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.