The electronic structure calculations performed on each of the systems
listed in Table 
yield charge- and spin-density
distributions for the ground states. Examples of these can be seen for
ligand-free and substrate-bound systems in Figures
and respectively. From these
figures we can see that the spin density is localised near to the haem
iron and that the spin density in the substrate-bound case is
significantly higher than that in the ligand-free system. Spin
population analysis of the results of the electronic structure
calculations gives a quantitative measurement of the spin and charge
on the iron for these systems and the other systems studied. The
results of these analyses are presented in Table . It
should be noted that these results relate only to the ground state of
each system and do not include any thermodynamic or entropic
effects. It should also be noted that the charge on an 
ion
embedded in a porphyrin ring is expected to be approximately
1e. This compares favourably with the charge of 1.10e calculated
for the ligand-free system.
Figure: Calculated charge- (left) and spin-densities
of the ligand-free active site of 
. Charge isosurfaces
are shown for 
, 
and 
. A spin
density isosurface is shown at 
. Carbon atoms are coloured green, oxygen red, nitrogen
light blue, sulphur yellow and iron dark blue. Hydrogen atoms have
been omitted.
Figure: Calculated charge-
(left) and spin-densities of the camphor-bound (substrate-bound)
active site of . Charge isosurfaces are shown as in
Figure . Spin density isosurface are shown at 
and . Atom species are represented as in
Figure .
Table: Calculated ground state spins and charges, spin equilibrium
constants ( 
) and high-spin fractions for P450 systems
studied. In some cases multiple values have been reported for the spin
equilibrium constants resulting in a range of possible HS
fractions. No spin equilibrium data was available for the
metyrapone-bound system.
Figure shows a graph of the fraction of HS component
against the calculated ground-state spin on the ion for the
ligand-free, substrate-bound and substrate analogue-bound
systems. This shows an excellent correlation between the calculated
ground-state iron spin and the spin equilibrium. This indicates that
the ab initio approach accurately predicts the spin state of the
iron in the P450 system. The results also demonstrate that the
majority of the difference in the spin state between complexes can be
accounted for by the difference in the ground state spin
distributions.
Figure: Graph of fraction of HS character against calculated
spin. Where more than one value for the fraction of HS has been
reported, a range of possible values has been plotted.
The spin on the haem iron in the inhibitor-bound system is expected to
be less than that for the ligand-free case, as this would explain the
type II spectral change on binding of the inhibitor
[131]. While the results show that the ground state is LS in
character, the spin on the iron is found to be higher than that in the
ligand-free system and of a similar magnitude to that found in the
substrate analogue-bound complexes. The difference in the HS fraction
seen in experiments may be due to thermodynamic
mixing of high spin states which are accessible to the ligand-free and
substrate analogue-bound systems, but not in the inhibitor bound
case. For example, the dissociation of the water molecule or inhibitor
from the ion would make the HS state more favourable. However, the
dissociation of the water is probably much more likely than the
dissociation of the inhibitor. This would lead to a larger proportion
of high spin states in thermodynamic equilibrium for the ligand-free
and substrate-analogue bound cases than in the inhibitor-bound case,
even if the ground state spins were similar.
