WHAT IF Check report

This file was created 2011-12-28 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 pdb1fkt.ent

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

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

Warning: B-factors outside the range 0.0 - 100.0

In principle, B-factors can have a very wide range of values, but in practice, B-factors should not be zero while B-factors above 100.0 are a good indicator that the location of that atom is meaningless. Be aware that the cutoff at 100.0 is arbitrary. 'High' indicates that atoms with a B-factor > 100.0 were observed; 'Zero' indicates that atoms with a B-factor of zero were observed.

   7 ILE   (   7-)  A    Zero
  56 ILE   (  56-)  A    Zero
  76 ILE   (  76-)  A    Zero
  90 ILE   (  90-)  A    Zero
  91 ILE   (  91-)  A    Zero

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:

Temperature not mentioned in PDB file. This most likely means that the temperature record is absent.
Room temperature assumed

Warning: Temperature factors given as "U", not as "B"

The average temperature factor found is very low. Probably they are given as "U" values, and not as "B" values. Values will be multiplied by 8-pi-squared for the analysis of B-factors.

Warning: More than 5 percent of buried atoms has low B-factor

For normal protein structures, no more than about 1 percent of the B factors of buried atoms is below 5.0. The fact that this value is much higher in the current structure could be a signal that the B-factors were restraints or constraints to too-low values, misuse of B-factor field in the PDB file, or a TLS/scaling problem. If the average B factor is low too, it is probably a low temperature structure determination.

Percentage of buried atoms with B less than 5 : 45.34

Note: B-factor plot

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

Chain identifier: A

Geometric checks

Warning: Unusual bond lengths

The bond lengths listed in the table below were found to deviate more than 4 sigma from standard bond lengths (both standard values and sigmas for amino acid residues have been taken from Engh and Huber [REF], for DNA they were taken from Parkinson et al [REF]). In the table below for each unusual bond the bond length and the number of standard deviations it differs from the normal value is given.

Atom names starting with "-" belong to the previous residue in the chain. If the second atom name is "-SG*", the disulphide bridge has a deviating length.

  25 HIS   (  25-)  A      CG   ND1   1.30   -4.1
  59 TRP   (  59-)  A      CG   CD2   1.34   -5.4
  87 HIS   (  87-)  A      CG   ND1   1.29   -4.5
  94 HIS   (  94-)  A      CG   ND1   1.29   -4.8

Warning: Directionality in bond lengths and no X-ray cell

Comparison of bond distances with Engh and Huber [REF] standard values for protein residues and Parkinson et al [REF] standard values for DNA/RNA shows a significant systematic deviation.

You have most probably seen symmetry problems earlier. Please correct these and rerun this check to see the possible implications on the X-ray cell axes.

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.

   3 GLN   (   3-)  A      NE2  CD   OE1 118.50   -4.1
   6 THR   (   6-)  A      CA   CB   OG1 102.74   -4.6
   8 SER   (   8-)  A      N    CA   C    99.92   -4.0
  21 THR   (  21-)  A      N    CA   C    99.48   -4.2
  23 VAL   (  23-)  A      N    CA   C    99.75   -4.1
  25 HIS   (  25-)  A      N    CA   C    97.99   -4.7
  25 HIS   (  25-)  A      CA   CB   CG  108.19   -5.6
  27 THR   (  27-)  A      N    CA   C    98.72   -4.5
  28 GLY   (  28-)  A      N    CA   C    99.97   -4.3
  43 ASN   (  43-)  A      ND2  CG   OD1 117.51   -5.1
  46 PHE   (  46-)  A      CA   CB   CG  109.58   -4.2
  59 TRP   (  59-)  A      CG   CD1  NE1 102.95   -5.6
  59 TRP   (  59-)  A      CD1  NE1  CE2 117.35    4.7
  59 TRP   (  59-)  A      NE1  CE2  CZ2 142.47    8.2
  59 TRP   (  59-)  A      NE1  CE2  CD2  99.83   -5.8
  59 TRP   (  59-)  A      CZ2  CE2  CD2 117.70   -4.7
  59 TRP   (  59-)  A      CE3  CD2  CG  127.41   -6.5
  65 GLN   (  65-)  A      N    CA   CB  102.23   -4.9
  65 GLN   (  65-)  A      NE2  CD   OE1 118.43   -4.2
  73 LYS   (  73-)  A      N    CA   C    94.11   -6.1
  75 THR   (  75-)  A      N    CA   C    97.42   -4.9
  75 THR   (  75-)  A      CA   CB   OG1 103.59   -4.0
  77 SER   (  77-)  A      CA   CB   OG  101.57   -4.8
  87 HIS   (  87-)  A      CA   CB   CG  105.72   -8.1
  94 HIS   (  94-)  A      CG   ND1  CE1 109.68    4.1
 100 ASP   ( 100-)  A      N    CA   C    97.73   -4.8
 100 ASP   ( 100-)  A      CA   CB   CG  108.30   -4.3
 103 LEU   ( 103-)  A      N    CA   C    97.27   -5.0
 107 GLU   ( 107-)  A      N    CA   C    92.84   -6.6

