Signal to noise considerations for single crystal femtosecond time resolved crystallography of the Photoactive Yellow Protein

By Jasper J. Van Thor, Mark M. Warren, Craig N. Lincoln, Matthieu Chollet, Henrik Till Lemke, David M. Fritz, Marius Schmidt1, Jason Tenboer, Zhong Ren, Vukica Srajer, Keith Moffat, Tim Graber

1. University of Wisconsin - Milwaukee

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journal-article

Author

Jasper J. van Thor and Mark M. Warren and Craig N. Lincoln and Matthieu Chollet and Henrik Till Lemke and David M. Fritz and Marius Schmidt and Jason Tenboer and Zhong Ren and Vukica Srajer and Keith Moffat and Tim Graber

Citation

Van Thor, J.J. et al., 2014. Signal to noise considerations for single crystal femtosecond time resolved crystallography of the Photoactive Yellow Protein. Faraday Discuss., 171, pp.439–455. Available at: http://dx.doi.org/10.1039/c4fd00011k.

Abstract

Femtosecond time resolved pump–probe protein X-ray crystallography requires highly accurate measurements of the photoinduced structure factor amplitude differences. In the case of femtosecond photolysis of single P63 crystals of the Photoactive Yellow Protein, it is shown that photochemical dynamics place a considerable restraint on the achievable time resolution due to the requirement to stretch and add second order dispersion in order to generate threshold concentration levels in the interaction region. Here, we report on using a ‘quasi-cw’ approach to use the rotation method with monochromatic radiation and 2 eV bandwidth at 9.465 keV at the Linac Coherent Light Source operated in SASE mode. A source of significant Bragg reflection intensity noise is identified from the combination of mode structure and jitter with very small mosaic spread of the crystals and very low convergence of the XFEL source. The accuracy with which the three dimensional reflection is approximated by the ‘quasi-cw’ rotation method with the pulsed source is modelled from the experimentally collected X-ray pulse intensities together with the measured rocking curves. This model is extended to predict merging statistics for recently demonstrated self seeded mode generated pulse train with improved stability, in addition to extrapolating to single crystal experiments with increased mosaic spread. The results show that the noise level can be adequately modelled in this manner, indicating that the large intensity fluctuations dominate the merged signal-to-noise (I/σI) value. Furthermore, these results predict that using the self seeded mode together with more mosaic crystals, sufficient accuracy may be obtained in order to resolve typical photoinduced structure factor amplitude differences, as taken from representative synchrotron results.

DOI

Funding

NSF-STC Biology with X-ray Lasers (NSF-1231306)