Slices of unripe avocado fruit placed in 2H2O (45 atom %) synthesized abscisic acid enriched 18.7% with 2H at C-5′. 500 MHz 1H NMR showed that the signal of the axial, 5′-pro-S H atom was decreased by 20%. The signal of the uncoupled, equatorial H atom was 9 Hz upfield owing to shielding from its geminal 2H partner, the reciprocal shielding was 8.9 Hz. The H+ added during cyclization, therefore, is at the C-5′-pro-S position of ABA. Unlabelled ABA leaked into the medium, the newly synthesized, 2H labelled ABA was retained in the tissue.
- H shielding
- abscisic acid