NO orientation and tilting in (nitrosyl)iron(II) deuteroporphyrin IX.

To investigate issues concerning the coordination of the nitrosyl ligand in naturally occurring hemes, we report the spectroscopy and X-ray structure of five-coordinate [Fe(Deut)(NO)]. Bonding parameters are comparable with those observed for previously characterized synthetic porphyrin complexes of this type. The asymmetric pattern of the peripheral substitution of the porphyrin core allows us to examine aspects associated with ligand binding and orientation previously unobserved in the symmetrical synthetic porphyrins. The nitrosyl is found to be oriented in the direction of the less basic pyrrole rings. This observed orientation of the NO is considered in reference to those orientations reported in a series of related protein structures. Off-axis tilting, a property associated with ordered (nitrosyl)iron(II) porphyrinates, is also investigated.