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.

  73 LYS   (  73-)  A    7.04
 107 GLU   ( 107-)  A    6.35
 103 LEU   ( 103-)  A    5.26
  75 THR   (  75-)  A    5.08
  25 HIS   (  25-)  A    5.05
  28 GLY   (  28-)  A    5.01
  27 THR   (  27-)  A    4.53
   8 SER   (   8-)  A    4.42
 100 ASP   ( 100-)  A    4.22
  21 THR   (  21-)  A    4.21
  23 VAL   (  23-)  A    4.09
 102 GLU   ( 102-)  A    4.03

Warning: High tau angle deviations

The RMS Z-score for the tau angles (N-Calpha-C) in the structure is too high. For well refined structures this number is expected to be near 1.0. The fact that it is higher than 1.5 worries us. However, we determined the tau normal distributions from 500 high-resolution X-ray structures, rather than from CSD data, so we cannot be 100 percent certain about these numbers.

Tau angle RMS Z-score : 2.316

Error: Side chain planarity problems

The side chains of the residues listed in the table below contain a planar group that was found to deviate from planarity by more than 4.0 times the expected value. For an amino acid residue that has a side chain with a planar group, the RMS deviation of the atoms to a least squares plane was determined. The number in the table is the number of standard deviations this RMS value deviates from the expected value. Not knowing better yet, we assume that planarity of the groups analyzed should be perfect.

  18 ARG   (  18-)  A    8.38
  57 ARG   (  57-)  A    8.32
  71 ARG   (  71-)  A    7.88
  13 ARG   (  13-)  A    7.84
  40 ARG   (  40-)  A    7.04
  42 ARG   (  42-)  A    6.56

Torsion-related checks

Error: Ramachandran Z-score very low

The score expressing how well the backbone conformations of all residues correspond to the known allowed areas in the Ramachandran plot is very low.

Ramachandran Z-score : -5.190

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.

  85 THR   (  85-)  A    -2.5
  78 PRO   (  78-)  A    -2.5
  83 GLY   (  83-)  A    -2.5
  34 LYS   (  34-)  A    -2.1
  33 GLY   (  33-)  A    -2.1
  62 GLY   (  62-)  A    -2.1

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.

  13 ARG   (  13-)  A  Poor phi/psi
  31 GLU   (  31-)  A  Poor phi/psi
  42 ARG   (  42-)  A  Poor phi/psi
  58 GLY   (  58-)  A  Poor phi/psi
  62 GLY   (  62-)  A  Poor phi/psi
  81 ALA   (  81-)  A  Poor phi/psi
  83 GLY   (  83-)  A  Poor phi/psi
  94 HIS   (  94-)  A  Poor phi/psi
 chi-1/chi-2 correlation Z-score : -2.466

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!

   6 THR   (   6-)  A      0
   7 ILE   (   7-)  A      0
  11 ASP   (  11-)  A      0
  14 THR   (  14-)  A      0
  16 PRO   (  16-)  A      0
  18 ARG   (  18-)  A      0
  25 HIS   (  25-)  A      0
  30 LEU   (  30-)  A      0
  31 GLU   (  31-)  A      0
  32 ASP   (  32-)  A      0
  34 LYS   (  34-)  A      0
  35 LYS   (  35-)  A      0
  37 ASP   (  37-)  A      0
  39 SER   (  39-)  A      0
  40 ARG   (  40-)  A      0
  42 ARG   (  42-)  A      0
  43 ASN   (  43-)  A      0
  47 LYS   (  47-)  A      0
  50 LEU   (  50-)  A      0
  52 LYS   (  52-)  A      0
  54 GLU   (  54-)  A      0
  65 GLN   (  65-)  A      0
  68 VAL   (  68-)  A      0
  80 TYR   (  80-)  A      0
  81 ALA   (  81-)  A      0
And so on for a total of 51 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 : 2.911

