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G.M. Roberts and V.G. Stavros
timescale can provide unparalleled insights into the mechanisms underpinning a diverse range of fundamental processes in biology.
The examples mentioned above highlight how light induced biochemistry can be
essential for life. In contrast though, light’s interaction with living organisms can
also be highly detrimental. In particular, following photoexcitation with ultraviolet (UV) radiation, the electronically excited states of UV chromophores in DNA
(DNA bases) can potentially trigger ultrafast structural changes (termed photolesions), which modify or terminate its genetic function [7]. These UV triggered processes can often be the fundamental precursors to mutagenesis, carcinogenesis and
apoptosis [8]. Through evolution, nature has developed methods to combat these
potentially devastating UV induced photoreactions. Light triggered photolyase enzymes, for example, act as one of the final bastions of defense against UV photodamage, by actively repairing photolesion sites in DNA [9]. Fortunately, many of the
molecular building-blocks nature has selected for life, particularly the DNA bases,
exhibit a built-in resistance against UV triggered damage. For this reason, they are
commonly classed as photostable species. This photostable behavior is postulated
to act as one of the front-line defenses against photodamage, and is so efficient that
less than 1 % of UV induced excitation processes in DNA result in the formation
of photolesions [10]. Their photostability is believed to be intimately linked to how
efficiently electronically excited states can undergo non-radiative relaxation back to
the electronic ground state, transforming electronic energy into less harmful thermal (vibrational) energy, which can be dissipated into surrounding solvent. It is this
concept of photostability, and the role that specific excited electronic states play in
this behavior, which we explore further in this chapter.
6.2 Excited Electronic States and Photostability
With the themes discussed in Sect. 6.1 in mind, a large interest towards understanding the intrinsic photostability of isolated DNA bases and, more generally, molecular subunits of biomolecules (specifically UV chromophores of the DNA bases and
amino acids) has ensued. The ultimate vision of this work has been to link photostability to electronic structure. Excited electronic states, by and large, are much more
reactive than the ground state and proficiency for rapidly diffusing this harmful energy will invariably aid the photostability of the molecule. Over the last decade,
a multitude of studies have been carried out to elicit the details of photochemical
pathways that can facilitate photostable behavior. A full description of the numerous reaction pathways that have been identified to date is beyond the scope of this
chapter and for further comprehensive discussion the reader is referred to references
[10–13] (and references therein).
This chapter discusses the role of 1 πσ ∗ states in the excited state dynamics of
heteroaromatic biomolecules and their UV chromophore subunits. In the seminal
theoretical work by Sobolewski et al. [14], the potential role of 1 πσ ∗ states in photostability was postulated through ab initio electronic structure calculations. These
G.M. Roberts and V.G. Stavros
timescale can provide unparalleled insights into the mechanisms underpinning a diverse range of fundamental processes in biology.
The examples mentioned above highlight how light induced biochemistry can be
essential for life. In contrast though, light’s interaction with living organisms can
also be highly detrimental. In particular, following photoexcitation with ultraviolet (UV) radiation, the electronically excited states of UV chromophores in DNA
(DNA bases) can potentially trigger ultrafast structural changes (termed photolesions), which modify or terminate its genetic function [7]. These UV triggered processes can often be the fundamental precursors to mutagenesis, carcinogenesis and
apoptosis [8]. Through evolution, nature has developed methods to combat these
potentially devastating UV induced photoreactions. Light triggered photolyase enzymes, for example, act as one of the final bastions of defense against UV photodamage, by actively repairing photolesion sites in DNA [9]. Fortunately, many of the
molecular building-blocks nature has selected for life, particularly the DNA bases,
exhibit a built-in resistance against UV triggered damage. For this reason, they are
commonly classed as photostable species. This photostable behavior is postulated
to act as one of the front-line defenses against photodamage, and is so efficient that
less than 1 % of UV induced excitation processes in DNA result in the formation
of photolesions [10]. Their photostability is believed to be intimately linked to how
efficiently electronically excited states can undergo non-radiative relaxation back to
the electronic ground state, transforming electronic energy into less harmful thermal (vibrational) energy, which can be dissipated into surrounding solvent. It is this
concept of photostability, and the role that specific excited electronic states play in
this behavior, which we explore further in this chapter.
6.2 Excited Electronic States and Photostability
With the themes discussed in Sect. 6.1 in mind, a large interest towards understanding the intrinsic photostability of isolated DNA bases and, more generally, molecular subunits of biomolecules (specifically UV chromophores of the DNA bases and
amino acids) has ensued. The ultimate vision of this work has been to link photostability to electronic structure. Excited electronic states, by and large, are much more
reactive than the ground state and proficiency for rapidly diffusing this harmful energy will invariably aid the photostability of the molecule. Over the last decade,
a multitude of studies have been carried out to elicit the details of photochemical
pathways that can facilitate photostable behavior. A full description of the numerous reaction pathways that have been identified to date is beyond the scope of this
chapter and for further comprehensive discussion the reader is referred to references
[10–13] (and references therein).
This chapter discusses the role of 1 πσ ∗ states in the excited state dynamics of
heteroaromatic biomolecules and their UV chromophore subunits. In the seminal
theoretical work by Sobolewski et al. [14], the potential role of 1 πσ ∗ states in photostability was postulated through ab initio electronic structure calculations. These