The Role of Substituent Functionality in the Photophysics of Model Biological Systems

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Over the course of billions of years of evolution, nature has selected specific molecules for use as the 'building blocks' of life. One important example of this may be found in DNA, where the sugar-phosphate linkages that constitute the backbone of the now famous double helix structure are held together by the interactions between just four molecules: Adenine (A), thymine (T), guanine (G) and cytosine (C) - collectively known as the DNA bases. It is therefore an important question to ask what is inherently special about these specific molecules that has given rise to their preferred use over all others. One key idea centres on the issue of photostability: In the early years of life on Earth there was no ozone layer to protect simple organisms from potentially damaging ultraviolet radiation. It has therefore been postulated that there must be rapid and efficient mechanisms for the dissipation of excess absorbed energy in these systems and that this provides an element of in-built 'self-protection' that has proved vital in evolutionary selection. More specifically, the dissipation process proceeds via an internal coupling between the electronic and vibrational degrees of freedom, and this is then followed by the subsequent intermolecular transfer of excess energy into the surrounding local environment. This initial steps in this process occur on so-called 'ultrafast' timescales on the order of a few hundred femtoseconds (1 femtosecond = 10^-15 s). The use of laser pulses with similar temporal durations allows one to follow the evolution of these processes in real time as they unfold. The aim of this proposal is to investigate the first steps of the energy dissipation process in a series of synthetically prepared model DNA systems using ultrafast laser pulses in conjunction with photoelectron spectroscopy techniques. The highly detailed measurements which are possible with this methodology, along with the novel stepwise approach we will use in building up the complexity of the model systems under study, will offer a new level insight into the photoresistive mechanisms present in this fundamentally important class of molecules. Self protection mechanisms within DNA represent the body's last line of defence against ultraviolet radiation. However, the first-line of defence against such radiation is provided by pigmentation molecules known as eumelanins, which are found in the skin, hair and retina. Although eumelanins are complex biopolymers, it is interesting to note that the basic constituent units of these systems have a striking structural resemblance to some of the absorption sites within DNA. It has therefore been postulated that the mechanisms for dissipating excess energy in eumelanin are similar to those within DNA. A detailed series of ultrafast laser experiments will be carried out in order to investigate this assertion further. In particular, evidence for the role of pathways recently predicted by theory will be sought.