Time-resolved spectroscopy of non-volatile biomolecule analogues

Abstract

A time-resolved photoion yield spectrometer, employing a soft thermal desorption source for low vapour pressure sample volatilisation, has been constructed, benchmarked and used to discern the non-adibatic dynamics of biologically relevant molecules. Extensive time-resolved photoion yield data collection and analysis software has also been developed and will be of use to the group for many years to come. The dynamics of the the RNA base uracil and its sulphur containing analogue 2-thiouracil have been investigated at an excitation wavelength of 267 nm. These results are compared to molecular beam studies employing similar excitation/ionisation schemes in order to benchmark the new spectrometer. Additional studies of uracil at pump wavelengths of 220 nm and 200 nm are the first reported for uracil < 250 nm. This study also looked for evidence of a theoretically predicted ultrafast ringopening mechanism, however signatures of this process were not observed within the time window investigated The non-adibatic dynamics operating in 5,6-dihydroxyindole, a sub-unit of the skin pigment eumelanin, have been studied in the gas-phase for the first time employing excitation wavelengths between 241 nm - 296 nm. This investigation revealed a significant change in dynamical behaviour when compared to the related indole and 5-hydorxyindole systems, with the molecule displaying dynamics more akin to phenol/catechol. The addition of a hydroxyl group at the O6H position opens an energy dissipation mechanism via H-atom barrier tunnelling along the O5H coordinate, considered as a spectator in the 5-HI system. Overall, the new spectroscopic instrument developed facilitates the study of low-vapour pressure molecular species in ultrafast dynamics experiments, broadening the range of molecules which may be investigated in the gas-phase. This allows for the study of structure-dynamics-function relationships to be extended to more complex and challenging systems.