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  • Title: Photodissociation of isolated ferric heme and heme-His cations in an electrostatic ion storage ring.
    Author: Lykkegaard MK, Zettergren H, Kirketerp MB, Ehlerding A, Wyer JA, Kadhane U, Brøndsted Nielsen S.
    Journal: J Phys Chem A; 2009 Feb 26; 113(8):1440-4. PubMed ID: 19182963.
    Abstract:
    Photodissociation of isolated Fe(III)-heme(+) and Fe(III)-heme(+)(His) ions in the gas phase has been investigated using an electrostatic storage ring. The experiment provides three pieces of information, namely fragmentation channels, dissociation times, and absorption spectra. After photoexcitation with either 390 or 532 nm light, we find that the fragmentation takes place on a microsecond to millisecond time scale, and the channels are CH(2)COOH loss (beta-cleavage reaction) and histidine loss from Fe(III)-heme(+) and Fe(III)-heme(+)(His), respectively. These channels were also observed by means of collision-induced dissociation. Significant information on the nonradiative processes that occur after photoexcitation was revealed from the decay spectra. At early times (first two to three milliseconds), the decay of the photoexcited ions is well-described by a statistical model based on an Arrhenius-type expression for the rate constant. The activation energy and preexponential factor are 1.9 +/- 0.2 eV and 1 x 10(17) to 1 x 10(21) s(-1) for heme(+) and 1.4 +/- 0.2 eV and 1 x 10(16) to 1 x 10(19) s(-1) for heme(+)(His). Decay on a longer time scale was also observed and is ascribed to the population of lower-lying states with higher spin multiplicity because intersystem crossing back to the electronic ground-state is a bottleneck for the dissociation. The measurements give lifetimes for these lower-lying states of about 10 ms after 390 nm excitation and we estimate the probability of spin flip to be 0.3 and 0.8 for heme(+) and heme(+)(His), respectively.
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