ASSIGNMENT BY 2D NMR BOND CORRELATION SPECTROSCOPY OF HYPERFINE-SHIFTED AND STRONGLY RELAXED PROTONS IN IRON PORPHYRIN AND CHLORIN COMPLEXES

Two-dimensional magnitude COSY experiments have been carried out on a variety of paramagnetic iron complexes of porphyrins and chlorins, including the ferric low-spin and both ferric and ferrous high-spin states. Numerous cross peaks that reflect spin connectivity between resonances with line widths to 150 Hz are clearly detectable. For low-spin ferric complexes of pyropheophorbide a, essentially all spin connectivities that would bc found in a diamagnetic analogue are observed. In the high-spin ferric complexes of both protoporphyrin IX and pyropheophorbide a, COSY cross peaks provide assignments for pairs of protons with expected coupling constants > 5 Hz. While the cross-peak intensities in the high-spin ferric complexes are strongly diminished relative to low-spin ferric systems due to both relaxation and antiphase cancellation, the repetition rates of the experiment can bc significantly increased due to the shorter relaxation times. This compensation allows the detection of cross peaks within a reasonable time. In the high-spin ferrous complexes of N-methylporphines, the COSY data indicate that the spin density distribution is as asymmetric within a pyrrole as it is between the nonequivalent pyrroles. The surprising success of magnitude COSY experiments with the -worst case scenario- high-spin ferric systems indicates that 2D COSY can be routinely incorporated into the assignment strategies for paramagnetic iron tetrapyrrole complexes.