WHAT IF Check report

This file was created 2012-01-19 from WHAT_CHECK output by a conversion script. If you are new to WHAT_CHECK, please study the pdbreport pages. There also exists a legend to the output.

Please note that you are looking at an abridged version of the output (all checks that gave normal results have been removed from this report). You can have a look at the Full report instead.

Verification log for pdb1yz4.ent

Checks that need to be done early-on in validation

Note: Non crystallographic symmetry RMS plot

The plot shows the RMS differences between two similar chains on a residue- by-residue basis. Individual "spikes" can be indicative of interesting or wrong residues. If all residues show a high RMS value, the structure could be incorrectly refined.

Chain identifiers of the two chains: A and B

All-atom RMS fit for the two chains : 1.323
CA-only RMS fit for the two chains : 0.837

Note: Non crystallographic symmetry backbone difference plot

The plot shows the differences in backbone torsion angles between two similar chains on a residue-by-residue basis. Individual "spikes" can be indicative of interesting or wrong residues. If all residues show high differences, the structure could be incorrectly refined.

Chain identifiers of the two chains: A and B

Warning: Ligands for which a topology was generated automatically

The topology for the ligands in the table below were determined automatically. WHAT IF uses a local copy of Daan van Aalten's Dundee PRODRG server to automatically generate topology information for ligands. For this PDB file that seems to have gone fine, but be aware that automatic topology generation is a complicated task. So, if you get messages that you fail to understand or that you believe are wrong, and one of these ligands is involved, then check the ligand topology first.

 321 BOG   ( 210-)  A  -

Non-validating, descriptive output paragraph

Note: Ramachandran plot

In this Ramachandran plot x-signs represent glycines, squares represent prolines, and plus-signs represent the other residues. If too many plus- signs fall outside the contoured areas then the molecule is poorly refined (or worse). Proline can only occur in the narrow region around phi=-60 that also falls within the other contour islands.

In a colour picture, the residues that are part of a helix are shown in blue, strand residues in red. Preferred regions for helical residues are drawn in blue, for strand residues in red, and for all other residues in green. A full explanation of the Ramachandran plot together with a series of examples can be found at the WHAT_CHECK website.

Chain identifier: A

Note: Ramachandran plot

Chain identifier: B

Coordinate problems, unexpected atoms, B-factor and occupancy checks

Warning: What type of B-factor?

WHAT IF does not yet know well how to cope with B-factors in case TLS has been used. It simply assumes that the B-factor listed on the ATOM and HETATM cards are the total B-factors. When TLS refinement is used that assumption sometimes is not correct. TLS seems not mentioned in the header of the PDB file. But anyway, if WHAT IF complains about your B-factors, and you think that they are OK, then check for TLS related B-factor problems first.

Obviously, the temperature at which the X-ray data was collected has some importance too:

Crystal temperature (K) :100.000

Note: B-factor plot

The average atomic B-factor per residue is plotted as function of the residue number.

Chain identifier: A

Note: B-factor plot

Chain identifier: B

Nomenclature related problems

Warning: Phenylalanine convention problem

The phenylalanine residues listed in the table below have their chi-2 not between -90.0 and 90.0.

  20 PHE   (  17-)  A
 157 PHE   ( 154-)  A
 175 PHE   (  17-)  B

Warning: Aspartic acid convention problem

The aspartic acid residues listed in the table below have their chi-2 not between -90.0 and 90.0, or their proton on OD1 instead of OD2.

 205 ASP   (  47-)  B

Warning: Glutamic acid convention problem

The glutamic acid residues listed in the table below have their chi-3 outside the -90.0 to 90.0 range, or their proton on OE1 instead of OE2.

  42 GLU   (  39-)  A
 119 GLU   ( 116-)  A
 218 GLU   (  60-)  B
 274 GLU   ( 116-)  B
 294 GLU   ( 136-)  B

Geometric checks

Warning: Unusual bond angles

The bond angles listed in the table below were found to deviate more than 4 sigma from standard bond angles (both standard values and sigma for protein residues have been taken from Engh and Huber [REF], for DNA/RNA from Parkinson et al [REF]). In the table below for each strange angle the bond angle and the number of standard deviations it differs from the standard values is given. Please note that disulphide bridges are neglected. Atoms starting with "-" belong to the previous residue in the sequence.

