Use of the nuclear Overhauser effect to assign 1H NMR resonances in a low-spin paramagnetic hemin

The nuclear Overhauser effect has been used to assign the 1H resonances of the paramagnetic low-spin biscyano complex of Fe(III) protoporphyrin. When the meso protons are irradiated, changes in integrated signal intensity are seen at neighboring methyl or methylene groups and vice versa. Although the changes ar small (less than 1% negative NOEs for Fe(III)protoporphyrin(CN)2 in Me2SO-d6 at 30 degrees C and 360 MHz), they can be seen clearly. This technique has been used to assign the 6-alpha-CH2 (6.21), 7-alpha-CH2 (5.82), beta-meso (0.50) and delta-meso (0.03 ppm) resonances of this species. The nuclear Overhauser effect will allow rapid assignment of 1H NMR resonances in a wide variety of low-spin paramagnetic hemins.