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Structure elucidation of chlorophyll catabolites (phyllobilins) by ESI-mass spectrometry-Pseudo-molecular ions and fragmentation analysis of a nonfluorescent chlorophyll catabolite (NCC).

Müller T, Vergeiner S, Kräutler B - Int J Mass Spectrom (2014)

Bottom Line: A recent application of this methodology concerned the investigation of the annually occurring degradation of green plant pigments.Since the first structural elucidation of a breakdown product in the early 1990s, a number of similarly structured, tetrapyrrolic catabolites have been discovered with the help of chromatographic, spectroscopic and spectrometric methods.Still, a thorough investigation of the common fragmentation behavior of these ubiquitous, naturally occurring chlorophyll breakdown products is lacking.

View Article: PubMed Central - PubMed

Affiliation: Institute of Organic Chemistry and Center for Molecular Biosciences, University of Innsbruck, 6020 Innsbruck, Austria.

ABSTRACT

The hyphenation of high performance chromatography with modern mass spectrometric techniques providing high-resolution data as well as structural information from MS/MS experiments has become a versatile tool for rapid natural product identification and characterization. A recent application of this methodology concerned the investigation of the annually occurring degradation of green plant pigments. Since the first structural elucidation of a breakdown product in the early 1990s, a number of similarly structured, tetrapyrrolic catabolites have been discovered with the help of chromatographic, spectroscopic and spectrometric methods. A prerequisite for a satisfactory, manually operated or database supported analysis of mass spectrometric fragmentation patterns is a deeper knowledge of the underlying gas phase chemistry. Still, a thorough investigation of the common fragmentation behavior of these ubiquitous, naturally occurring chlorophyll breakdown products is lacking. This study closes the gap and gives a comprehensive overview of collision-induced fragmentation reactions of a tetrapyrrolic nonfluorescent chlorophyll catabolite, which is intended to serve as a model compound for the substance class of phyllobilins.

No MeSH data available.


Related in: MedlinePlus

Diagnostic fragmentations of (+)-ions of NCC 1 (R1 = H, Na or K): Loss of ring A; a similar mechanism could be relevant for the cleavage between the pyrrole rings C and E.
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fig0040: Diagnostic fragmentations of (+)-ions of NCC 1 (R1 = H, Na or K): Loss of ring A; a similar mechanism could be relevant for the cleavage between the pyrrole rings C and E.

Mentions: All MS/MS experiments in the positive as well as in the negative ion-mode showed loss of ring D (−123 Da; (C15–C16)-bond was cleaved) as a very favored fragmentation mode, either of the pseudo-molecular ion directly (Fig. 3A and Fig. 4), or in a row with the loss MeOH and CO2 (Fig. 3A–C). It is assumed that protonation of the carbonyl group at ring D (the 1,2-dihydropyrrol-5-one) initiates the loss of a neutral (aromatic), 2-hydroxy-pyrrol unit (Scheme 3; top), while a positive charge remains at the other fragment, where it gains stabilization from interaction with the electron-rich ring C hetero-cycle. Due to the fact that the loss a neutral 2-hydroxy-pyrrol unit (−123 Da) is also observed in the negative ion-mode, a different mechanism is supposed to contribute to the loss of ring D. This mode of cleavage makes it likely that the nitrogen bound hydrogen from pyrrolic ring C is a direct or indirect intramolecular proton source for ring D. As a consequence the C15–C16 bond can be cleaved heterolytically while no charge remains on either part (see Scheme 3; bottom). This also would explain why ring D can be cleaved after the loss of ring A and vice versa (Fig. 3A). Interestingly, in case of [M + Na]+ and [M + K]+ (Fig. 3B and C) the loss of methanol comes first and the loss of ring D is not occurring directly from the sodiated and potassiated molecules. We hypothesize that the sodium or potassium ions are stabilized by complexation involving all oxygens of the carboxlic acid as well as the β-keto ester on the ring C and ring E, respectively. This special arrangement might favor the neutral loss of methanol over any other reaction channel. In contrast, signals from loss of ring A (−153 Da; (C4–C5)-bond was cleaved) were of comparably low intensity (Fig. 3A andB). As shown in Scheme 4, in this fragmentation process, an initial protonation is suggested at the α-position of ring A. Although a neutral, stable pyrrole unit is lost and the (+)-charge associated with the ring B moiety is delocalized and stabilized electronically, the initial protonation disrupts the aromaticity of the pyrrole unit of ring A and is, therefore, an energetically disfavored step.


