Interestingly, the positive charge on NO when it coordinates to ferric heme
implies stronger interactions with the environment than those for the isoelectronic
Fe(II)–CO adduct. Thus, DFT modelling has revealed that the lone pair of a nearby
imidazole would be directed toward the NO with an interaction energy of 0.05 eV
[15]. Indeed, when the distal histidine in Mb is replaced by apolar residues, the Fe
(III)–NO dissociation rate has experimentally been found to increase seven to
tenfold [16]. The lone pair actually pushes electron density out of the NO π* orbital
and onto the porphyrin (Fig. 7.3) [17]. A structured water in the protein cavity can
display a similar effect. These electronic perturbations are likely to show up in the
electronic absorption spectrum, in the Soret and/or Q-band regions.
It is a great advantage and highly exploited fact that the absorption by heme
proteins strongly depends on the microenvironment of the heme, with spectral
features depending on the iron oxidation state, peripheral substituents, axial
ligands, coordination state, spin state, and nearby amino acid residues. Sample
absorption spectra are shown in Fig. 7.4 for metMb with and without NO bound to
the prosthetic ferric heme group. The spectral features change upon NO binding,
and both the Soret and Q bands redshift. The band between 600 nm and 650 nm seen
for metMb is ascribed to porphyrin π–iron(III) charge transfer transitions. It
disappears when NO is bound because the electron donation from NO to iron
reduces the formal oxidation state of the iron to +2. Thus, with appropriate
reference spectra, spectroscopy can provide important information on the environment of the heme. While some of the required reference spectra are easily obtainable, others are not as will be discussed next.
7.2
Spectroscopy of 4c Ferric Heme, Complexes with Amino
Acids and NO, and Proteins In Vacuo
Under certain conditions it has been speculated that 5-coordinated (5c) ferric heme
proteins undergo histidine deligation. Two possible pathways for lowering the
coordination state to four are acid-induced rupture of the iron–histidine bond due
to histidine protonation [19], or local heating after multiple photon absorption.
Fig. 7.3 The lone pair of a
distal imidazole (ImH) group
that is near to an Fe(II)–NO
+
heme adduct interacts with
the HOMO-6 of the latter.
This orbital interaction leads
to electron density being
pushed out of the NO π*
orbital and onto the
porphyrin. Reprinted from
[17], with permission from
Elsevier
120
J.A. Wyer and S.B. Nielsen
implies stronger interactions with the environment than those for the isoelectronic
Fe(II)–CO adduct. Thus, DFT modelling has revealed that the lone pair of a nearby
imidazole would be directed toward the NO with an interaction energy of 0.05 eV
[15]. Indeed, when the distal histidine in Mb is replaced by apolar residues, the Fe
(III)–NO dissociation rate has experimentally been found to increase seven to
tenfold [16]. The lone pair actually pushes electron density out of the NO π* orbital
and onto the porphyrin (Fig. 7.3) [17]. A structured water in the protein cavity can
display a similar effect. These electronic perturbations are likely to show up in the
electronic absorption spectrum, in the Soret and/or Q-band regions.
It is a great advantage and highly exploited fact that the absorption by heme
proteins strongly depends on the microenvironment of the heme, with spectral
features depending on the iron oxidation state, peripheral substituents, axial
ligands, coordination state, spin state, and nearby amino acid residues. Sample
absorption spectra are shown in Fig. 7.4 for metMb with and without NO bound to
the prosthetic ferric heme group. The spectral features change upon NO binding,
and both the Soret and Q bands redshift. The band between 600 nm and 650 nm seen
for metMb is ascribed to porphyrin π–iron(III) charge transfer transitions. It
disappears when NO is bound because the electron donation from NO to iron
reduces the formal oxidation state of the iron to +2. Thus, with appropriate
reference spectra, spectroscopy can provide important information on the environment of the heme. While some of the required reference spectra are easily obtainable, others are not as will be discussed next.
7.2
Spectroscopy of 4c Ferric Heme, Complexes with Amino
Acids and NO, and Proteins In Vacuo
Under certain conditions it has been speculated that 5-coordinated (5c) ferric heme
proteins undergo histidine deligation. Two possible pathways for lowering the
coordination state to four are acid-induced rupture of the iron–histidine bond due
to histidine protonation [19], or local heating after multiple photon absorption.
Fig. 7.3 The lone pair of a
distal imidazole (ImH) group
that is near to an Fe(II)–NO
+
heme adduct interacts with
the HOMO-6 of the latter.
This orbital interaction leads
to electron density being
pushed out of the NO π*
orbital and onto the
porphyrin. Reprinted from
[17], with permission from
Elsevier
120
J.A. Wyer and S.B. Nielsen