A relevant question is whether dissociation can compete with heat dissipation
when the heme is located within a protein cavity. In the case of horse heart
cytochrome c, Negrerie et al. [40] reported the time constant for vibrational cooling
to be ca. 4 ps based on time-resolved spectroscopy. Furthermore, no photodissociation of axial ligands took place in agreement with the long timescale for breakup of
the gas-phase Fe(III)–heme
+
(His) complex. Thus, from a heme protein point of
view, the heme is highly photostable.
Finally, the photodissociation mass spectrum of Fe(III)–heme
+
(NO) recorded at
an accelerator mass spectrum is shown in Fig. 7.15. The by far dominant reaction
channel was loss of NO in agreement with a 1 eV breakup energy of the complex,
and there was no indication of the formation of NO
+
. That NO
+ was not observed is
in accordance with a crossing after excitation from the Fe(II)–NO
+ electronic
ground state to the nearby Fe(III)–NO (radical) state that is dissociative along the
Fe–NO coordinate, which accounts for the weak binding of NO to ferric heme [9,
41]. From a spectroscopic point of view it appears as if NO is bound strongly to the
iron (strong σ bond between Fe
3+ and N), while in reality it is thermodynamically
weakly bound!
In very recent work, Park et al. [42] showed based on time-resolved vibrational
spectroscopy of ferric Mb(NO) that the majority of proteins (86 %) undergoes NO
deligation immediately after photoexcitation in the Q band. This can proceed faster
than 100 fs [42, 43]. The remaining 14 % of the population returns to the ground
state and thermally relaxes without NO deligation [43]. For the gas-phase complex
Fig. 7.12 Binding of
cysteine to iron (a) in an
iron–thiolate structure and (b)
in its canonical form. (c)
Binding of methionine to iron
Table 7.1 Band maxima for Fe(III)–porphyrin, ferric heme ions and complexes
Soret band (nm)
Q 1 band (nm)
Q 0 band (nm)
Reference
4c
(TipsiPP)]
+ [CB 11 H 6 Br 6 ]
À
400
505
[20]
heme
+ (room temp)
380
497
524
[21, 22]
heme
+ (16 K)
378
–
[25]
5c
heme
+ (His)
379
497
524
[21, 22]
hemin
+ (DMSO)
–
500
[23]
cytochrome c
410
–
[26]
heme
+ (NO)
–
523
561
[14]
6c
heme
+ (Met)(NO)
–
524
563
[14]
7 Spectroscopy of Ferric Heme and Protoporphyrin IX Ions In Vacuo
129
when the heme is located within a protein cavity. In the case of horse heart
cytochrome c, Negrerie et al. [40] reported the time constant for vibrational cooling
to be ca. 4 ps based on time-resolved spectroscopy. Furthermore, no photodissociation of axial ligands took place in agreement with the long timescale for breakup of
the gas-phase Fe(III)–heme
+
(His) complex. Thus, from a heme protein point of
view, the heme is highly photostable.
Finally, the photodissociation mass spectrum of Fe(III)–heme
+
(NO) recorded at
an accelerator mass spectrum is shown in Fig. 7.15. The by far dominant reaction
channel was loss of NO in agreement with a 1 eV breakup energy of the complex,
and there was no indication of the formation of NO
+
. That NO
+ was not observed is
in accordance with a crossing after excitation from the Fe(II)–NO
+ electronic
ground state to the nearby Fe(III)–NO (radical) state that is dissociative along the
Fe–NO coordinate, which accounts for the weak binding of NO to ferric heme [9,
41]. From a spectroscopic point of view it appears as if NO is bound strongly to the
iron (strong σ bond between Fe
3+ and N), while in reality it is thermodynamically
weakly bound!
In very recent work, Park et al. [42] showed based on time-resolved vibrational
spectroscopy of ferric Mb(NO) that the majority of proteins (86 %) undergoes NO
deligation immediately after photoexcitation in the Q band. This can proceed faster
than 100 fs [42, 43]. The remaining 14 % of the population returns to the ground
state and thermally relaxes without NO deligation [43]. For the gas-phase complex
Fig. 7.12 Binding of
cysteine to iron (a) in an
iron–thiolate structure and (b)
in its canonical form. (c)
Binding of methionine to iron
Table 7.1 Band maxima for Fe(III)–porphyrin, ferric heme ions and complexes
Soret band (nm)
Q 1 band (nm)
Q 0 band (nm)
Reference
4c
(TipsiPP)]
+ [CB 11 H 6 Br 6 ]
À
400
505
[20]
heme
+ (room temp)
380
497
524
[21, 22]
heme
+ (16 K)
378
–
[25]
5c
heme
+ (His)
379
497
524
[21, 22]
hemin
+ (DMSO)
–
500
[23]
cytochrome c
410
–
[26]
heme
+ (NO)
–
523
561
[14]
6c
heme
+ (Met)(NO)
–
524
563
[14]
7 Spectroscopy of Ferric Heme and Protoporphyrin IX Ions In Vacuo
129