Time-resolved structural photochemistry

X-ray scattering from disordered substances, such as dissolved photochemicals, contains information on the distances characteristic of the solvent & solute. That time-resolved x-ray scattering could be used to resolve structural changes within a light-activated photochemical was recognised theoretically more than two decades ago. However, it was not until single x-ray pulses were isolated from third generation synchrotron x-ray sources that the technology to realise this method was available. Exploiting this technology we provided a proof-of-principle experimental demonstration of the method through time-resolved studies on photo-excited iodine in solution with 100 ps resolution. We were also the first to extend this approach to study a more complex molecule, diiodomethane. Several other researchers have also begun to apply these methods & this structural technique is making an increasingly recognised contribution to ultrafast photo-chemistry.

Schematic of the concept of time-resolved x-ray diffraction from a disordered sample. X-rays scattered from a randomly oriented sample produce a series of concentric rings characteristic of the distance between atoms in the sample, in this case two atoms of iodine. As the interatomic distances between iodine molecules increase (below) the x-ray scattering rings become closer together. Thus the x-ray scattering data contains information on the bond-distances of a photo-intermediate of iodine.

Almost all biology & most industrial chemistry occurs in the liquid phase. The concept of time-resolved x-ray diffraction of simple photo-chemical systems in the solution phase explore this question we calculated the x-ray scattering from the sample over the entire x-ray pulse & compared this with that from a molecule which suffered no radiation damage. To quantify the extent to which the "radiation damage experiment" differed from a "radiation damage free experiment" we used an R-factor, similar to that used in crystallography. For conditions under which a relatively low value of the R-factor was recovered (somewhat arbitarily chosen as 15 %) it was judged that the radiation damage suffered was "acceptable" & there was a reasonable chance to record useful structural information.

The results of this analysis are shown above: the x-axis gives the pulse-duration in femtoseconds & the y-axis gives the number of 12 keV x-ray photons which passed through a 0.1 um focal spot. As would be expected, for a given x-ray pulse intensity a shorter pulse duration yeilded better quality x-ray diffraction data, reflected by the fact that the R-factor is lower in the bottom left corner of the diagram. The up-shot of our analysis was that, for x-ray pulses significantly shorter than 100 fs, it appears to be possible to record useful x-ray diffraction data from single-shot experiments on crystals of only a few nm in dimensions, & potentially from regular arrays like a virus capsid without the need for crystals. One would need to record & average data from numberous samples in order to build up complete data, but the result opens the possibility of a radically different approach to data-collection from very small protien samples. Since the publication of these simulations the work has formed a component of the scientific case for the developing x-ray free electron lasers, & Janos Hajdu in particular has been active in building collaborations aimed towards realising these long term goals.