SAXS and SANS facilities and experimental practice. Clement Blanchet

Transcription

1 SAXS and SANS facilities and experimental practice Clement Blanchet

2 SAS experiment Detector X-ray or neutron Beam Sample 2 s Buffer

3 X-rays Roengten, 1895

4 Electromagnetic wave

5 The electromagnetic spectrum

6 Why these wavelength? λ<<dimension of the object Transmission λ dimension of the object Diffraction λ >object, object invisible

7 Neutrons Matter wave (De Broglie, 1924) =h/mv

8 SAXS and SANS Common analysis methods but: The scattered particles are different Different interactions X-rays interact with electrons via electromagnetic forces Neutrons interact with nucleus via nuclear forces Different instruments

9 Outlines X-rays and neutrons sources SAXS and SANS instruments Optics Sample area Detectors Sample requirements and collection strategy

10 X-rays and neutron sources

11 X-rays: how are they produced? Principle Maxwell equation: accelerated charge -> radiation

12 X-ray sources Synchrotrons

13 X-ray sources Synchrotron radiation

14

15 Insertion devices Dipole bending magnet (APS) Undulator (PetraIII)

16 Synchrotrons around the world

17 X-ray sources

18 Lab sources Principle : electron beam send on a target Brehmstrahlung Fluorescence

19 X-ray sources Lab source (rotating anode, liquid jet)

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21 Neutron production Nuclear reactor

22 Neutron sources Spallation source Accelerated protons hit a target.

23 Neutron Sources

24 SAXS and SANS Instruments

25 Optics Prepare the beam coming from the source Monochromatic beam Focus/collimated beam

26 Monochromatic X-ray Bragg diffraction on a crystal n = 2dsin

27 Monochromator Before After Polychromatic One wavelength + harmonics

28 Focusing/low divergence 2 Small beam at the detector position Small beam at the sample position

29 Focusing X-ray Focussing mirror Reflectivity 1,0 0,8 Transmission 0,6 0, Degree 0.25 Degree 1 Degree 0,2 0, Energy [ev]

30 Focussing mirror harmonics filter

31 Monochromatic neutrons De Broglie equation λ=h/mv The wavelength of a neutron is related to its velocity. Velocity selector

32 Collimation neutrons A collimator is used to obtain a parallel beam

33 Sample environment As many sample environment as there is sample For biological macromolecules in solution: Liquid containing cell Preferably in vacuum thermostated

34 Sample cell Cell material, low absorption and low scattering Mica, polycarbonate Cell thickness: compromise between absorption and scattering

35 Sample environment On dedicated beamline, Sample handling is now automated: Faster measurement Better cleaning Unattended operation

36 Flight tube

37 Beamstop Prevent the direct beam from hitting the detector Big enough to stop the direct beam Small enough to collect the small angle Measure transmitted beam

38 Detectors

39 Single photon counting detector principle

40 Single photon counting detector Pilatus High dynamic range No background noise Fast framing Ideal for SAXS

41 Neutron detection He3 detector: n + 3 He 3 H + 1 H MeV

42 Experimental practice

43 Experiment SAS applicable to many type of samples. Biological macromolecules in solution Isotropic scattering Weakly scattering

44 SAS Experiment

45 Buffer subtraction Biological sample scatters very weakly Care should be taken for the buffer subtraction Exactly matching buffer (dialysis, elution buffer) Sample and buffer measured in the same cell

46 Monodispersity SAS is very sensible to aggregation, the sample should be monodisperse

47 Monodispersity Check the monodispersity of your sample before coming to the beamline. (native gel, dynamic light scattering, ultracentrifugation, )

48 Monodispersity Improving monodispersity: online size exclusion column

49 SEC + SAXS Defined buffer region

50 Inter-particle interactions

51 Inter-particle interactions Change solution (ph, salt concentration) to limit interactions Measure different concentrations and extrapolate

52 Measure also water and/or standard protein to estimate the molecular mass of your sample using the forward scattering For data on an absolute scale (water measurement) M=I(0)*N A /(C* Using a protein standard M=M BSA *I(0)/I BSA (0)

53 X-rays - Radiation damage!!! With intense third generation synchrotons: creation of free radicals in solution, that degrades protein and causes aggregation. Monitor radiation damage: collect several frames and compare them. Limit the radiation damage Use of scavengers DTT, Glycerol. Flow measurements.

54 Contrast in neutron Neutrons interact with the nucleus of atoms Each atoms has its own scattering length: H D C N O P S

55

56 Labeling Deuteration of sample Expression of the protein in deuterated medium

57 Collection strategy Solvent matching Find the solvent composition that match the contrast of the component you want to hide Measure the sample in this solvent Contrast variation Measure the sample in solvent with different contrast Using Stuhrmann analysis you can access the curves of the different components.

58 Conclusion - Instruments SAXS and SANS facilities: great high tech instruments. Apply for beamtime (dedicated biosaxs beamline in PetraIII/EMBL, ESRF, Soleil, SSRL, Australian synchrotron, spring8, diamond) Lab source also available for SAXS

59 SAS sample Protein concentration: 1-10 mg/ml Volume: 5-50 microliter (SAXS), microliter (SANS) Time: lab source: 5-60 min Synchrotron: seconds Neutrons: 30 minutes - hours

60 SAS sample Pure and monodisperse sample Exactly matching buffer Measure concentration series For SAXS: Be aware of radiation damage For SANS: Carefully design your experiment, think of your collection strategy.