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 . 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.