Applications of mass spectrometry in studies of porphyrins and related tetrapyrroles

Abstract

Electron impact mass spectrometry has played a vital role in the structure determination of a variety of porphyrin and chlorophyll derivatives, especially those occurring in small quantities, and which are related to important intermediates in both normal and abnormal metabolism. More recently, other newer mass spectral techniques have also become of increasing importance, i.e. soft ionization methods and g.c.-m.s. of degradation products. Field desorption mass spectrometry is particularly useful for porphyrins because of their low volatility and it is now very useful for preliminary analyses of crude mixtures of porphyrins, as well as for direct analysis of individual fractions obtained in high pressure liquid chromatography h.p.l.c. separations. Examples include porphyrins excreted by porphyric patients, petro-porphyrins, chlorophyll derivatives, and synthetic porphyrins. The coupling of a high pressure liquid chromatograph by means of a moving belt system also appears to be a promising technique for the direct mass spectromatic analysis of porphyrins in liquid chromatography fractions, and preliminary results are described. Chromic acid oxidation, or hydriodic acid reduction, of porphyrins to monopyrrolic fragments (the classical methods used for porphyrin structure determination) have now been refined by use of g.c.-m.s. and can be applied to sub-milligram quantities of porphyrins, or bile pigments; examples include isocoproporphyrin, ( d -urobilin and stercobilin. Biosynthetic studies have also been facilitated by degradative techniques combined with mass spectrometry. The final section of the paper describes preliminary studies of some complex metastable fragmentation processes occurring with chlorophyll derivatives with the use of the 'Dadi’ and linked B/E scan methods, together with field desorption and collisional activation.