 145 SER   ( 142-)  A      N    CA   C   122.40    4.0

Error: Nomenclature error(s)

Checking for a hand-check. WHAT IF has over the course of this session already corrected the handedness of atoms in several residues. These were administrative corrections. These residues are listed here.

  42 GLU   (  39-)  A
 119 GLU   ( 116-)  A
 205 ASP   (  47-)  B
 218 GLU   (  60-)  B
 274 GLU   ( 116-)  B
 294 GLU   ( 136-)  B

Error: Tau angle problems

The side chains of the residues listed in the table below contain a tau angle (N-Calpha-C) that was found to deviate from te expected value by more than 4.0 times the expected standard deviation. The number in the table is the number of standard deviations this RMS value deviates from the expected value.

 145 SER   ( 142-)  A    6.73
 311 ARG   ( 153-)  B    4.81

Torsion-related checks

Warning: Torsion angle evaluation shows unusual residues

The residues listed in the table below contain bad or abnormal torsion angles.

These scores give an impression of how `normal' the torsion angles in protein residues are. All torsion angles except omega are used for calculating a `normality' score. Average values and standard deviations were obtained from the residues in the WHAT IF database. These are used to calculate Z-scores. A residue with a Z-score of below -2.0 is poor, and a score of less than -3.0 is worrying. For such residues more than one torsion angle is in a highly unlikely position.

 221 ILE   (  63-)  B    -2.6
 167 LEU   (   9-)  B    -2.5
 220 PRO   (  62-)  B    -2.3
 146 SER   ( 143-)  A    -2.2
  12 LEU   (   9-)  A    -2.1
   5 GLY   (   2-)  A    -2.1
 172 LEU   (  14-)  B    -2.0
 284 ASN   ( 126-)  B    -2.0

Warning: Backbone evaluation reveals unusual conformations

The residues listed in the table below have abnormal backbone torsion angles.

Residues with `forbidden' phi-psi combinations are listed, as well as residues with unusual omega angles (deviating by more than 3 sigma from the normal value). Please note that it is normal if about 5 percent of the residues is listed here as having unusual phi-psi combinations.

  50 ASP   (  47-)  A  Poor phi/psi
 125 ARG   ( 122-)  A  Poor phi/psi
 144 ALA   ( 141-)  A  Poor phi/psi
 161 ASN   (   3-)  B  Poor phi/psi
 205 ASP   (  47-)  B  Poor phi/psi
 220 PRO   (  62-)  B  Poor phi/psi
 chi-1/chi-2 correlation Z-score : -1.725

Warning: Unusual backbone conformations

For the residues listed in the table below, the backbone formed by itself and two neighbouring residues on either side is in a conformation that is not seen very often in the database of solved protein structures. The number given in the table is the number of similar backbone conformations in the database with the same amino acid in the centre.

For this check, backbone conformations are compared with database structures using C-alpha superpositions with some restraints on the backbone oxygen positions.

A residue mentioned in the table can be part of a strange loop, or there might be something wrong with it or its directly surrounding residues. There are a few of these in every protein, but in any case it is worth looking at!

   3 HIS   (   0-)  A      0
   4 MET   (   1-)  A      0
   6 ASN   (   3-)  A      0
   8 MET   (   5-)  A      0
  10 LYS   (   7-)  A      0
  12 LEU   (   9-)  A      0
  13 PRO   (  10-)  A      0
  34 ILE   (  31-)  A      0
  45 GLN   (  42-)  A      0
  49 GLN   (  46-)  A      0
  60 ASP   (  57-)  A      0
  65 PRO   (  62-)  A      0
  66 ILE   (  63-)  A      0
  90 HIS   (  87-)  A      0
  93 ALA   (  90-)  A      0
  95 ILE   (  92-)  A      0
  96 SER   (  93-)  A      0
  97 ARG   (  94-)  A      0
 114 TRP   ( 111-)  A      0
 124 THR   ( 121-)  A      0
 125 ARG   ( 122-)  A      0
 127 ILE   ( 124-)  A      0
 128 ALA   ( 125-)  A      0
 129 ASN   ( 126-)  A      0
 130 PRO   ( 127-)  A      0
And so on for a total of 101 lines.

