discussion on strategies introductory notes - omega vs. phi scans - beam polarization - single sweep...

Discussion on StrategiesIntroductory Notes

- omega vs. phi scans

- beam polarization

- single sweep vs. multi sweep

- xtal shape as re-orientation/re-centering factor

ˆ J ( h ) 1

s J u (h )Exp( h B h T )

Reconstruction of the mean reflection intensities using limited experimental data set:

<E2> profiles – a feature of PROTEINS, NOT APPLICABLE TO SMALL MOLECULES

10 5 3.33 2.5 2 1.67 1.43 1.25 1.11 1 0.9099

9.5

10

10.5

11

11.5

12

12.5

resolution d (A)

ln <I>

(a)o

1hq3 : [ ]=0.63, []=0.06

1at0 : [ ]=0.00, []=0.60

1d5t : [ ]=0.27, []=0.23

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 cells @ high mosaicity

• Intrinsic diffraction anisotropyeach 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 dataanomalous difference error model (radiation inducednon-isomorphism) accounts for the difference in dose between the observed Bijvoet mates

Minimal RFriedel= <|<E+>-<E->|> vs. Resolution and Orientation (error

contribution to the difference only, no anomalous scattering contribution

P2

0

2

4

6

8

10

12

14

16

0 0.05 0.1 0.15 0.2 0.25

1/d^2

Rfr

ied

el, %

0 1 01 0 01 1 0Random

P222

0

2

4

6

8

10

12

14

16

0 0.05 0.1 0.15 0.2 0.25

1/d^2

Rfr

ied

el,

%

random1 0 01 1 01 1 1

P4

0

2

4

6

8

10

12

14

16

0 0.05 0.1 0.15 0.2 0.25

1/d^2

Rfr

ied

el, %

0 0 11 0 0 1 1 1

P23

0

2

4

6

8

10

12

14

16

0 0.05 0.1 0.15 0.2 0.251/d^2

Rfr

iede

l, % 1 0 0

1 1 01 1 1random

Data collection using multiple crystals

Reference images

Auto-indexing

BESTCrystal characterization and ranking

Determination of maximal achievable resolution

Optimal crystal orientation(s)

Experimental aim

Crystal 3Crystal 5

Crystal 1Crystal 8

D.C. planCompleteness 23%

Completeness 58%

Completeness 91%

Completeness 99.7%

Omega vs. Phi scans

Omega scans - orientation wrt scan axis is optimized

OverlapsRadiation-induced non-isomorphismMulti-crystalsAAS

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 completionMultiplicity

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 sweepRadiation damage

Disadvantage – Inhomogeneous S/N

Single sweepRD 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