Structure elucidation of chlorophyll catabolites (phyllobilins) by ESI-mass spectrometry-Pseudo-molecular ions and fragmentation analysis of a nonfluorescent chlorophyll catabolite (NCC).

Müller T, Vergeiner S, Kräutler B - Int J Mass Spectrom (2014)

Diagnostic fragmentations of (+)-ions of NCC 1 (R1 = H, Na or K): Loss of ring A; a similar mechanism could be relevant for the cleavage between the pyrrole rings C and E.
© Copyright Policy - CC BY-NC-ND
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4375672&req=5

fig0040: Diagnostic fragmentations of (+)-ions of NCC 1 (R1 = H, Na or K): Loss of ring A; a similar mechanism could be relevant for the cleavage between the pyrrole rings C and E.
Mentions: All MS/MS experiments in the positive as well as in the negative ion-mode showed loss of ring D (−123 Da; (C15–C16)-bond was cleaved) as a very favored fragmentation mode, either of the pseudo-molecular ion directly (Fig. 3A and Fig. 4), or in a row with the loss MeOH and CO2 (Fig. 3A–C). It is assumed that protonation of the carbonyl group at ring D (the 1,2-dihydropyrrol-5-one) initiates the loss of a neutral (aromatic), 2-hydroxy-pyrrol unit (Scheme 3; top), while a positive charge remains at the other fragment, where it gains stabilization from interaction with the electron-rich ring C hetero-cycle. Due to the fact that the loss a neutral 2-hydroxy-pyrrol unit (−123 Da) is also observed in the negative ion-mode, a different mechanism is supposed to contribute to the loss of ring D. This mode of cleavage makes it likely that the nitrogen bound hydrogen from pyrrolic ring C is a direct or indirect intramolecular proton source for ring D. As a consequence the C15–C16 bond can be cleaved heterolytically while no charge remains on either part (see Scheme 3; bottom). This also would explain why ring D can be cleaved after the loss of ring A and vice versa (Fig. 3A). Interestingly, in case of [M + Na]+ and [M + K]+ (Fig. 3B and C) the loss of methanol comes first and the loss of ring D is not occurring directly from the sodiated and potassiated molecules. We hypothesize that the sodium or potassium ions are stabilized by complexation involving all oxygens of the carboxlic acid as well as the β-keto ester on the ring C and ring E, respectively. This special arrangement might favor the neutral loss of methanol over any other reaction channel. In contrast, signals from loss of ring A (−153 Da; (C4–C5)-bond was cleaved) were of comparably low intensity (Fig. 3A andB). As shown in Scheme 4, in this fragmentation process, an initial protonation is suggested at the α-position of ring A. Although a neutral, stable pyrrole unit is lost and the (+)-charge associated with the ring B moiety is delocalized and stabilized electronically, the initial protonation disrupts the aromaticity of the pyrrole unit of ring A and is, therefore, an energetically disfavored step.

Bottom Line: A recent application of this methodology concerned the investigation of the annually occurring degradation of green plant pigments.Since the first structural elucidation of a breakdown product in the early 1990s, a number of similarly structured, tetrapyrrolic catabolites have been discovered with the help of chromatographic, spectroscopic and spectrometric methods.Still, a thorough investigation of the common fragmentation behavior of these ubiquitous, naturally occurring chlorophyll breakdown products is lacking.

View Article: PubMed Central - PubMed

Affiliation: Institute of Organic Chemistry and Center for Molecular Biosciences, University of Innsbruck, 6020 Innsbruck, Austria.

ABSTRACT

The hyphenation of high performance chromatography with modern mass spectrometric techniques providing high-resolution data as well as structural information from MS/MS experiments has become a versatile tool for rapid natural product identification and characterization. A recent application of this methodology concerned the investigation of the annually occurring degradation of green plant pigments. Since the first structural elucidation of a breakdown product in the early 1990s, a number of similarly structured, tetrapyrrolic catabolites have been discovered with the help of chromatographic, spectroscopic and spectrometric methods. A prerequisite for a satisfactory, manually operated or database supported analysis of mass spectrometric fragmentation patterns is a deeper knowledge of the underlying gas phase chemistry. Still, a thorough investigation of the common fragmentation behavior of these ubiquitous, naturally occurring chlorophyll breakdown products is lacking. This study closes the gap and gives a comprehensive overview of collision-induced fragmentation reactions of a tetrapyrrolic nonfluorescent chlorophyll catabolite, which is intended to serve as a model compound for the substance class of phyllobilins.

No MeSH data available.


Related in: MedlinePlus