The structure of chlorophyllonic acid a methyl ester (3), a new chlorophyll a related compound was determined as shown in Fig. 2 by NMR spectral and X-ray crystallographic analysis. In the course of our screening for antioxidants from marine organisms we isolated new chlorophylia related compounds, chlorophyllone a (l),') chlorophyllonelactone a (2)2) and chlorophyllonic acid a methyl ester (3), as antioxidants in the dark from the short-necked clam. In this paper, ,we wish to describe the_structure determination of 3. Compound 3 was obtained as a brown crystal, HR-FABMS'm/z 563.2686 (MH+, +2.8 mmu for C34H3SN404). .By direct comparison of 1 H-NMR data of 33) with those of 2.4) 3 proved to have a chlorin ring system like 2 and a methoxy group (6 4.04) in place of the hydroxy group (6 5.86) of 2. 'f'he splitting pattern of H-7, -7a; -7a', -7b and -7b' of 2 and those of 3 are quite similar to each other. In the HMBC measurement of 3 long-range couplings from methoxy group (b 4.04) to carbonyl carbon (5 166.9) and from H-7b' to two other carbonyl carbon signals (5 196.9, 192) were observed. These results postulated the structure of 3 with a methoxy carbonyl and 1,2-diketone group as shown in Fig. 1. There. seems, however, to be very serious steric congestion between C-9 and C-IO carbonyls. The structure of 3 was confirmed by single crystal X-ray analysis,s) as shown in Fig. 2. The C-IO carbonyl of 3 is directed downward out of chlorin ring to reduce the steric congestion between the C-9 and C-IO carbonyls. Coupling constants of H-7a and H-7b and H-7a' and H-7b' in the lH-NMR of 3 in the solution exhibited eclipsed conformation of all these protons-the same conformation seen in the crystal structure. Compound 3 was confirmed to be present in the extracts of scallop and oyster, too. Compound 3 (3 up) exhibited stronger antioxidative activity than a-tocopherol (20 ug) by a slightly modified thiocyanate method.@