The optical properties of P450 have been investigated by means
of polarized absorption spectroscopy of single crystals of camphor-
bound P450CAM in the oxidized, reduced, and CO-reduced states,
and iterative extended Ruckel (IEH) calculations. The heme chromophores
are orientated such that transitions polarized in the heme
plane (x,y-polarized) can be readily distinguished from transitions
polarized perpendicular to the heme plane (z-polarized) . High spin
oxidized P450 exhibits two broad z-polarized bands, at 567 and
323 nm. IEH calculations suggest that these bands arise from
cysteine mercaptide sulfur-to-iron charge transfer transitions.
High spin reduced P450 has no z-polarized bands. IEH calculations
suggest that loss of these bands occurs because the cysteine sulfur
is protonated to a mercaptan. Low spin CO-P450 has an intense
x,y-polarized band at 363 nm. This transition, assigned as a mercaptide
sulfur-to-porphyrin charge transfer transition, has the
correct symmetry to mix with the Soret and may cause the
anomalous red shift of the Soret
