7.3
Time Constants and Channels for Photo-induced
Dissociation
Dissociation of photoexcited Fe(III)–heme
+ and Fe(III)–heme
+ (His) ions occurs on
the microsecond to millisecond time scale with the main channels being ascribed to
loss of either one or two CH 2 COOH groups through β-cleavage, and loss of
histidine, respectively (see Figs. 7.13 and 7.14) [31]. The former reaction is
associated with a reaction energy of about 2 eV while histidine loss costs less
energy, only about 1.4 eV. In agreement with this, the dissociation of Fe(III)–heme
+
was found to result from the absorption of either one or two photons, somewhat
dependent on the excitation wavelength, while that of Fe(III)–heme
+ (His) could be
accounted for purely by one-photon absorption. In this regard, the formation of the
m/z-498 fragment ion from heme
+ (loss of two CH 2 COOH groups) right after
photoexcitation is clear evidence of two-photon absorption as one visible photon
is not enough to cause loss of both CH 2 COOH groups (Fig. 7.14c).
0
1
2
3
4
5
0
1
2
3
4
5
Absorption (arb. units)
b
a
Protein absorption
450
500
550
600
650
700
Wavelength (nm)
Fig. 7.11 (a) Action spectra
of 4c Fe(III)–heme
+ (green
line) [22], and Fe
(III)–heme
+ (NO) (red solid
circles) [14]. (b) Action
spectra of Fe
(III)–heme
+ (Met)(NO) (blue
circles) and Fe
(III)–heme
+ (Cys)(NO) (green
diamonds) [14]. The action
spectrum of Fe
(III)–heme
+ (NO) (red line)
and the range most measured
proteins absorb in
(560–572 nm, proximal
histidine ligand) are also
included in (b) for
comparative purposes
128
J.A. Wyer and S.B. Nielsen