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

在数十亿年的进化过程中，大自然选择了特定的分子作为生命的“基石”。一个重要的例子可以在DNA中找到，其中糖-磷酸键构成了现在著名的双螺旋结构的主干，通过四个分子之间的相互作用结合在一起：腺嘌呤(A)，胸腺嘧啶(T)，鸟嘌呤(G)和胞嘧啶(C) -统称为DNA碱基。因此，一个重要的问题是，这些特定的分子有什么内在的特殊之处，使它们比所有其他分子更受青睐。其中一个关键的观点集中在光稳定性问题上：在地球生命的早期，没有臭氧层来保护简单的有机体免受潜在的破坏性紫外线辐射。因此，我们假设，在这些系统中一定存在快速有效的机制来耗散多余的吸收能量，这提供了一种内在的“自我保护”元素，这在进化选择中已被证明是至关重要的。更具体地说，耗散过程是通过电子自由度和振动自由度之间的内部耦合进行的，然后是随后多余能量的分子间转移到周围的局部环境中。这个过程的最初步骤发生在所谓的“超快”时间尺度上，大约几百飞秒（1飞秒= 10^-15秒）。使用具有类似时间持续时间的激光脉冲，可以在这些过程展开时实时跟踪它们的演变。本提案的目的是研究利用超快激光脉冲结合光电子能谱技术在一系列合成的DNA模型系统中能量耗散过程的第一步。使用这种方法可以进行非常详细的测量，以及我们将用于建立所研究模型系统复杂性的新颖逐步方法，将为这类重要分子中存在的光敏机制提供一个新的层次的见解。DNA内的自我保护机制是人体抵御紫外线辐射的最后一道防线。然而，抵抗这种辐射的第一道防线是被称为真黑色素的色素沉着分子，它存在于皮肤、头发和视网膜中。虽然真黑素是复杂的生物聚合物，但有趣的是，这些系统的基本组成单位与DNA中的一些吸收位点具有惊人的结构相似性。因此，人们假设真黑素中消耗多余能量的机制与DNA中的机制相似。为了进一步研究这一论断，将进行一系列详细的超快激光实验。特别是，将寻求理论最近预测的途径的作用的证据。

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.