| | |
Summary: 272 J. Am. Chem. SOC.1987, 109, 272-273
from the nature of the lone spin-containing d orbital.
The fact that the pyrrole substituent shifts exhibit primarily
K contact shifts is evidenced by the similar magnitude of the
downfield methyl and upfield pyrrole H shifts in deuterohemin
reconstituted HRPCN'O as well as by the downfield 4-H, and
upfield 4-H, shifts in HRPCN.'O The large asymmetry in the
hyperfine shifts among the four pyrroles, however, is considerably
greater than could conceivably arise from only rhombic dipolar
shifts and arises from the fact that the spin-containing d orbital
in HRPCN is primarily d,, in character.I0 This predominantly
dyzcan interact with the K orbitals of only pyrroles I1and IV. Thus
the increase in K contact shifts for two pyrroles and the decrease
in the shifts for the other two pyrroles, as compared to a model
complex,is observed in every low-spincyanide derivativeof a ferric
hemoprotein and has been proposed to arise from K bonding with
the axial histidyl imidazole, as discussed p r e v i o u ~ l y . ~ ~ ~ ~ ~In
HRPCN, this indicates that the axial imidazole plane is in the
xz plane, as depicted schematically in Figure 1. In order that
the spin-containing d orbital be primarily dyr,this orbital must
|
