted laser desorption/ionization and liquid secondary ion mass spectrometry

Mass spectrometry has proven to be a powerful technique applicable on trace amounts for the identification of known hemes and cyclic tetrapyrroles, and for providing critical information for the structure of new and novel versions. This report describes investigations of the practical limits of detection for such bioinorganic prosthetic groups, primarily by liquid secondary ion mass spectrometry (LSIMS) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS), including a survey of the utility of common matrices. The lower limit of detection under favorable conditions extends to low picomole amounts. Certain derivatization techniques, such as methyl esterification and chelation to zinc, both increase the sensitivity of analyses and provide spectroscopic signatures that enable heme/cyclic tetrapyrrole ions to be identified in the presence of contaminants.

Analyte incorporation and ionization in matrix-assisted laser desorption/ionization visualized by pH indicator molecular probes

Despite the spreading applications of matrix-assisted laser desorption/ionization (MALDI), its fundamental understanding is still limited and under constant debate. This report focuses on the initial state of the analyte in the host matrix. pH indicator dyes serve as molecular probes since their color directly indicates their (de)protonation state. For a set of matrixes at their intrinsic pH, solution color was maintained, delivering clear proof for analyte incorporation in the solution charge state. Moreover, substantial solvent inclusion is determined by 1H NMR spectroscopy. MALDI mass spectra show a clear correlation to the dye charge state. However, the dominant solution species are not observed exclusively in the mass spectra, pointing to a proton transfer or proton neutralization activity of the matrix.

A fast atom bombardment and matrix-assisted laser desorption/ionization mass spectrometry study of doubly charged porphyrins

In this mass spectrometry (MS) study of doubly charged porphyrin salts, fast atom bombardment (FAB) and matrix-assisted laser desorption/ionization (MALDI) MS techniques are utilized to examine several unique ionic species. The predominant transformation of preformed doubly charged ions in the desorption/ionization mechanism of FAB and MALDI is the result of deprotonation reactions to form singly charged ions of the type [M(2+) - H(+)](+) and of one-electron reductions to form radical cations [M(2+) + e(-)](+.). The dependence of this phenomenon and the formation of a number of additional ionic species on the different matrices and the FAB-matrix additive benzoquinone is examined. The significant analogous behavior of doubly charged porphyrins in FAB- and MALDI-MS leads to the conclusion that one-electron reductions are of distinct relevance in the desorption/ionization mechanism of MALDI.

Liquid secondary ion mass spectrometry of porphyrin dimers: reduction reactions and structural characterisation

New synthesised porphyrin dimers, with an amide or ester linkage between the two porphyrin units, were studied using liquid secondary ion mass spectrometry (LSIMS). The formation of reduced species was observed for all the compounds and it was found that the extent of reduction is dependent on the matrix used and on the structure of the porphyrin dimer. The main fragmentation pathways lead to monomer fragments resulting from cleavage of the amide or ester linkage between the two porphyrin units. The consistency of the fragmentations for all the dimers studied leads to the proposal of a common designation for the fragment ions. LSIMS, in addition to molecular weight determination, can provide important structural information for this type of compound.

Siroamide: a prosthetic group isolated from sulfite reductases in the genus Desulfovibrio

While isolating siroheme from enzymes or whole cells of Desulfovibrio species, it was discovered that the main product after metal removal and esterification was not the octamethyl ester derivative of sirohydrochlorin, but a monoamide, heptamethyl ester derivative. The structure of this derivative was established by mass spectrometry and NMR. Nuclear Overhauser enhancement measurements in combination with chemical shift analogy arguments indicate that the 2(1)-acetate has been stereospecifically amidated. Other cellular sources of siroheme were investigated, but only the octamethyl ester derivative was found, with no traces of the amide derivative. The results suggest that, in Desulfovibrio, the physiologically active prosthetic group may be an amidated form of siroheme.

