Discussion on Strategies Introductory Notes - omega vs. phi scans - beam polarization - single sweep vs. multi sweep - xtal shape as re-orientation/re-centering factor

Reconstruction of the mean reflection intensities using limited experimental data set: profiles – a feature of PROTEINS, NOT APPLICABLE TO SMALL MOLECULES

Optimization target: Signal/Noise NOT the time to be spent for experiment, number of frames to collect, etc … ALL the data collection parameters (multi- sub-wedge, variable exposure time, etc.) are optimized simultaneously - Example: multiplicity vs exposure time

Radiation Damage - Compensation of intensity decay by adjusting (increasing) the exposure time / frame is essential : Total dose per data set is not important –defined by the long exposure of the LAST frames –short exposures of the FIRST frames are critical

What works in BEST now? optimal orientation with respect to: Overlaps (~90% of failing experiments – J. Holton ) - also with isometric high mosaicity Intrinsic diffraction anisotropy each diffraction pattern is maximally isotropic, S/N in a weak direction compensated by exposure (small effect when judged by standard "resolution shell" statistics) Low noise in anomalous difference data anomalous difference error model (radiation induced non-isomorphism) accounts for the difference in dose between the observed Bijvoet mates

Minimal R Friedel = - |> vs. Resolution and Orientation (error contribution to the difference only, no anomalous scattering contribution

Data collection using multiple crystals Reference images Auto-indexing BEST Crystal characterization and ranking Determination of maximal achievable resolution Optimal crystal orientation(s) Experimental aim Crystal 3 Crystal 5 Crystal 1 Crystal 8 D.C. plan Completeness 23% Completeness 58% Completeness 91% Completeness 99.7%

Omega vs. Phi scans Omega scans - orientation wrt scan axis is optimized Overlaps Radiation-induced non-isomorphism Multi-crystals AAS Phi scans - orientation wrt BEAM (direction/electric field vector) is varied "true redundancy" (– no advantage wrt. Omega, but - may be - less limitations) Blind region reduction ( - when in a symmetric setting) AAS?

Beam polarization Isotropic scattering – Scan axis || Electic Filed vector is optimal, though only important at high resolution ( < 2*wavelength) Vertical OMEGA is of advantage for the microbeam (gravity) PHI is mechanically non-micro AAS BEST minimizes the noise in anomalous diffrence data (fully applicable to AAS data) the target describing the AAS signal is required

Single Sweep vs. Multi Sweep Multi sweep on a single crystal: Blind region completion Multiplicity Partial data set completion (disaster scenario) From the point of view of implementation in BEST, Multi- Sweep strategy is a particular case of multiple crystal data collection optimization with the goniometric limitations

Single Sweep vs. Multi Sweep "Fast" coverage of an asymmetric unit on a single crystal – no advantage in signal-to-noise! Single sweep Radiation damage Disadvantage – Inhomogeneous S/N Single sweep RD compensation Multiple sweeps

xtal shape as re-orientation/re- centering factor Exploiting ALL of the crystal volume is critically important Severe mismatch of Xtal/Beam size – major limitation to sample characterization, strategy and data quality in general Use Kappa to match the Xtal/Beam size (at least in a vertical direction), Simplify line scans along Omega