PARENT SESSION
Posters P8B Supermolecular organization of the photosynthetic apparatus. Abstracts (592-611)

Synthesis of ¹⁸O-labelled (pyro)pheophytins including Pheophytin-d. Hidetada Morishita^*,1, Hitoshi Tamiaki¹, ¹ Department of Bioscience and Biotechnology, Faculty of Science and Engineering, Kusatsu, Shiga, Japan

ABSTRACT- Pheophytins are demetalated pigments of chlorophylls, which are naturally occurring magnesium complexes of cyclic tetrapyrroles possessing oxygen atoms at their peripheral positions. In most natural oxygenic photosynthetic organisms, pheophytin-a plays important roles in various apparatus of the initial stages in their photosynthesis, and is fixed on membranous proteins through specific interaction of its 13¹-keto-oxygen atom with their surrounded peptide residues. Pheophytin-d possessing the 3-formyl group instead of the 3-vinyl group of pheophytin-a is situated in the apparatus of Acaryochloris marina. Vibrational spectroscopies including isotope labelling techniques give useful information on the local structure around the photosynthetic pigments and/or peptide scaffolds. Here, we report preparation of 3¹- and/or 13¹-¹⁸O-labelled (pyro)pheophytins at a high regioselectivity by a simple one-step procedure. The 3- and/or 13-CO of (pyro)pheophytins derived from chlorophyll-a, reacted with H₂¹⁸O under acidic conditions to give the regioselectively ¹⁸O-oxo-labelled compounds. In the case of pheophytins possessing 13²-COOMe, the oxygen atom of the 13-CO was resisted toward attack by proton because of a decrease in the basicity and an increase in the steric hindrance of the 13-CO group, compared with the 13-CO in pyropheophytins lacking the 13²-COOMe. Therefore, the ¹⁸O-label at the 13-CO of pheophytin-a was difficult to be achieved. The ¹⁸O-exchanging reaction of pheophytin-d possessed 13²-COOMe and 3-CHO, led to 3¹-single-¹⁸O-oxo-labelled pheophytin-d as expected. These isotopic labels induced an about 30-cm^-1 down-shift of the C=O stretching bands in infrared spectroscopy, and an about 0.04 ppm high field shift of the carbonyl carbon chemical shift in ¹³C-NMR measurements, compared with the corresponding unlabelled (pyro)pheophytins.

KEY WORDS: ¹⁸O-label, ¹³C-NMR, (pyro)pheophytins, infrared