Competitive ionization of tetraphenylporphyrin in a laser-generated metal ion plasma

The ionization of tetraphenylporphyrin (TPP) in a laser-desorbed metal ion plasma is examined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Competitive reaction pathways observed to generate abundant molecular ion species include electron detachment, cation attachment, charge exchange, metallation, and transmetallation in the positive ion mode and electron capture, metallation, and transmetallation in the negative ion mode. In general, cation attachment reactions dominate positive ion spectra below the laser irradiance threshold for plasma ignition, although the metallation product from [TPP](+) reaction with the metal atom, M, is observed. Negative ion products are not observed in the FT-ICR spectrum when a plasma is not formed. Under plasma ignition conditions, positive ion spectra include [TPP](+) formed by charge exchange with M(+), which is also present in the spectrum. Negative ion spectra are dominated by [TPP](-); which is formed by attachment to thermal electrons generated in the plasma. Metallation reactions involving TPP and the metal substrate are examined. Positive ion metallation products are observed both in the absence of a plasma through reaction of [TPP](+) with M and by a second pathway under plasma ignition conditions through reaction of TPP with M(+). In negative ion mode, metallation is only observed under plasma ignition conditions through reaction of [TPP](-) with M. Observation of metallated products is found to be consistent with formation of stable metal oxidation states in the metallated porphyrin.

A novel derivative of the prosthetic group heme d1: S-methylporphyrindione d1

Several derivatives of heme d1 have been characterized by ultraviolet-visible, NMR, and mass spectrometry. Most arise from side reactions during the isolation of d1 from the enzyme. One, however, has now been shown to correspond to the replacement of a meso proton by an S-methyl group. Since the porphyrin is not exposed to S-methyl-containing reagents during its isolation, this raises hypotheses that it has its origin in vivo.

Desorption of ions from rat membranes: selectivity of different ionization techniques

Complex lipid biomarkers, including phosphatidylcholines, cerebrosides and sulfatides, are shown to be desorbed intact from rat brain myelin and rat liver microsomes by liquid secondary ion mass spectrometry, by plasma desorption and by laser desorption. Different polar lipids are favored by the different desorption techniques and as negative or positive ions. These selectivities support current theories about ionization for the different techniques.

Fast atom bombardment mass spectrometry of high-molecular-weight fraction of porphyrin-based photodynamic therapy drugs

Photodynamic therapy (PDT) involves the treatment of tumor tissue with a photosensitizer and light to effect the delineation and/or eradication of the tumor. PDT is a two step process: (1) incorporation of photosensitizer into the cell where it must be retained by tumors in vivo; and (2) illumination of the tumor cell with light to effect cell death. Hematoporphyrin derivative (HPD) is a complex mixture of porphyrins currently used for PDT in the clinical setting. Hematoporphyrin-based oligomers of up to five subunits were determined using fast atom bombardment mass spectrometry of the high-molecular-weight fraction of the drug. Reduction of this fraction with LiAlH4 permitted determination of the covalent bond linking the monomers in these oligoporphyrins. Application of these analytical procedures to the determination of the composition of different preparations of HPD will be described.

Evidence that heme d1 is a 1,3-porphyrindione

Heme d1 is the noncovalently associated heme prosthetic group of the bacterial nitrite reductase known as cytochrome cd1. Additional evidence has been obtained in support of a dioxoisobacteriochlorin, or 1,3-porphyrindione, skeleton for this heme. The new data include the natural abundance 13C NMR spectrum of the free base methyl ester derivative of d1, mass spectrometric determinations of the molecular mass of the free base methyl ester and the Cu and the Zn chelates, visible and 1H NMR spectral comparisons between d1 and synthetic porphyrindione model compounds, and the isolation and characterization of several byproducts formed during the purification of the free base methyl ester of d1. The accumulated evidence strongly supports the following structure for the skeleton of d1: 1-oxo-2-methyl-2'-acetyl-3-oxo-4-methyl-4'-acetyl-5-methyl-6-acrylyl+ ++-7- propionyl-8-methylporphyrin.