O-18 and mass spectrometry in chlorophyll research: Derivation and loss ofoxygen atoms at the periphery of the chlorophyll macrocycle during biosynthesis, degradation and adaptation

Chlorophylls, magnesium-containing tetrapyrrolic pigments of photosynthesis , are widely-distributed in Nature and participate in both light harvesting and in the transduction of light energy to chemical energy for the photosy nthetic fixation of carbon dioxide. We briefly discuss the extensive role o f various isotopic labelling techniques in elucidating the pathway of tetra pyrrole-pigment biosynthesis and we acknowledge the classic and meticulous research of David Shemin who, approximately 50 years ago, introduced isotop ic tracer techniques with N-15 and C-14 isotopes to study the biosynthesis of the carbon/nitrogen macrocycle of haem, an iron tetrapyrrole. The main f ocus of this review is the application of mass spectrometry and O-18 labell ing to the study of the incorporation of oxygen atoms from molecular oxygen or water into the periphery of the chlorophyll macrocycle during biosynthe sis and their loss during degradation and light acclimation. In particular, we review the mechanism of formation of the isocyclic ring of chlorophylls , in higher plants, green algae and various photosynthetic bacteria, which concomitantly incurs formation of the 13(1)-oxo group that is present in al l photosynthetically-active chlorophylls. In addition we discuss the format ion of the ubiquitous 13(3)- and 17(3)-carboxyl groups and also the formati on of the 7-formyl group of chlorophyll b and the 3-acetyl group of bacteri ochlorophyll a.