respect to UV-induced reactions and the highly efficient repair mechanisms for DNA
damage. In the prebiotic era, the protective stratospheric ozone layer had not yet
developed, leading to high UV radiation flux on the surface of the young Earth. The
high photostability of DNA and RNA must have been a key factor for the evolution of
life and is in fact one of the reasons why DNA can be used to encode genetic
information in the first place. It is the nucleobases (adenine, thymine, cytosine,
guanine, and uracil) that absorb the UV light transmitted through the atmosphere,
so it comes as no surprise that their photophysics has been studied extensively.
Indeed, it has been realised that the particular chemical structures of DNA bases
serve several roles: They account beautifully for DNA replication, RNA transcription
and translation according to the specific pairing of bases (Watson-Crick base pairs)
and the genetic code, and they are the reason for the extraordinary photostability of
DNA, protecting against UV induced damage [1, 2]. Still, photoinduced modification
of the bases through photodamage can lead to mutations which ultimately may
compromise cell function and result in cell death or cancer.
Single bases absorb most strongly around 260 nm. The corresponding electronic
transitions are due to excitations into π* orbitals from either π orbitals (denoted as
ππ* transitions) or non-bonding orbitals (denoted as nπ* transitions) [3–5]. The
oscillator strength for ππ* transitions is significantly higher than that for nπ*
transitions. Relaxation of excited electronic states can occur by internal conversion
(IC) via conical intersections (where electronic potential energy surfaces touch)
within a few hundred femtoseconds [1, 2, 6]. Consequently, fluorescence quantum
yields are very small, 10
À4 or less, and intersystem crossing to lower-lying triplet
states is negligible [7]. Initially, vibrational energy resulting from the conversion of
electronic excitation is deposited in the base, but intramolecular vibrational redistribution (IVR) is assumed to occur rapidly, followed by dissipation of the excess
energy to surrounding water molecules within a few picoseconds. This scheme
protects the molecules as the electronic excitation that generates transient reactive
species is (mostly) converted to harmless heat.
The situation is more complicated for multichromophore DNA and RNA
strands, where each base is connected to a furanose sugar and two sugars are linked
by a negatively-charged phosphate group. The building blocks, each of which is
composed of (at least) a nucleobase, a sugar and a phosphate group, are called
nucleotides. In double strands, the distance between two adjacent bases is short
(3.4 A ˚ ), and their aromatic rings are parallel, which leads to the interaction of their
π-electron systems. Because the stacked bases are electronically coupled when
photoexcited, deexcitation times can span tens of picoseconds to nanoseconds
[8–10], and the energy is therefore not always distributed rapidly to the surrounding
medium. The characteristics of the bright excited states, such as the number of
bases that are collectively excited and the quantum dynamics involving dark
charge-transfer states, have been much debated (see [11] and references therein
for examples). The interbase electronic coupling depends on conformation and
environmental perturbations, which makes theoretical descriptions inherently complicated. Whether such superposition states are actually of importance in the
context of photoreactivity of nucleic acids and their protection from radiation
damage is still an open question.
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J.M. Weber et al.
damage. In the prebiotic era, the protective stratospheric ozone layer had not yet
developed, leading to high UV radiation flux on the surface of the young Earth. The
high photostability of DNA and RNA must have been a key factor for the evolution of
life and is in fact one of the reasons why DNA can be used to encode genetic
information in the first place. It is the nucleobases (adenine, thymine, cytosine,
guanine, and uracil) that absorb the UV light transmitted through the atmosphere,
so it comes as no surprise that their photophysics has been studied extensively.
Indeed, it has been realised that the particular chemical structures of DNA bases
serve several roles: They account beautifully for DNA replication, RNA transcription
and translation according to the specific pairing of bases (Watson-Crick base pairs)
and the genetic code, and they are the reason for the extraordinary photostability of
DNA, protecting against UV induced damage [1, 2]. Still, photoinduced modification
of the bases through photodamage can lead to mutations which ultimately may
compromise cell function and result in cell death or cancer.
Single bases absorb most strongly around 260 nm. The corresponding electronic
transitions are due to excitations into π* orbitals from either π orbitals (denoted as
ππ* transitions) or non-bonding orbitals (denoted as nπ* transitions) [3–5]. The
oscillator strength for ππ* transitions is significantly higher than that for nπ*
transitions. Relaxation of excited electronic states can occur by internal conversion
(IC) via conical intersections (where electronic potential energy surfaces touch)
within a few hundred femtoseconds [1, 2, 6]. Consequently, fluorescence quantum
yields are very small, 10
À4 or less, and intersystem crossing to lower-lying triplet
states is negligible [7]. Initially, vibrational energy resulting from the conversion of
electronic excitation is deposited in the base, but intramolecular vibrational redistribution (IVR) is assumed to occur rapidly, followed by dissipation of the excess
energy to surrounding water molecules within a few picoseconds. This scheme
protects the molecules as the electronic excitation that generates transient reactive
species is (mostly) converted to harmless heat.
The situation is more complicated for multichromophore DNA and RNA
strands, where each base is connected to a furanose sugar and two sugars are linked
by a negatively-charged phosphate group. The building blocks, each of which is
composed of (at least) a nucleobase, a sugar and a phosphate group, are called
nucleotides. In double strands, the distance between two adjacent bases is short
(3.4 A ˚ ), and their aromatic rings are parallel, which leads to the interaction of their
π-electron systems. Because the stacked bases are electronically coupled when
photoexcited, deexcitation times can span tens of picoseconds to nanoseconds
[8–10], and the energy is therefore not always distributed rapidly to the surrounding
medium. The characteristics of the bright excited states, such as the number of
bases that are collectively excited and the quantum dynamics involving dark
charge-transfer states, have been much debated (see [11] and references therein
for examples). The interbase electronic coupling depends on conformation and
environmental perturbations, which makes theoretical descriptions inherently complicated. Whether such superposition states are actually of importance in the
context of photoreactivity of nucleic acids and their protection from radiation
damage is still an open question.
182
J.M. Weber et al.