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Phasing Today’s goal is to calculate phases (ap) for proteinase K using MIRAS method (PCMBS and GdCl3). What experimental data do we need? 1) from native crystal we measured |Fp| for native crystal 2) from PCMBS derivative we measured |FPH1(h k l)| 3) also from PCMBS derivative we measured |FPH1(-h-k-l)| 4) FH1 for Hg atom of PCMBS 5) from GdCl3 derivative we measured |FPH2(h k l)| 6) also from GdCl3 derivative we measured |FPH2(-h-k-l)| 7) FH2 for Gd atom of GdCl3 How many phasing triangles will we have for each structure factor if we use all the data sets ? FP ( h k l) = FPH ( h k l) - FH ( h k l) is one type of phase triangle. FP ( h k l) = FPH(-h-k-l) * - FH (-h-k-l)* is another type of phase triangle. Four Phase Relationships PCMBS FP ( h k l) = FPH1 ( h k l) - FH1 ( h k l) isomorphous differences FP ( h k l) = FPH1(-h-k-l)* - FH1 (-h-k-l)* from Friedel mates (anomalous) GdCl3 FP ( h k l) = FPH2( h k l) - FH2 ( h k l) isomorphous differences FP ( h k l) = FPH2 (-h-k-l)* - FH2 (-h-k-l)* from Friedel mates (anomalous) FP ( h k l) = FPH1 ( h k l) - FH1 (h k l) Imaginary axis |Fp ( h k l) | = 1.8 2 1 Real axis -2 -1 1 2 -1 -2 Harker construction 1) |FP ( h k l) | –native measurement FP ( h k l) = FPH1 ( h k l) - FH1 ( h k l) Imaginary axis |Fp ( h k l) | = 1.8 |FH ( h k l) | = 1.4 aH (hkl) = 45° |Fp | Real axis FH Harker construction 1) |FP ( h k l) | –native measurement 2) FH (h k l) calculated from heavy atom position. FP ( h k l) = FPH1 ( h k l) - FH1 ( h k l) Imaginary axis |FP ( hkl) | = 1.8 |FH1 ( hkl) | = 1.4 aH1 (hkl) = 45° |Fp | |FPH1 (hkl) | = 2.8 |FPH| Real axis FH |FPH| Harker construction Let’s look at the quality 1) |FP ( h k l) | –native measurement 2) FH1 (h k l) calculated from heavy of the phasing statistics atom position. 3) |FP1(hkl)|–measured from up to this point. derivative. Which of the following SIR graphs best represents the phase probability Imaginary axis distribution, P(a)? |Fp | a) 0 90 180 270 360 |FPH| Real axis FH b) |FPH| 0 90 180 270 360 c) 0 90 180 270 360 The phase probability SIR distribution, P(a) is sometimes shown as being wrapped Imaginary axis around the phasing circle. |Fp | 0 90 180 270 360 |FPH| Real axis FH |FPH| 90 180 0 270 Which of the following is SIR the best choice of Fp? Imaginary axis 90 |Fp | a) 180 0 90 270 |FPH| 180 0 Real axis 90 FH b) 180 0 |FPH| 270 270 90 c) 180 0 270 Radius of circle is approximately |Fp| SIR Imaginary axis |Fp | best F = |Fp|eia•P(a)da 90 a |FPH| 180 0 Real axis Sum of FH probability |FPH| 270 weighted vectors Fp Usually shorter than Fp SIR best F = |Fp|eia•P(a)da 90 a 180 0 Sum of probability 270 weighted vectors Fp Best phase SIR Which of the following is the best approximation to the Figure Of Merit (FOM) for this reflection? 90 a) 1.00 b) 2.00 180 0 c) 0.50 d) -0.10 270 FOM=|Fbest|/|FP| Radius of circle is approximately |Fp| Which phase probability distribution would yield the most desirable Figure of Merit? a) b) 0 90 0 270 + + 90 180 180 270 0 c) 270 + + 90 180 SIR Which of the following is the best approximation Imaginary axis to the phasing power for this reflection? |Fp | a) 2.50 |FPH| Fbest b) 1.00 Real axis |Fp | c) 0.50 FH d) -0.50 |FPH| Phasing Power = |FH| Lack of closure |FH ( h k l) | = 1.4 Lack of closure = |FPH|-|FP+FH| = 0.5 (at the aP of Fbest) SIR Which of the Imaginary axis following is the most desirable phasing power? |Fp | a) 2.50 |FPH| Fbest b) 1.00 Real axis |Fp | c) 0.50 FH d) -0.50 |FPH| Phasing Power = |FH| Lack of closure What Phasing Power is sufficient to solve the structure? >1 |FP ( hkl) | = 1.8 SIR Which of the following is Imaginary axis |FPHg (hkl) | = 2.8 the RCullis for this reflection? |Fp | a) -0.5 |FPH| Fbest b) 0.5 Real axis |Fp | c) 1.30 FH d) 2.00 |FPH| RCullis = Lack of closure isomorphous difference From previous page, LoC=0.5 Isomorphous difference= |FPH| - |FP| 1.0 = 2.8-1.8 SIRAS Isomorphous differences Anomalous differences Imaginary axis FP ( h k l) = FPH(-h-k-l)* - FH (-h-k-l)* Real axis We will calculate SIRAS phases using the PCMBS Hg site. FP –native measurement FH (hkl) and FH(-h-k-l) calculated from heavy atom position. FPH(hkl) and FPH(-h-k-l) –measured from derivative. Point to these on graph. To Resolve the phase ambiguity SIRAS Isomorphous differences Anomalous differences Imaginary axis Which P(a) corresponds to SIR? Real axis Which P(a) corresponds to SIRAS? 0 90 180 270 360 SIRAS Isomorphous differences Anomalous differences Imaginary axis Which graph represents FOM of SIRAS phase? Which represents FOM of SIR phase? Which is better? Why? Real axis 90 90 180 0 180 0 270 270 Calculate phases and refine parameters (MLPhaRe) A Tale of Two Ambiguities • We can solve both ambiguities in one experiment. Center of inversion ambiguity Remember, because the position of Hg was determined using a Patterson map there is an ambiguity in handedness. The Patterson map has an additional center of symmetry not present in the real crystal. Therefore, both the site x,y,z and -x,-y,-z are equally consistent with Patterson peaks. Handedness can be resolved by calculating both electron density maps and choosing the map which contains structural features of real proteins (L- amino acids, right handed a-helices). If anomalous data is included, then one map will appear significantly better than the other. Note: Inversion of the space group symmetry (P43212 →P41212) accompanies inversion of the Patterson map coordinates (x,y,z→ -x,-y,-z) Choice of origin ambiguity • I want to include the Gd data (derivative 2) in phase calculation. • I can determine the Gd site x,y,z coordinates using a difference Patterson map. • But, how can I guarantee the set of coordinates I obtain are referred to the same origin as Hg (derivative 1)? • Do I have to try all 48 possibilities? Use a Cross difference Fourier to resolve the handedness ambiguity With newly calculated protein phases, fP, a protein electron density map could be calculated. The amplitudes would be |FP|, the phases would be fP. r(x)=1/V*S|FP|e-2pi(hx+ky+lz-fP) Answer: If we replace the coefficients with |FPH2-FP|, the result is an electron density map corresponding to this structural feature. r(x)=1/V*S|FPH2-FP|e-2pi(hx-fP) What is the second heavy atom, Alex. When the difference FPH2-FP is taken, the protein component is removed and we are left with only the contribution from the second heavy atom. This cross difference Fourier will help us in two ways: 1) It will resolve the handedness ambiguity by producing a very high peak when phases are calculated in the correct hand, but only noise when phases are calculated in the incorrect hand. 2) It will allow us to find the position of the second heavy atom and combine this data set into our phasing. Thus improving our phases. Phasing Procedures 1) Calculate phases for site x,y,z of Hg and run cross difference Fourier to find the Gd site. Note the height of the peak and Gd coordinates. 2) Negate x,y,z of Hg and invert the space group from P43212 to P41212. Calculate a second set of phases and run a second cross difference Fourier to find the Gd site. Compare the height of the peak with step 1. 