lower absorption of light by Fe(III)–heme
+ here. A similar anomaly was seen at the
second harmonic (532 nm) of the pump laser (vide supra). If this anomalous peak is
ignored, it can be seen that the complex displays a broad absorption band with a
maximum at 379 nm and a pronounced shoulder on the high energy side. This
shoulder is likely due to charge-transfer transitions. In relation to the bare
Fe(III)–heme
+ ion, the maximum of the Soret band for Fe(III)–heme
+
(His) is at
approximately the same position but the band has broadened considerably and
displays the aforementioned high-energy shoulder. The broadening is likely due
to the different electronic structures of the porphyrin in the two molecules: In heme
the iron occupies the central hole of the porphyrin ring (planar heme), while in
Fe(III)–heme
+
(His) it is displaced out of the plane by 0.28 A ˚ to maximise chemical
bonding with the histidine (Fig. 7.8c) [27].
The spectrum of the adduct is similar in shape to that of 5c high-spin
Fe(III)–heme in metmyoglobin variants in which the distal histidine is replaced by
a nonpolar residue. The latter prevents water from taking up the available sixth
coordinate site [27, 28]. However, such protein variants display maximum absorption at about 395 nm, which is a redshift of 16 nm compared to the isolated complex.
Likewise, the adduct between 1,2-dimethyl-imidazole and ferric heme embedded in
SDS (sodium dodecyl sulfate) micelles has a maximum at 400 nm, which is again
significantly redshifted compared to the isolated Fe(III)–heme
+ (His) [29]. The
environment within the micelle mimics the hydrophobic cavity of heme proteins.
In other work, Nonose et al. [23] measured the absorption by the adduct between
Fe(III)–heme
+ and DMSO (dimethylsulfoxide) in the visible light region and found
a broad absorption band at ~500 nm (Fig. 7.6c). While the position of the band is
similar to that of Fe(III)–heme(His)
+ , the relative intensities of absorption in the Q
band to that at higher wavelengths are different.
Excitations at wavelengths higher than in the Q-band region are, as mentioned
previously, ascribed to porphyrin π-iron charge-transfer transitions, likely
transitions to the iron e g (d xy ,d yz ) orbitals [4, 24]. For Fe(III) there are three possible
spin states; 1/2, 3/2 and 5/2. A comparison of the gas-phase Fe(III)–heme
+ and
Fe(III)–heme
+
(His) data to spectra for myoglobin indicates that the iron is not high
spin [30]. The similarities between the two spectra indicate that isolated 4c and 5c
ferric heme ions are in the same spin state. This is consistent with quantumchemical modelling which find that the electronic ground states of both
Fe(III)–heme
+ and Fe(III)–heme
+ (His) are quartet states [31–34].
In a recent study Dugourd and co-workers [26] produced the entire ferricytochrome c protein in the gas phase and recorded its absorption spectrum in the
Soret-band region by monitoring the electron photodetachment yield of cytochrome
c
6À from measurements of the production of cytochrome c
5À (Fig. 7.9). Maximal
absorption was found to be at 410 nm, which is similar to the position of the band
for the native cytochrome c in solution. Thus, the protein pocket induced a shift of
30 nm from that of the bare Fe(III)–heme
+ in vacuo. As one histidine residue
broadened the absorption spectrum but did not induce a significant shift, the authors
conclude that the entire protein pocket is important in restoring the optical
properties of the heme protein. Furthermore, their results suggest that the protein
environments of the heme group are similar in both solution and gas phase, and that
7 Spectroscopy of Ferric Heme and Protoporphyrin IX Ions In Vacuo
125
+ here. A similar anomaly was seen at the
second harmonic (532 nm) of the pump laser (vide supra). If this anomalous peak is
ignored, it can be seen that the complex displays a broad absorption band with a
maximum at 379 nm and a pronounced shoulder on the high energy side. This
shoulder is likely due to charge-transfer transitions. In relation to the bare
Fe(III)–heme
+ ion, the maximum of the Soret band for Fe(III)–heme
+
(His) is at
approximately the same position but the band has broadened considerably and
displays the aforementioned high-energy shoulder. The broadening is likely due
to the different electronic structures of the porphyrin in the two molecules: In heme
the iron occupies the central hole of the porphyrin ring (planar heme), while in
Fe(III)–heme
+
(His) it is displaced out of the plane by 0.28 A ˚ to maximise chemical
bonding with the histidine (Fig. 7.8c) [27].
The spectrum of the adduct is similar in shape to that of 5c high-spin
Fe(III)–heme in metmyoglobin variants in which the distal histidine is replaced by
a nonpolar residue. The latter prevents water from taking up the available sixth
coordinate site [27, 28]. However, such protein variants display maximum absorption at about 395 nm, which is a redshift of 16 nm compared to the isolated complex.
Likewise, the adduct between 1,2-dimethyl-imidazole and ferric heme embedded in
SDS (sodium dodecyl sulfate) micelles has a maximum at 400 nm, which is again
significantly redshifted compared to the isolated Fe(III)–heme
+ (His) [29]. The
environment within the micelle mimics the hydrophobic cavity of heme proteins.
In other work, Nonose et al. [23] measured the absorption by the adduct between
Fe(III)–heme
+ and DMSO (dimethylsulfoxide) in the visible light region and found
a broad absorption band at ~500 nm (Fig. 7.6c). While the position of the band is
similar to that of Fe(III)–heme(His)
+ , the relative intensities of absorption in the Q
band to that at higher wavelengths are different.
Excitations at wavelengths higher than in the Q-band region are, as mentioned
previously, ascribed to porphyrin π-iron charge-transfer transitions, likely
transitions to the iron e g (d xy ,d yz ) orbitals [4, 24]. For Fe(III) there are three possible
spin states; 1/2, 3/2 and 5/2. A comparison of the gas-phase Fe(III)–heme
+ and
Fe(III)–heme
+
(His) data to spectra for myoglobin indicates that the iron is not high
spin [30]. The similarities between the two spectra indicate that isolated 4c and 5c
ferric heme ions are in the same spin state. This is consistent with quantumchemical modelling which find that the electronic ground states of both
Fe(III)–heme
+ and Fe(III)–heme
+ (His) are quartet states [31–34].
In a recent study Dugourd and co-workers [26] produced the entire ferricytochrome c protein in the gas phase and recorded its absorption spectrum in the
Soret-band region by monitoring the electron photodetachment yield of cytochrome
c
6À from measurements of the production of cytochrome c
5À (Fig. 7.9). Maximal
absorption was found to be at 410 nm, which is similar to the position of the band
for the native cytochrome c in solution. Thus, the protein pocket induced a shift of
30 nm from that of the bare Fe(III)–heme
+ in vacuo. As one histidine residue
broadened the absorption spectrum but did not induce a significant shift, the authors
conclude that the entire protein pocket is important in restoring the optical
properties of the heme protein. Furthermore, their results suggest that the protein
environments of the heme group are similar in both solution and gas phase, and that
7 Spectroscopy of Ferric Heme and Protoporphyrin IX Ions In Vacuo
125