Complete assignment of the carbon-13 NMR spectrum of chlorophyl a

The ¹³C NMR spectrum (at natural abundance) of monomeric chlorophyll α in acetone-d₆ has been recorded to re-examine the assignments of the low field (aromatic-olefinic) region of the spectrum. The assignments, made by the examination of the fully coupled spectrum and by the use of long-range selective ¹H decoupling (LSPD) with low-power irradiation, were compared with those of the previous reports. The results of the present work clarify the ambiguities previously encountered in the assignment of the 10a-ester, 7c-propionyl, P-2-phytyl, 2b-vinyl, γ- and β-methine carbon atoms, as well as the β-pyrrolic carbon-6 and α-pyrrolic carbons ?16 and ?17 of chlorophyll α. Reassignment of the three last carbons was found necessary. Knowledge of the chemical shifts of these carbon atoms was considered to be particularly valuable, as it yields relevant information on the delocalized π electron system which is crucial for the function of chlorophyll in photosynthesis.