3) Chose the handedness which produces the highest peak for Gd. Use the corresponding hand of space group and Hg, and Gd coordinates to make a combined set of phases. FH1(hkl) FPH1(hkl) FH1(-h-k-l)* FPH1(-h-k-l)* MIRAS Imaginary axis FH2(hkl) Isomorphous Deriv 2 FPH2(hkl) Anomalous Fp (hkl) Deriv 1 Real axis Isomorphous Deriv 1 GdCl3 FPH = FP+FH for isomorphous differences FH1(hkl) FPH1(hkl) GdCl3 FP ( h k l) = FPH1 (-h-k-l)* - FH1 (-h-k-l)* FH1(-h-k-l)* FPH1(-h-k-l)* Imaginary axis FH2(hkl) Isomorphous Deriv 2 FPH2(hkl) FH2 (-h-k-l)* FPH2(-h-k-l)* Anomalous Fp (hkl) Deriv 1 Real axis Isomorphous Anomalous Deriv 1 Deriv 2 Harker Construction for MIRAS phasing (Multiple Isomorphous Replacement with Anomalous Scattering) FH1(hkl) FPH1(hkl) For opposite hand FH1(-h-k-l)* FPH1(-h-k-l)* Isomorphous Deriv 1 Imaginary axis FH2(hkl) FPH2(hkl) Anomalous Deriv 2 FH2 (-h-k-l)* FPH2(-h-k-l)* Fp (hkl) Real axis Anomalous Deriv 1 Isomorphous Deriv 2 Note: in the presence of anomalous scattering, FH(HKL) ≠ FH(-H-K-L)* so if this opposite hand is the incorrect hand, the phase angle (aopposite) will be significantly different than (-aoriginal). The electron density would not just be inverted, but inverted and more noisy. Density modification A) Solvent flattening. • Calculate an electron density map. • If r<threshold, -> solvent • If r>threshold -> protein • Build a mask • Set density value in solvent region to a constant (low). • Transform flattened map to structure factors • Combine modified phases with original phases. • Iterate MIR phased map + MIRAS phased map Solvent Flattening + Histogram Matching MIR phased map + MIRAS phased map Solvent Flattening + Histogram Matching Density modification B) Histogram matching. • Calculate an electron density map. • Calculate the electron density distribution. It’s a histogram. How many grid points on map have an electron density falling between 0.2 and 0.3 etc? • Compare this histogram with ideal protein electron density map. • Modify electron density to resemble an ideal distribution. Number of times a particular electron density value is observed. Electron density value Each grid point has a value r(x,y,z) Y X Z represent r(x,y,z) in shades of gray Y X Z How many grid points have density 0? How many grid points have density 1? How many grid points have density x? ntotal=130 pixels total r=0 n=60 r=1 n=6 r=2 n=17 r=3 n=5 r=4 n=1 r=5 n=2 r=6 n=12 r=7 n=2 r=8 n=4 r=9 n=1 r=10 n=25 Histogram of electron density distribution P(r). Number of pixels with density (r) 0 1 2 3 4 5 6 7 8 9 10 Density (r) Cumulative density distribution function N(r). Number of pixels with density < r Number of pixels with density (r) 0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10 Density (r) Cumulative density Histogram matching density distribution function N(r). Number of pixels with density < r Number of pixels with density < r Cumulative density Cumulative density observed ideal Pixels with r=4 in original map will be modified to r= 2.5 in the histogram matched map. HOMEWORK Barriers to combining phase information from 2 derivatives 1) Initial Phasing with PCMBS 1) Calculate phases using coordinates you determined. 2) Refine heavy atom coordinates 2) Find Gd site using Cross Difference Fourier map. 1) Easier than Patterson methods. 2) Want to combine PCMBS and Gd to make MIRAS phases. 3) Determine handedness (P43212 or P41212 ?) 1) Repeat calculation above, but in P41212. 2) Compare map features with P43212 map to determine handedness. 4) Combine PCMBS and Gd sites (use correct hand of space group) for improved phases. 5) Density modification (solvent flattening & histogram matching) 1) Improves Phases 6) View electron density map

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posted: | 9/2/2012 |

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