Warning: Omega angles too tightly restrained

The omega angles for trans-peptide bonds in a structure are expected to give a gaussian distribution with the average around +178 degrees and a standard deviation around 5.5 degrees. These expected values were obtained from very accurately determined structures. Many protein structures are too tightly restrained. This seems to be the case with the current structure too, as the observed standard deviation is below 4.0 degrees.

Standard deviation of omega values : 1.172

Bump checks

Error: Abnormally short interatomic distances

The pairs of atoms listed in the table below have an unusually short interactomic distance; each bump is listed in only one direction.

The contact distances of all atom pairs have been checked. Two atoms are said to `bump' if they are closer than the sum of their Van der Waals radii minus 0.40 Angstrom. For hydrogen bonded pairs a tolerance of 0.55 Angstrom is used. The first number in the table tells you how much shorter that specific contact is than the acceptable limit. The second distance is the distance between the centres of the two atoms. Although we believe that two water atoms at 2.4 A distance are too close, we only report water pairs that are closer than this rather short distance.

The last text-item on each line represents the status of the atom pair. If the final column contains the text 'HB', the bump criterion was relaxed because there could be a hydrogen bond. Similarly relaxed criteria are used for 1-3 and 1-4 interactions (listed as 'B2' and 'B3', respectively). BL indicates that the B-factors of the clashing atoms have a low B-factor thereby making this clash even more worrisome. INTRA and INTER indicate whether the clashes are between atoms in the same asymmetric unit, or atoms in symmetry related asymmetric units, respectively.

 280 ARG   ( 122-)  B      NH2 <->  323 HOH   ( 165 )  B      O      0.26    2.44  INTRA
 286 ASN   ( 128-)  B      ND2 <->  288 GLY   ( 130-)  B      N      0.24    2.61  INTRA BL
 300 SER   ( 142-)  B      N   <->  323 HOH   ( 182 )  B      O      0.23    2.47  INTRA BL
 163 MET   (   5-)  B      CE  <->  187 ASN   (  29-)  B      ND2    0.21    2.89  INTRA
 200 GLN   (  42-)  B      NE2 <->  210 ARG   (  52-)  B      CB     0.21    2.89  INTRA BF
   3 HIS   (   0-)  A      O   <->    5 GLY   (   2-)  A      N      0.20    2.50  INTRA BF
  79 CYS   (  76-)  A      SG  <->  207 THR   (  49-)  B      CG2    0.16    3.24  INTRA BL
 150 ARG   ( 147-)  A      NH2 <->  322 HOH   ( 234 )  A      O      0.16    2.54  INTRA
 286 ASN   ( 128-)  B      ND2 <->  289 PHE   ( 131-)  B      N      0.16    2.69  INTRA BL
 160 GLY   (   2-)  B      O   <->  162 GLY   (   4-)  B      N      0.15    2.55  INTRA BF
 135 ARG   ( 132-)  A      NH2 <->  322 HOH   ( 240 )  A      O      0.13    2.57  INTRA
 190 THR   (  32-)  B      OG1 <->  191 HIS   (  33-)  B      ND1    0.12    2.58  INTRA BL
 131 ASN   ( 128-)  A      ND2 <->  134 PHE   ( 131-)  A      CE2    0.12    2.98  INTRA BL
 254 THR   (  96-)  B      O   <->  257 VAL   (  99-)  B      CG1    0.11    2.69  INTRA BL
 269 TRP   ( 111-)  B      NE1 <->  294 GLU   ( 136-)  B      OE1    0.11    2.59  INTRA
 104 ALA   ( 101-)  A      O   <->  108 THR   ( 105-)  A      CG2    0.09    2.71  INTRA BL
 150 ARG   ( 147-)  A      NE  <->  322 HOH   ( 234 )  A      O      0.09    2.61  INTRA
   8 MET   (   5-)  A      N   <->   28 GLN   (  25-)  A      OE1    0.09    2.61  INTRA
  41 HIS   (  38-)  A      ND1 <->  322 HOH   ( 225 )  A      O      0.08    2.62  INTRA BL
 210 ARG   (  52-)  B      NE  <->  212 PRO   (  54-)  B      CG     0.07    3.03  INTRA
 169 GLY   (  11-)  B      CA  <->  241 ASN   (  83-)  B      ND2    0.07    3.03  INTRA BL
 107 MET   ( 104-)  A      O   <->  111 GLY   ( 108-)  A      N      0.07    2.63  INTRA BL
 156 ARG   ( 153-)  A      NH2 <->  227 GLU   (  69-)  B      OE2    0.06    2.64  INTRA
 268 GLY   ( 110-)  B      N   <->  323 HOH   ( 168 )  B      O      0.06    2.64  INTRA BL
  58 VAL   (  55-)  A      CG1 <->   64 VAL   (  61-)  A      CG1    0.06    3.14  INTRA
 197 GLU   (  39-)  B      N   <->  320 SO4   ( 160-)  B      O1     0.06    2.64  INTRA BF
  79 CYS   (  76-)  A      O   <->   83 ASN   (  80-)  A      ND2    0.06    2.64  INTRA BL
 167 LEU   (   9-)  B      O   <->  168 PRO   (  10-)  B      C      0.06    2.54  INTRA BL
 167 LEU   (   9-)  B      C   <->  168 PRO   (  10-)  B      C      0.06    2.74  INTRA BL
 122 LYS   ( 119-)  A      NZ  <->  128 ALA   ( 125-)  A      O      0.05    2.65  INTRA
 196 HIS   (  38-)  B      CE1 <->  246 SER   (  88-)  B      C      0.05    3.15  INTRA
 235 CYS   (  77-)  B      SG  <->  240 GLY   (  82-)  B      C      0.05    3.35  INTRA BL
 180 ASP   (  22-)  B      CG  <->  183 GLN   (  25-)  B      CG     0.05    3.15  INTRA
 299 ALA   ( 141-)  B      N   <->  323 HOH   ( 182 )  B      O      0.05    2.65  INTRA BL
 131 ASN   ( 128-)  A      ND2 <->  134 PHE   ( 131-)  A      CD2    0.04    3.06  INTRA BL
 147 GLN   ( 144-)  A      N   <->  322 HOH   ( 231 )  A      O      0.04    2.66  INTRA BL
  97 ARG   (  94-)  A      NE  <->  317 SO4   ( 211-)  A      O2     0.04    2.66  INTRA BL
  35 THR   (  32-)  A      OG1 <->   36 HIS   (  33-)  A      ND1    0.03    2.67  INTRA BL
  78 HIS   (  75-)  A      ND1 <->  105 TYR   ( 102-)  A      OH     0.03    2.67  INTRA BL
 197 GLU   (  39-)  B      N   <->  198 SER   (  40-)  B      N      0.03    2.57  INTRA BF
 154 GLU   ( 151-)  A      O   <->  158 GLY   ( 155-)  A      N      0.03    2.67  INTRA
 262 MET   ( 104-)  B      O   <->  266 GLY   ( 108-)  B      N      0.02    2.68  INTRA BL
 151 ARG   ( 148-)  A      NH2 <->  152 GLN   ( 149-)  A      CG     0.01    3.09  INTRA BF
  25 ASP   (  22-)  A      OD1 <->   28 GLN   (  25-)  A      N      0.01    2.69  INTRA

