WHAT IF Check report

This file was created 2012-01-13 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 pdb1w74.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 : 0.146
CA-only RMS fit for the two chains : 0.052

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. The header of the PDB file states that TLS groups were used. So, if WHAT IF complains about your B-factors, while 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:


Number of TLS groups mentione in PDB file header: 0

Crystal temperature (K) :110.000

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: Tyrosine convention problem

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

  41 TYR   (  52-)  A
  84 TYR   (  95-)  A
 100 TYR   ( 111-)  A
 212 TYR   (  52-)  B
 255 TYR   (  95-)  B
 271 TYR   ( 111-)  B

Warning: Phenylalanine convention problem

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

  31 PHE   (  42-)  A
  67 PHE   (  78-)  A
  96 PHE   ( 107-)  A
 117 PHE   ( 128-)  A
 202 PHE   (  42-)  B
 238 PHE   (  78-)  B
 267 PHE   ( 107-)  B
 287 PHE   ( 127-)  B
 288 PHE   ( 128-)  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.

  88 ASP   (  99-)  A
  97 ASP   ( 108-)  A
 161 ASP   ( 172-)  A
 259 ASP   (  99-)  B
 268 ASP   ( 108-)  B
 332 ASP   ( 172-)  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.

  93 GLU   ( 104-)  A
 135 GLU   ( 146-)  A
 146 GLU   ( 157-)  A
 264 GLU   ( 104-)  B
 306 GLU   ( 146-)  B
 317 GLU   ( 157-)  B
 337 GLU   ( 177-)  B

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.

  61 HIS   (  72-)  A      CG   CD2   1.31   -4.4
 232 HIS   (  72-)  B      CG   CD2   1.31   -4.4

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.

  12 ARG   (  23-)  A      CD   NE   CZ  129.28    4.2
  61 HIS   (  72-)  A      CB   CG   ND1 129.81    5.5
 183 ARG   (  23-)  B      CG   CD   NE  117.36    4.0
 183 ARG   (  23-)  B      CD   NE   CZ  129.30    4.2
 232 HIS   (  72-)  B      CB   CG   ND1 128.99    4.9
 296 HIS   ( 136-)  B      CA   CB   CG  117.85    4.1

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.

  88 ASP   (  99-)  A
  93 GLU   ( 104-)  A
  97 ASP   ( 108-)  A
 135 GLU   ( 146-)  A
 146 GLU   ( 157-)  A
 161 ASP   ( 172-)  A
 259 ASP   (  99-)  B
 264 GLU   ( 104-)  B
 268 ASP   ( 108-)  B
 306 GLU   ( 146-)  B
 317 GLU   ( 157-)  B
 332 ASP   ( 172-)  B
 337 GLU   ( 177-)  B

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.

 128 ARG   ( 139-)  A    -2.8
 271 TYR   ( 111-)  B    -2.8
 299 ARG   ( 139-)  B    -2.7
 100 TYR   ( 111-)  A    -2.7
  82 PRO   (  93-)  A    -2.5
 253 PRO   (  93-)  B    -2.5
  68 MET   (  79-)  A    -2.3
 239 MET   (  79-)  B    -2.2
 284 GLY   ( 124-)  B    -2.2
  12 ARG   (  23-)  A    -2.2
 113 GLY   ( 124-)  A    -2.1
 183 ARG   (  23-)  B    -2.1
  67 PHE   (  78-)  A    -2.0
 238 PHE   (  78-)  B    -2.0
  98 LYS   ( 109-)  A    -2.0
 269 LYS   ( 109-)  B    -2.0
 233 ARG   (  73-)  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.

   4 ALA   (  15-)  A  Poor phi/psi
  59 VAL   (  70-)  A  omega poor
  67 PHE   (  78-)  A  Poor phi/psi
  87 ALA   (  98-)  A  omega poor
 100 TYR   ( 111-)  A  Poor phi/psi
 114 SER   ( 125-)  A  Poor phi/psi
 128 ARG   ( 139-)  A  Poor phi/psi
 157 ASP   ( 168-)  A  Poor phi/psi
 214 THR   (  54-)  B  Poor phi/psi
 230 VAL   (  70-)  B  omega poor
 238 PHE   (  78-)  B  Poor phi/psi
 271 TYR   ( 111-)  B  Poor phi/psi
 285 SER   ( 125-)  B  Poor phi/psi
 299 ARG   ( 139-)  B  Poor phi/psi
 328 ASP   ( 168-)  B  Poor phi/psi
 chi-1/chi-2 correlation Z-score : -2.064

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!

