WHAT IF Check report

This file was created 2012-01-05 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 pdb1qls.ent

Checks that need to be done early-on in validation

Warning: Matthews Coefficient (Vm) high

The Matthews coefficient [REF] is defined as the density of the protein structure in cubic Angstroms per Dalton. Normal values are between 1.5 (tightly packed, little room for solvent) and 4.0 (loosely packed, much space for solvent). Some very loosely packed structures can get values a bit higher than that.

Very high numbers are most often caused by giving the wrong value for Z on the CRYST1 card (or not giving this number at all), but can also result from large fractions missing out of the molecular weight (e.g. a lot of UNK residues, or DNA/RNA missing from virus structures).

Molecular weight of all polymer chains: 12060.996
Volume of the Unit Cell V= 580724.063
Space group multiplicity: 12
No NCS symmetry matrices (MTRIX records) found in PDB file
Matthews coefficient for observed atoms and Z a bit high: Vm= 4.012
Vm by authors and this calculated Vm agree well
Matthews coefficient read from REMARK 280 Vm= 3.900

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.

 108 ACE   (   0-)  D  -

Administrative problems that can generate validation failures

Warning: Groups attached to potentially hydrogenbonding atoms

Residues were observed with groups attached to (or very near to) atoms that potentially can form hydrogen bonds. WHAT IF is not very good at dealing with such exceptional cases (Mainly because it's author is not...). So be warned that the hydrogenbonding-related analyses of these residues might be in error.

For example, an aspartic acid can be protonated on one of its delta oxygens. This is possible because the one delta oxygen 'helps' the other one holding that proton. However, if a delta oxygen has a group bound to it, then it can no longer 'help' the other delta oxygen bind the proton. However, both delta oxygens, in principle, can still be hydrogen bond acceptors. Such problems can occur in the amino acids Asp, Glu, and His. I have opted, for now to simply allow no hydrogen bonds at all for any atom in any side chain that somewhere has a 'funny' group attached to it. I know this is wrong, but there are only 12 hours in a day.

  96 ALA   (   1-)  D  -   N   bound to  108 ACE   (   0-)  D  -   C

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: D

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.

  48 GLN   (  52-)  A    High
 106 TRP   (  11-)  D    High

Warning: Occupancies atoms do not add up to 1.0.

In principle, the occupancy of all alternates of one atom should add up till 1.0. A valid exception is the missing atom (i.e. an atom not seen in the electron density) that is allowed to have a 0.0 occupancy. Sometimes this even happens when there are no alternate atoms given...

Atoms want to move. That is the direct result of the second law of thermodynamics, in a somewhat weird way of thinking. Any way, many atoms seem to have more than one position where they like to sit, and they jump between them. The population difference between those sites (which is related to their energy differences) is seen in the occupancy factors. As also for atoms it is 'to be or not to be', these occupancies should add up to 1.0. Obviously, it is possible that they add up to a number less than 1.0, in cases where there are yet more, but undetected' rotamers/positions in play, but also in those cases a warning is in place as the information shown in the PDB file is less certain than it could have been. The residues listed below contain atoms that have an occupancy greater than zero, but all their alternates do not add up to one.

WARNING. Presently WHAT CHECK only deals with a maximum of two alternate positions. A small number of atoms in the PDB has three alternates. In those cases the warning given here should obviously be neglected! In a next release we will try to fix this.

  96 ALA   (   1-)  D    0.50
  97 MET   (   2-)  D    0.50
  98 VAL   (   3-)  D    0.50
  99 SER   (   4-)  D    0.50
 100 ALA   (   5-)  D    0.50
 101 PHE   (   6-)  D    0.50
 102 LEU   (   7-)  D    0.50
 103 LYS   (   8-)  D    0.50
 104 GLN   (   9-)  D    0.50
 105 ALA   (  10-)  D    0.50
 106 TRP   (  11-)  D    0.50

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) :280.000

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: D

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.