Packing, accessibility and threading

Note: Inside/Outside RMS Z-score plot

The Inside/Outside distribution normality RMS Z-score over a 15 residue window is plotted as function of the residue number. High areas in the plot (above 1.5) indicate unusual inside/outside patterns.

Chain identifier: A

Note: Inside/Outside RMS Z-score plot

Chain identifier: B

Warning: Abnormal packing environment for some residues

The residues listed in the table below have an unusual packing environment.

The packing environment of the residues is compared with the average packing environment for all residues of the same type in good PDB files. A low packing score can indicate one of several things: Poor packing, misthreading of the sequence through the density, crystal contacts, contacts with a co-factor, or the residue is part of the active site. It is not uncommon to see a few of these, but in any case this requires further inspection of the residue.

   4 MET   (   1-)  A      -7.07
 143 TRP   ( 140-)  A      -6.08
 202 LEU   (  44-)  B      -5.89
   3 HIS   (   0-)  A      -5.81
  49 GLN   (  46-)  A      -5.70
 204 GLN   (  46-)  B      -5.66
   6 ASN   (   3-)  A      -5.42
 161 ASN   (   3-)  B      -5.36
 247 PHE   (  89-)  B      -5.19
  92 PHE   (  89-)  A      -5.19
 156 ARG   ( 153-)  A      -5.14

Warning: Abnormal packing environment for sequential residues

A stretch of at least three sequential residues with a questionable packing environment was found. This could indicate that these residues are part of a strange loop. It might also be an indication of misthreading in the density. However, it can also indicate that one or more residues in this stretch have other problems such as, for example, missing atoms, very weird angles or bond lengths, etc.