  12 ARG   (  23-)  A      0
  20 PHE   (  31-)  A      0
  23 HIS   (  34-)  A      0
  36 GLN   (  47-)  A      0
  38 THR   (  49-)  A      0
  47 SER   (  58-)  A      0
  50 PRO   (  61-)  A      0
  51 SER   (  62-)  A      0
  53 PRO   (  64-)  A      0
  54 PHE   (  65-)  A      0
  55 TYR   (  66-)  A      0
  56 ASP   (  67-)  A      0
  60 PHE   (  71-)  A      0
  61 HIS   (  72-)  A      0
  62 ARG   (  73-)  A      0
  65 GLN   (  76-)  A      0
  67 PHE   (  78-)  A      0
  68 MET   (  79-)  A      0
  77 THR   (  88-)  A      0
  82 PRO   (  93-)  A      0
  84 TYR   (  95-)  A      0
  86 PHE   (  97-)  A      0
  95 GLN   ( 106-)  A      0
  96 PHE   ( 107-)  A      0
  97 ASP   ( 108-)  A      0
And so on for a total of 149 lines.

Warning: Unusual PRO puckering amplitudes

The proline residues listed in the table below have a puckering amplitude that is outside of normal ranges. Puckering parameters were calculated by the method of Cremer and Pople [REF]. Normal PRO rings have a puckering amplitude Q between 0.20 and 0.45 Angstrom. If Q is lower than 0.20 Angstrom for a PRO residue, this could indicate disorder between the two different normal ring forms (with C-gamma below and above the ring, respectively). If Q is higher than 0.45 Angstrom something could have gone wrong during the refinement. 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]

  25 PRO   (  36-)  A    0.09 LOW
 109 PRO   ( 120-)  A    0.16 LOW
 196 PRO   (  36-)  B    0.08 LOW
 280 PRO   ( 120-)  B    0.14 LOW

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].

  82 PRO   (  93-)  A   -63.7 envelop C-beta (-72 degrees)
 253 PRO   (  93-)  B   -62.7 half-chair C-beta/C-alpha (-54 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.

  42 SER   (  53-)  A      O   <->  256 LYS   (  96-)  B      NZ     0.34    2.36  INTRA BF
  44 GLN   (  55-)  A      NE2 <->   48 GLY   (  59-)  A      O      0.28    2.42  INTRA BF
 178 THR   (  18-)  B      CG2 <->  180 HIS   (  20-)  B      CE1    0.28    2.92  INTRA BF
 180 HIS   (  20-)  B      NE2 <->  340 THR   ( 180-)  B      OG1    0.19    2.51  INTRA BF
   9 HIS   (  20-)  A      NE2 <->  169 THR   ( 180-)  A      OG1    0.17    2.53  INTRA BF
  43 THR   (  54-)  A      C   <->  256 LYS   (  96-)  B      NZ     0.12    2.98  INTRA BF
 244 ASP   (  84-)  B      OD2 <->  249 GLY   (  89-)  B      N      0.10    2.60  INTRA BL
 106 ASN   ( 117-)  A      ND2 <->  130 HIS   ( 141-)  A      ND1    0.10    2.90  INTRA BL
  58 ALA   (  69-)  A      O   <->  165 ILE   ( 176-)  A      N      0.08    2.62  INTRA BF
  42 SER   (  53-)  A      C   <->  256 LYS   (  96-)  B      NZ     0.08    3.02  INTRA BF
 325 ASP   ( 165-)  B      OD1 <->  327 ASN   ( 167-)  B      N      0.06    2.64  INTRA BF
  73 ASP   (  84-)  A      OD2 <->   78 GLY   (  89-)  A      N      0.06    2.64  INTRA BL
 229 ALA   (  69-)  B      O   <->  336 ILE   ( 176-)  B      N      0.05    2.65  INTRA BF
 177 ALA   (  17-)  B      O   <->  188 ILE   (  28-)  B      N      0.05    2.65  INTRA BL
   6 ALA   (  17-)  A      O   <->   17 ILE   (  28-)  A      N      0.04    2.66  INTRA BL
 215 GLN   (  55-)  B      NE2 <->  219 GLY   (  59-)  B      O      0.03    2.67  INTRA BF
 120 VAL   ( 131-)  A      N   <->  121 GLY   ( 132-)  A      N      0.02    2.58  INTRA BF
 262 HIS   ( 102-)  B      ND1 <->  263 PRO   ( 103-)  B      CD     0.02    3.08  INTRA BF
  43 THR   (  54-)  A      O   <->  256 LYS   (  96-)  B      NZ     0.02    2.68  INTRA BF

