was based on comparisons of the total ion beam depletion with a statistical decay
model. The total beam depletion for the anions was essentially explained by
completely statistical decay, while for the cations the ion beam depletion due to
statistical decay was less than 1 % of the total. Similarly, for singly-protonated
DNA dinucleotides (dA 2 , dG 2 , and dC 2 ) where light is absorbed by either a
protonated or a neutral base, only 2–3 % of the ions decayed statistically after
260-nm photoexcitation [21]. Interestingly, in the case of protonated dT 2 , the
propensity of statistical decay is higher (10 %), which is attributed to the lower
proton affinity of thymine compared to the other bases, and that the phosphoric acid
group can compete with the thymine base for the excess proton. In this picture, a
protonated-base nucleotide is more prone to prompt dissociation than a neutral one.
For deprotonated dinucleotides (dA 2 and dT 2 ) where both bases are neutral, about
20 % of the decay could be accounted for by statistical dissociation, which is much
less than that seen for the mononucleotide. The reason for this is unclear.
Experiments were also carried out on a series of singly-protonated DNA
homopolymers of adenine (dA 2 , dA 3 , dA 4 , and dA 5 ) using 266-nm light [19].
Here the importance of nonstatistical dissociation decreased with the size of the
oligonucleotide or, in other words, with the number of neutral adenines. Taken
together, the overall picture is that photoexcitation of neutral bases causes more
dissociation on a long time scale (microseconds) than photoexcitation of protonated
bases does.
Additionally, we note that calculations by Brøndsted Nielsen and Sølling [18] on
neutral and protonated adenine indicate that the conical intersection between the S 1
and S 0 states reached after out-of-plane deformation is located significantly lower
in energy for the cation than for the neutral, which may provide an explanation for
the very different photodissociation chemistry of nucleotide anions and cations.
Femtosecond pump-probe experiments by Nolting et al. [51] on protonated adenine, monitoring fragment ion formation as a function of time, revealed that internal
conversion takes place in less than 161 fs for protonated adenine, but that it takes
place on longer time scales (picoseconds) for neutral adenine, as reported by others
[52, 53]. This difference was ascribed to the intermediate nπ* state that is spectroscopically not accessible for the cation, and that traps the excited-state population in
the neutral adenine case. It is possible that the lack of this intermediate trapping
state leads to population of a repulsive electronic curve, which could explain the
increase in non-statistical decay with the size of the nucleotide [19]. However, this
is a hypothesis that remains to be tested more thoroughly.
To further explore the occurrence of rapid dissociation processes, Andersen and
co-workers in Aarhus [31] and Fayeton and co-workers in Orsay [29] performed
photoexcitation experiments on AMP and dAMP nucleotide anions, respectively, in
an electric field and using time-of-flight mass spectrometry. After light absorption,
the ions would spend up to one hundred nanoseconds in the electric field. Fragment
ions formed within this field have different energies and velocities than those
formed outside of the field. By simply measuring the arrival times of the neutral
fragments to the detector, the groups could disentangle rapid dissociation from
slow, statistical dissociation. The Orsay group observed, in addition to neutral
fragments, ionic fragments formed in coincidence [29]. An electrostatic analyser
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J.M. Weber et al.
model. The total beam depletion for the anions was essentially explained by
completely statistical decay, while for the cations the ion beam depletion due to
statistical decay was less than 1 % of the total. Similarly, for singly-protonated
DNA dinucleotides (dA 2 , dG 2 , and dC 2 ) where light is absorbed by either a
protonated or a neutral base, only 2–3 % of the ions decayed statistically after
260-nm photoexcitation [21]. Interestingly, in the case of protonated dT 2 , the
propensity of statistical decay is higher (10 %), which is attributed to the lower
proton affinity of thymine compared to the other bases, and that the phosphoric acid
group can compete with the thymine base for the excess proton. In this picture, a
protonated-base nucleotide is more prone to prompt dissociation than a neutral one.
For deprotonated dinucleotides (dA 2 and dT 2 ) where both bases are neutral, about
20 % of the decay could be accounted for by statistical dissociation, which is much
less than that seen for the mononucleotide. The reason for this is unclear.
Experiments were also carried out on a series of singly-protonated DNA
homopolymers of adenine (dA 2 , dA 3 , dA 4 , and dA 5 ) using 266-nm light [19].
Here the importance of nonstatistical dissociation decreased with the size of the
oligonucleotide or, in other words, with the number of neutral adenines. Taken
together, the overall picture is that photoexcitation of neutral bases causes more
dissociation on a long time scale (microseconds) than photoexcitation of protonated
bases does.
Additionally, we note that calculations by Brøndsted Nielsen and Sølling [18] on
neutral and protonated adenine indicate that the conical intersection between the S 1
and S 0 states reached after out-of-plane deformation is located significantly lower
in energy for the cation than for the neutral, which may provide an explanation for
the very different photodissociation chemistry of nucleotide anions and cations.
Femtosecond pump-probe experiments by Nolting et al. [51] on protonated adenine, monitoring fragment ion formation as a function of time, revealed that internal
conversion takes place in less than 161 fs for protonated adenine, but that it takes
place on longer time scales (picoseconds) for neutral adenine, as reported by others
[52, 53]. This difference was ascribed to the intermediate nπ* state that is spectroscopically not accessible for the cation, and that traps the excited-state population in
the neutral adenine case. It is possible that the lack of this intermediate trapping
state leads to population of a repulsive electronic curve, which could explain the
increase in non-statistical decay with the size of the nucleotide [19]. However, this
is a hypothesis that remains to be tested more thoroughly.
To further explore the occurrence of rapid dissociation processes, Andersen and
co-workers in Aarhus [31] and Fayeton and co-workers in Orsay [29] performed
photoexcitation experiments on AMP and dAMP nucleotide anions, respectively, in
an electric field and using time-of-flight mass spectrometry. After light absorption,
the ions would spend up to one hundred nanoseconds in the electric field. Fragment
ions formed within this field have different energies and velocities than those
formed outside of the field. By simply measuring the arrival times of the neutral
fragments to the detector, the groups could disentangle rapid dissociation from
slow, statistical dissociation. The Orsay group observed, in addition to neutral
fragments, ionic fragments formed in coincidence [29]. An electrostatic analyser
190
J.M. Weber et al.