Warning: Unusual PRO puckering phases

The proline residues listed in the table below have a puckering phase that is not expected to occur in protein structures. Puckering parameters were calculated by the method of Cremer and Pople [REF]. Normal PRO rings approximately show a so-called envelope conformation with the C-gamma atom above the plane of the ring (phi=+72 degrees), or a half-chair conformation with C-gamma below and C-beta above the plane of the ring (phi=-90 degrees). If phi deviates strongly from these values, this is indicative of a very strange conformation for a PRO residue, and definitely requires a manual check of the data. Be aware that this is a warning with a low confidence level. See: Who checks the checkers? Four validation tools applied to eight atomic resolution structures [REF].

  78 PRO   (  78-)  A  -123.6 half-chair C-delta/C-gamma (-126 degrees)

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.

   5 GLU   (   5-)  A      CD  <->   73 LYS   (  73-)  A      NZ     0.49    2.61  INTRA BF
  28 GLY   (  28-)  A      N   <->   37 ASP   (  37-)  A      O      0.26    2.44  INTRA BL
   1 GLY   (   1-)  A      CA  <->   77 SER   (  77-)  A      OG     0.26    2.54  INTRA BL
  11 ASP   (  11-)  A      CG  <->   12 GLY   (  12-)  A      N      0.21    2.79  INTRA BL
  87 HIS   (  87-)  A      N   <->   91 ILE   (  91-)  A      O      0.17    2.53  INTRA BF
  57 ARG   (  57-)  A      O   <->   59 TRP   (  59-)  A      N      0.16    2.54  INTRA BL
 105 LYS   ( 105-)  A      NZ  <->  106 LEU   ( 106-)  A      O      0.13    2.47  INTRA BF
  45 PRO   (  45-)  A      O   <->   47 LYS   (  47-)  A      NZ     0.13    2.57  INTRA BF
  71 ARG   (  71-)  A      CD  <->  102 GLU   ( 102-)  A      CD     0.12    3.08  INTRA BL
   1 GLY   (   1-)  A      CA  <->   79 ASP   (  79-)  A      OD2    0.11    2.69  INTRA BL
   5 GLU   (   5-)  A      OE2 <->   73 LYS   (  73-)  A      NZ     0.11    2.59  INTRA BF
  57 ARG   (  57-)  A      C   <->   59 TRP   (  59-)  A      N      0.09    2.81  INTRA BL
  82 TYR   (  82-)  A      CE2 <->   91 ILE   (  91-)  A      CD1    0.09    3.11  INTRA BL
  40 ARG   (  40-)  A      O   <->   42 ARG   (  42-)  A      NE     0.09    2.61  INTRA BF
  36 PHE   (  36-)  A      CG  <->   37 ASP   (  37-)  A      N      0.08    2.92  INTRA BL
  28 GLY   (  28-)  A      C   <->   36 PHE   (  36-)  A      CE1    0.08    3.12  INTRA BL
  56 ILE   (  56-)  A      CG2 <->   82 TYR   (  82-)  A      CE1    0.06    3.14  INTRA BL
  59 TRP   (  59-)  A      O   <->   63 VAL   (  63-)  A      N      0.06    2.64  INTRA BL
  52 LYS   (  52-)  A      CE  <->   54 GLU   (  54-)  A      OE2    0.05    2.75  INTRA BF
 105 LYS   ( 105-)  A      CG  <->  106 LEU   ( 106-)  A      N      0.05    2.95  INTRA BL
  26 TYR   (  26-)  A      N   <->   39 SER   (  39-)  A      OG     0.05    2.65  INTRA BL
  87 HIS   (  87-)  A      O   <->   89 GLY   (  89-)  A      N      0.03    2.67  INTRA BF
  54 GLU   (  54-)  A      N   <->   55 VAL   (  55-)  A      N      0.03    2.57  INTRA BL
  81 ALA   (  81-)  A      CB  <->   82 TYR   (  82-)  A      N      0.03    2.67  INTRA BL
  71 ARG   (  71-)  A      NH1 <->  102 GLU   ( 102-)  A      CD     0.03    3.07  INTRA BF
  76 ILE   (  76-)  A      O   <->   96 THR   (  96-)  A      CG2    0.02    2.78  INTRA BL
  64 ALA   (  64-)  A      N   <->   65 GLN   (  65-)  A      N      0.02    2.58  INTRA BL
  59 TRP   (  59-)  A      NE1 <->   99 PHE   (  99-)  A      CD2    0.02    3.08  INTRA BL

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

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.

