Synthesis of 18O-labeled photosynthetically active chlorophylls at the 3- or 7-carbonyl group with high regioselectivity
The 3- and 7-formyl groups of chlorophyll-d (Chl-d) and bacteriochlorophyll-e (BChl-e), respectively, were regioselectively labeled with an isotopically stable oxygen-18 (18O) atom to give 31-18O-labeled Chl-d and 71-18O-labeled BChl-e (ca. 90% 18O) by exchanging the carbonyl oxygen atoms in the presence of acidic H2 18O (ca. 95% 18O). Another photosynthetically active chlorophyll, BChl-a possessing the 3-acetyl group was treated under similar acidic conditions to afford a trace amount of 31-18O-labeled BChl-a and further demetallated compound, the corresponding 31-18O-labeled bacteriopheophytin-a as the major product with 55% 18O-degree. The FT-IR spectra of 18O-(un)labeled chlorophylls in the solution and the solid states showed that the 3- and 7-carbonyl stretching vibration modes moved to about a 30-cm−1 lower wavenumber by 18O-labeling at the 31- and 71-oxo moieties. In artificial chlorosome-like self-aggregates of BChl-e, the 18O-labeled 7-carbonyl stretching mode was completely resolved from the specially hydrogen-bonded 13-C=O stretching mode, evidently indicating no interaction of the 7-CHO with other functional groups in the supramolecules.
KeywordsBacteriochlorophyll Chlorophyll Isotope-18O-labeling Photosynthetic pigment Self-aggregation Vibrational spectroscopy
We thank Dr. Tomohiro Miyatake, Ryukoku University and Mr. Masaaki Amakawa, Ritsumeikan University, for their helpful discussions. This study was partially supported by a Grant-in-Aid for Scientific Research (A) (No. 22245030) from the Japan Society for the Promotion of Science (JSPS).
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