belonging to the same parent ion in most cases [26–28]. Part of the spectral
congestion observed is certainly due to the parent ions being at room temperature
[63] but if the relaxation of the electronically excited state proceeds at subpicosecond time scales [1, 18], lifetime broadening could play a role as well.
With these spectra at hand, it is now possible to elucidate the role of basestacking effects from gas-phase spectroscopy of DNA strands. Brøndsted Nielsen
and co-workers [19] studied small DNA homopolymers of adenine, and their
fragmentation induced by photon absorption was monitored from the yield of
neutral fragments formed as a function of time. An example for the spectrum of
the [dA 4 –H]
À anion is shown in Fig. 10.8 together with that of the [dAMP–H]
À
mononucleotide. It is evident that the two have quite similar photodestruction
spectra. The maximum of the strand spectrum is slightly blueshifted by 3 nm
from the peak at 253 nm reported for [dAMP–H]
À anions, which is indicative of
exciton coupling between interacting adenine bases. However, there is no tail to the
red as predicted by Lange and Herbert [16]. It should be mentioned that the
interpretation of the experimental results is complicated by the fact that different
Fig. 10.7 Parent ion depletion spectra (open circles) and aqueous solution absorption spectra
(solid lines) of the DNA mononucleotides (the base is indicated in each panel). Depletion data
(in arbitrary units) have been normalised to the peaks of the aqueous solution absorption spectra in
each case for easier comparison. Reprinted from [26]. Copyright 2009, Royal Society of
Chemistry
10 UV Photophysics of DNA and RNA Nucleotides In Vacuo: Dissociation. . .
195
congestion observed is certainly due to the parent ions being at room temperature
[63] but if the relaxation of the electronically excited state proceeds at subpicosecond time scales [1, 18], lifetime broadening could play a role as well.
With these spectra at hand, it is now possible to elucidate the role of basestacking effects from gas-phase spectroscopy of DNA strands. Brøndsted Nielsen
and co-workers [19] studied small DNA homopolymers of adenine, and their
fragmentation induced by photon absorption was monitored from the yield of
neutral fragments formed as a function of time. An example for the spectrum of
the [dA 4 –H]
À anion is shown in Fig. 10.8 together with that of the [dAMP–H]
À
mononucleotide. It is evident that the two have quite similar photodestruction
spectra. The maximum of the strand spectrum is slightly blueshifted by 3 nm
from the peak at 253 nm reported for [dAMP–H]
À anions, which is indicative of
exciton coupling between interacting adenine bases. However, there is no tail to the
red as predicted by Lange and Herbert [16]. It should be mentioned that the
interpretation of the experimental results is complicated by the fact that different
Fig. 10.7 Parent ion depletion spectra (open circles) and aqueous solution absorption spectra
(solid lines) of the DNA mononucleotides (the base is indicated in each panel). Depletion data
(in arbitrary units) have been normalised to the peaks of the aqueous solution absorption spectra in
each case for easier comparison. Reprinted from [26]. Copyright 2009, Royal Society of
Chemistry
10 UV Photophysics of DNA and RNA Nucleotides In Vacuo: Dissociation. . .
195