  18 ARG   (  18-)  A      -7.43
  42 ARG   (  42-)  A      -6.28
  53 GLN   (  53-)  A      -5.42
  49 MET   (  49-)  A      -5.17
  31 GLU   (  31-)  A      -5.02

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.

  42 ARG   (  42-)  A        44 - LYS     44- ( A)         -5.20

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: 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

Water, ion, and hydrogenbond related checks

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.

   2 VAL   (   2-)  A      N
  13 ARG   (  13-)  A      N
  14 THR   (  14-)  A      N
  17 LYS   (  17-)  A      N
  31 GLU   (  31-)  A      N
  34 LYS   (  34-)  A      N
  46 PHE   (  46-)  A      N
  54 GLU   (  54-)  A      N
  55 VAL   (  55-)  A      N
  59 TRP   (  59-)  A      NE1
  77 SER   (  77-)  A      OG
  82 TYR   (  82-)  A      N

Warning: No crystallisation information

No, or very inadequate, crystallisation information was observed upon reading the PDB file header records. This information should be available in the form of a series of REMARK 280 lines. Without this information a few things, such as checking ions in the structure, cannot be performed optimally.

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.

  41 ASP   (  41-)  A   H-bonding suggests Asn; but Alt-Rotamer
 100 ASP   ( 100-)  A   H-bonding suggests Asn; 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 :  -1.849
  2nd generation packing quality :  -1.117
  Ramachandran plot appearance   :  -5.190 (bad)
  chi-1/chi-2 rotamer normality  :  -2.466
  Backbone conformation          :  -0.923

RMS Z-scores, should be close to 1.0:
  Bond lengths                   :   1.227
  Bond angles                    :   1.585
  Omega angle restraints         :   0.529 (tight)
  Side chain planarity           :   3.114 (loose)
  Improper dihedral distribution :   0.891
  Inside/Outside distribution    :   0.931

      WHAT IF: a molecular modelling and drug design program,
    J. Mol. Graph. 8, 52--56 (1990).

WHAT_CHECK (verification routines from WHAT IF)
    R.W.W.Hooft, G.Vriend, C.Sander and E.E.Abola,
      Errors in protein structures
    Nature 381, 272 (1996).
    (see also http://swift.cmbi.ru.nl/gv/whatcheck for a course and extra inform

Bond lengths and angles, protein residues
    R.Engh and R.Huber,
      Accurate bond and angle parameters for X-ray protein structure
    Acta Crystallogr. A47, 392--400 (1991).

Bond lengths and angles, DNA/RNA
    G.Parkinson, J.Voitechovsky, L.Clowney, A.T.Bruenger and H.Berman,
      New parameters for the refinement of nucleic acid-containing structures
    Acta Crystallogr. D52, 57--64 (1996).

    W.Kabsch and C.Sander,
      Dictionary of protein secondary structure: pattern
      recognition of hydrogen bond and geometrical features
    Biopolymers 22, 2577--2637 (1983).

Hydrogen bond networks
    R.W.W.Hooft, C.Sander and G.Vriend,
      Positioning hydrogen atoms by optimizing hydrogen bond networks in
      protein structures
    PROTEINS, 26, 363--376 (1996).

Matthews' Coefficient
      Solvent content of Protein Crystals
    J. Mol. Biol. 33, 491--497 (1968).

Protein side chain planarity
    R.W.W. Hooft, C. Sander and G. Vriend,
      Verification of protein structures: side-chain planarity
    J. Appl. Cryst. 29, 714--716 (1996).

Puckering parameters
    D.Cremer and J.A.Pople,
      A general definition of ring puckering coordinates
    J. Am. Chem. Soc. 97, 1354--1358 (1975).

Quality Control
    G.Vriend and C.Sander,
      Quality control of protein models: directional atomic
      contact analysis,
    J. Appl. Cryst. 26, 47--60 (1993).

Ramachandran plot
    G.N.Ramachandran, C.Ramakrishnan and V.Sasisekharan,
      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
    I.D.Brown and K.K.Wu,
      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
    Acta Cryst. D 59 32--37 (2003).

Checking checks
    K.Wilson, C.Sander, R.W.W.Hooft, G.Vriend, et al.
      Who checks the checkers
    J.Mol.Biol. (1998) 276,417-436.