experiments for ferrous heme (Fig. 7.20). Ferrous heme can bind CO and O 2 in
addition to NO, and a comparison between ferric heme–NO and ferrous
heme–CO would be particularly interesting as they are isoelectronic. However,
to produce a charged species, which is needed for mass spectrometric
investigations, one or both of the carboxylic acid groups of ferrous heme have
to be deprotonated. This is of course not an obstacle since protoporphyrin IX
anions were easily brought into gas phase and subjected to investigations.
Gradually building up the microenvironment of both ferrous and ferric heme
0.0
0.2
0.4
0.6
0.8
1.0
Wavelength (nm)
650
700
750
800
850
900
950
0.0
0.2
0.4
0.6
0.8
200
300
400
500
600
0.0
0.2
0.4
0.6
0.8
1.0
c
b
a
Q
x (0,0)?
Yield of photoneutrals (arb. units)
Fig. 7.19 (a) Action
spectrum of ground-state PP
À
ions in vacuo [44]. In the Qband region the bands are
split due to the asymmetry in
the ring. Low statistics
prevented measurement of the
fourth Q band, but it was
estimated to be at ~630 nm
based on a vibronic splitting
of 1,350 cm
À1
. Band maxima
were observed at 291 nm,
375–386 nm, 499 nm, 535 nm
and 581 nm. (b) and (c) Twolaser experiment: Action
spectra of photoexcited PP
anions obtained from the
yield of photoneutrals due to
the probe laser pulse as a
function of probe wavelength
[45]. The pump laser was set
to produce light at 535 nm.
Time delays between the
pump and probe were (a)
0.19 ms and (b) 0.57 ms
7 Spectroscopy of Ferric Heme and Protoporphyrin IX Ions In Vacuo
135