Deducing fast electron density changes in randomly orientated uncrystallized biomolecules in a pump–probe experiment

• Published in: Philosophical transactions of the Royal Society of London. Series B. Biological sciences Vol. 369; no. 1647; p. 20130332
• Main Authors: Pande, K., Schwander, P., Schmidt, M., Saldin, D. K.
• Format: Journal Article
• Language: English
• Published: England The Royal Society 17-07-2014
• Subjects: Algorithms; Electrons; Lasers; Models, Theoretical; Molecular Conformation; Part II: Technique development; Proteins; Time Factors; Time-Resolved; X-Ray Diffraction - methods; X-Ray Free-Electron Laser; X-ray free-electron laser; time-resolved; proteins
• ISSN: 0962-8436; 1471-2970
• DOI: 10.1098/rstb.2013.0332

We propose a method for deducing time-resolved structural changes in uncrystallized biomolecules in solution. The method relies on measuring the angular correlations of the intensities, when averaged over a large number of diffraction patterns from randomly oriented biomolecules in solution in a liquid solvent. The experiment is somewhat like a pump–probe version of an experiment on small angle X-ray scattering, except that the data expected by the algorithm are not just the radial variation of the averaged intensities. The differences of these correlation functions as measured from a photoexcited and dark structure enable the direct calculation of the difference electron density with a knowledge of only the dark structure. We exploit a linear relation we derive between the difference in these correlation functions and the difference electron density, applicable for small structural changes.