The table below lists the first and last residue in each stretch found, as well as the average residue score of the series.

   2 SER   (  -1-)  A         4 - MET      1- ( A)         -5.71

Note: Quality value plot

The quality value smoothed over a 10 residue window is plotted as function of the residue number. Low areas in the plot (below -2.0) indicate unusual packing.

Chain identifier: A

Note: Quality value plot

The quality value smoothed over a 10 residue window is plotted as function of the residue number. Low areas in the plot (below -2.0) indicate unusual packing.

Chain identifier: B

Warning: Low packing Z-score for some residues

The residues listed in the table below have an unusual packing environment according to the 2nd generation packing check. The score listed in the table is a packing normality Z-score: positive means better than average, negative means worse than average. Only residues scoring less than -2.50 are listed here. These are the unusual residues in the structure, so it will be interesting to take a special look at them.

 144 ALA   ( 141-)  A   -2.71
  41 HIS   (  38-)  A   -2.62

Note: Second generation quality Z-score plot

The second generation quality Z-score smoothed over a 10 residue window is plotted as function of the residue number. Low areas in the plot (below -1.3) indicate unusual packing.

Chain identifier: A

Note: Second generation quality Z-score plot

Chain identifier: B

Water, ion, and hydrogenbond related checks

Error: HIS, ASN, GLN side chain flips

Listed here are Histidine, Asparagine or Glutamine residues for which the orientation determined from hydrogen bonding analysis are different from the assignment given in the input. Either they could form energetically more favourable hydrogen bonds if the terminal group was rotated by 180 degrees, or there is no assignment in the input file (atom type 'A') but an assignment could be made. Be aware, though, that if the topology could not be determined for one or more ligands, then this option will make errors.

  41 HIS   (  38-)  A
 196 HIS   (  38-)  B
 238 ASN   (  80-)  B
 241 ASN   (  83-)  B
 286 ASN   ( 128-)  B

Warning: Buried unsatisfied hydrogen bond donors

The buried hydrogen bond donors listed in the table below have a hydrogen atom that is not involved in a hydrogen bond in the optimized hydrogen bond network.

Hydrogen bond donors that are buried inside the protein normally use all of their hydrogens to form hydrogen bonds within the protein. If there are any non hydrogen bonded buried hydrogen bond donors in the structure they will be listed here. In very good structures the number of listed atoms will tend to zero.

Waters are not listed by this option.

  43 SER   (  40-)  A      N
  97 ARG   (  94-)  A      NH1
  97 ARG   (  94-)  A      NH2
 127 ILE   ( 124-)  A      N
 131 ASN   ( 128-)  A      N
 131 ASN   ( 128-)  A      ND2
 150 ARG   ( 147-)  A      NH1
 203 LEU   (  45-)  B      N
 210 ARG   (  52-)  B      NE
 216 THR   (  58-)  B      N
 247 PHE   (  89-)  B      N
 270 ARG   ( 112-)  B      N
 282 ILE   ( 124-)  B      N
 286 ASN   ( 128-)  B      ND2
 292 GLN   ( 134-)  B      NE2
 302 GLN   ( 144-)  B      NE2

Warning: Buried unsatisfied hydrogen bond acceptors

The buried side-chain hydrogen bond acceptors listed in the table below are not involved in a hydrogen bond in the optimized hydrogen bond network.

Side-chain hydrogen bond acceptors buried inside the protein normally form hydrogen bonds within the protein. If there are any not hydrogen bonded in the optimized hydrogen bond network they will be listed here.

Waters are not listed by this option.

   3 HIS   (   0-)  A      NE2
   6 ASN   (   3-)  A      OD1
  69 HIS   (  66-)  A      ND1
 224 HIS   (  66-)  B      ND1

Warning: Possible wrong residue type

The residues listed in the table below have a weird environment that cannot be improved by rotamer flips. This can mean one of three things, non of which WHAT CHECK really can do much about. 1) The side chain has actually another rotamer than is present in the PDB file; 2) A counter ion is present in the structure but is not given in the PDB file; 3) The residue actually is another amino acid type. The annotation 'Alt-rotamer' indicates that WHAT CHECK thinks you might want to find an alternate rotamer for this residue. The annotation 'Sym-induced' indicates that WHAT CHECK believes that symmetry contacts might have something to do with the difficulties of this residue's side chain. Determination of these two annotations is difficult, so their absence is less meaningful than their presence. The annotation Ligand-bound indicates that a ligand seems involved with this residue. In nine of ten of these cases this indicates that the ligand is causing the weird situation rather than the residue.