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.

  79 ARG   (  90-)  A      -8.12
 183 ARG   (  23-)  B      -5.80
  12 ARG   (  23-)  A      -5.65
  36 GLN   (  47-)  A      -5.52
 207 GLN   (  47-)  B      -5.51
 293 LYS   ( 133-)  B      -5.36
 128 ARG   ( 139-)  A      -5.33
 250 ARG   (  90-)  B      -5.32
 122 LYS   ( 133-)  A      -5.32
 299 ARG   ( 139-)  B      -5.30
  65 GLN   (  76-)  A      -5.28
 236 GLN   (  76-)  B      -5.22

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

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.

  61 HIS   (  72-)  A
 180 HIS   (  20-)  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.

   9 HIS   (  20-)  A      ND1
  12 ARG   (  23-)  A      NE
  22 ASN   (  33-)  A      N
  51 SER   (  62-)  A      N
  56 ASP   (  67-)  A      N
  98 LYS   ( 109-)  A      N
 106 ASN   ( 117-)  A      ND2
 121 GLY   ( 132-)  A      N
 131 THR   ( 142-)  A      N
 183 ARG   (  23-)  B      NE
 193 ASN   (  33-)  B      N
 222 SER   (  62-)  B      N
 246 THR   (  86-)  B      OG1
 250 ARG   (  90-)  B      NE
 255 TYR   (  95-)  B      N
 269 LYS   ( 109-)  B      N
 277 ASN   ( 117-)  B      ND2
 292 GLY   ( 132-)  B      N
 302 THR   ( 142-)  B      N
 334 VAL   ( 174-)  B      N

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.

  14 ASP   (  25-)  A      OD1
 112 ASN   ( 123-)  A      OD1
 185 ASP   (  25-)  B      OD1
 283 ASN   ( 123-)  B      OD1

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 :  -0.653
  2nd generation packing quality :  -0.261
  Ramachandran plot appearance   :  -1.409
  chi-1/chi-2 rotamer normality  :  -2.064
  Backbone conformation          :   0.220

RMS Z-scores, should be close to 1.0:
  Bond lengths                   :   0.582 (tight)
  Bond angles                    :   0.827
  Omega angle restraints         :   1.044
  Side chain planarity           :   0.625 (tight)
  Improper dihedral distribution :   0.805
  B-factor distribution          :   0.327
  Inside/Outside distribution    :   0.923

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 : 2.60


Structure Z-scores, positive is better than average:

  1st generation packing quality :   0.3
  2nd generation packing quality :   1.1
  Ramachandran plot appearance   :   0.8
  chi-1/chi-2 rotamer normality  :   0.1
  Backbone conformation          :   1.0

RMS Z-scores, should be close to 1.0:
  Bond lengths                   :   0.582 (tight)
  Bond angles                    :   0.827
  Omega angle restraints         :   1.044
  Side chain planarity           :   0.625 (tight)
  Improper dihedral distribution :   0.805
  B-factor distribution          :   0.327
  Inside/Outside distribution    :   0.923
==============

WHAT IF
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    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
      refinement,
    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).

DSSP
    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
    B.W.Matthews
      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
      structures?
    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.