Ultrafast biomolecular dynamics

This thesis describes experimental studies of ultrafast charge transfer processes in molecules which are the building blocks of proteins and DNA; the amino acid phenylalanine and the nucleoside adenosine respectively. This was achieved by using femtosecond (10-15 and attosecond (10-18) laser pulses with a range of wavelengths to create a positive charge in these molecules through ionisation and to probe the subsequent electron motion. This work has been performed using gas phase molecular targets which were liberated from solid samples using a technique known as laser induced acoustic desorption (LlAD). An experimental investigation was performed supported by simulations undertaken using a radiation transport code, HYADES, to optimise and understand this technique. The results presented for the adenosine and the nucleobase adenine suggest that fragmentation of the sugar group following ionisation (which corresponds to strand breakages in DNA) is mostly via relatively slow dissociation processes (nano or microsecond timescales). In vivo such a slow statistical process is likely to be quenched by cooling of the vibrational energy in the molecule by the surrounding environment. This shows that, to a degree, cells can be protected from the direct action of ionising radiation in DNA. In phenylalanine two different ultrafast processes were observed. The first of these was charge transfer to the phenyl group with a time constant of 80 fs, attributed to transfer between electronic states. A second process lasting 30 fs was observed, which is consistent with a number of theoretical predictions of a purely electronic process, termed charge migration. This work provides the first experimental evidence for charge migration in a complex molecule, and is one of the fastest processes ever measured in a biological system. It could ultimately pave the way for control of electron transport in natural or synthetic nanostructures.EThOS - Electronic Theses Online ServiceGBUnited Kingdo