  60 ASP   (  57-)  A   H-bonding suggests Asn
 154 GLU   ( 151-)  A   H-bonding suggests Gln; but Alt-Rotamer

Final summary

Note: Summary report for users of a structure

This is an overall summary of the quality of the structure as compared with current reliable structures. This summary is most useful for biologists seeking a good structure to use for modelling calculations.

The second part of the table mostly gives an impression of how well the model conforms to common refinement restraint values. The first part of the table shows a number of global quality indicators.


Structure Z-scores, positive is better than average:

  1st generation packing quality :  -0.041
  2nd generation packing quality :  -0.992
  Ramachandran plot appearance   :  -1.370
  chi-1/chi-2 rotamer normality  :  -1.725
  Backbone conformation          :   0.463

RMS Z-scores, should be close to 1.0:
  Bond lengths                   :   0.322 (tight)
  Bond angles                    :   0.646 (tight)
  Omega angle restraints         :   0.213 (tight)
  Side chain planarity           :   0.248 (tight)
  Improper dihedral distribution :   0.568
  B-factor distribution          :   1.020
  Inside/Outside distribution    :   0.978

Note: Summary report for depositors of a structure

This is an overall summary of the quality of the X-ray structure as compared with structures solved at similar resolutions. This summary can be useful for a crystallographer to see if the structure makes the best possible use of the data. Warning. This table works well for structures solved in the resolution range of the structures in the WHAT IF database, which is presently (summer 2008) mainly 1.1 - 1.3 Angstrom. The further the resolution of your file deviates from this range the more meaningless this table becomes.

The second part of the table mostly gives an impression of how well the model conforms to common refinement restraint values. The first part of the table shows a number of global quality indicators, which have been calibrated against structures of similar resolution.

Resolution found in PDB file : 2.40


Structure Z-scores, positive is better than average:

  1st generation packing quality :   0.9
  2nd generation packing quality :   0.1
  Ramachandran plot appearance   :   0.3
  chi-1/chi-2 rotamer normality  :  -0.0
  Backbone conformation          :   0.8

RMS Z-scores, should be close to 1.0:
  Bond lengths                   :   0.322 (tight)
  Bond angles                    :   0.646 (tight)
  Omega angle restraints         :   0.213 (tight)
  Side chain planarity           :   0.248 (tight)
  Improper dihedral distribution :   0.568
  B-factor distribution          :   1.020
  Inside/Outside distribution    :   0.978
==============

WHAT IF
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    (see also http://swift.cmbi.ru.nl/gv/whatcheck for a course and extra inform

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Bond lengths and angles, DNA/RNA
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DSSP
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Hydrogen bond networks
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Matthews' Coefficient
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Protein side chain planarity
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Puckering parameters
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Quality Control
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      contact analysis,
    J. Appl. Cryst. 26, 47--60 (1993).

Ramachandran plot
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      Stereochemistry of Polypeptide Chain Conformations
    J. Mol. Biol. 7, 95--99 (1963).

Symmetry Checks
    R.W.W.Hooft, C.Sander and G.Vriend,
      Reconstruction of symmetry related molecules from protein
      data bank (PDB) files
    J. Appl. Cryst. 27, 1006--1009 (1994).

Ion Checks
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      Empirical Parameters for Calculating Cation-Oxygen Bond Valences
    Acta Cryst. B32, 1957--1959 (1975).

    M.Nayal and E.Di Cera,
      Valence Screening of Water in Protein Crystals Reveals Potential Na+
      Binding Sites
    J.Mol.Biol. 256 228--234 (1996).

    P.Mueller, S.Koepke and G.M.Sheldrick,
      Is the bond-valence method able to identify metal atoms in protein
      structures?
    Acta Cryst. D 59 32--37 (2003).

Checking checks
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      Who checks the checkers
    J.Mol.Biol. (1998) 276,417-436.