   4 THR   (   8-)  A      N    CA   CB  118.92    5.0
   4 THR   (   8-)  A      C    CA   CB  117.75    4.0
   4 THR   (   8-)  A      CA   CB   OG1 121.05    7.6
   7 CYS   (  11-)  A      C    CA   CB  118.15    4.2
   9 GLU   (  13-)  A      CB   CG   CD  103.26   -5.5
  16 GLN   (  20-)  A      CB   CG   CD  120.27    4.5
  16 GLN   (  20-)  A      NE2  CD   OE1 117.68   -4.9
  18 HIS   (  22-)  A      CA   CB   CG  105.65   -8.2
  20 GLY   (  24-)  A     -CA  -C    N   124.98    4.4
  22 ASP   (  26-)  A      CB   CG   OD2 130.73    5.4
  25 ASN   (  29-)  A     -O   -C    N   115.56   -4.7
  25 ASN   (  29-)  A     -CA  -C    N   126.31    5.1
  25 ASN   (  29-)  A      CB   CG   ND2 123.07    4.4
  27 LYS   (  31-)  A      CA   CB   CG  105.73   -4.2
  29 SER   (  33-)  A     -C    N    CA  113.18   -4.7
  38 ASN   (  42-)  A      CB   CG   ND2 122.66    4.2
  47 ASN   (  51-)  A      CA   CB   CG  117.52    4.9
  56 ARG   (  60-)  A      CD   NE   CZ  132.20    5.8
  62 ASP   (  66-)  A      CB   CG   OD2 132.50    6.1
  64 ASP   (  68-)  A     -CA  -C    N   124.21    4.0
  66 ASP   (  70-)  A      CA   CB   CG  117.29    4.7
  68 GLN   (  72-)  A      CA   C    O   112.97   -4.6
  69 LEU   (  73-)  A     -CA  -C    N   128.82    6.3
  70 ASP   (  74-)  A     -C    N    CA  113.94   -4.3
  74 PHE   (  78-)  A      CA   CB   CG  119.06    5.3
  95 LYS   (  99-)  A      CA   C    O   109.65   -6.6
  96 ALA   (   1-)  D      N    CA   CB  117.16    4.5

Warning: Chirality deviations detected

The atoms listed in the table below have an improper dihedral value that is deviating from expected values. As the improper dihedral values are all getting very close to ideal values in recent X-ray structures, and as we actually do not know how big the spread around these values should be, this check only warns for 6 sigma deviations.

Improper dihedrals are a measure of the chirality/planarity of the structure at a specific atom. Values around -35 or +35 are expected for chiral atoms, and values around 0 for planar atoms. Planar side chains are left out of the calculations, these are better handled by the planarity checks.

Three numbers are given for each atom in the table. The first is the Z-score for the improper dihedral. The second number is the measured improper dihedral. The third number is the expected value for this atom type. A final column contains an extra warning if the chirality for an atom is opposite to the expected value.

   4 THR   (   8-)  A      CA    -7.4    21.52    33.84
The average deviation= 1.590

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.

  65 SER   (  69-)  A    -2.5
  49 LYS   (  53-)  A    -2.4
  47 ASN   (  51-)  A    -2.4

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.

  22 ASP   (  26-)  A  Poor phi/psi
 chi-1/chi-2 correlation Z-score : -2.420

Warning: Unusual rotamers

The residues listed in the table below have a rotamer that is not seen very often in the database of solved protein structures. This option determines for every residue the position specific chi-1 rotamer distribution. Thereafter it verified whether the actual residue in the molecule has the most preferred rotamer or not. If the actual rotamer is the preferred one, the score is 1.0. If the actual rotamer is unique, the score is 0.0. If there are two preferred rotamers, with a population distribution of 3:2 and your rotamer sits in the lesser populated rotamer, the score will be 0.667. No value will be given if insufficient hits are found in the database.

It is not necessarily an error if a few residues have rotamer values below 0.3, but careful inspection of all residues with these low values could be worth it.

  99 SER   (   4-)  D    0.36

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!

  21 ARG   (  25-)  A      0
  22 ASP   (  26-)  A      0
  25 ASN   (  29-)  A      0
  26 THR   (  30-)  A      0
  39 THR   (  43-)  A      0
  41 LEU   (  45-)  A      0
  48 GLN   (  52-)  A      0
  49 LYS   (  53-)  A      0
  62 ASP   (  66-)  A      0
  65 SER   (  69-)  A      0
  68 GLN   (  72-)  A      0
  94 GLN   (  98-)  A      0
  95 LYS   (  99-)  A      0
  96 ALA   (   1-)  D      0
  97 MET   (   2-)  D      0
 104 GLN   (   9-)  D      0
  24 ASN   (  28-)  A      1
  27 LYS   (  31-)  A      1
  61 LEU   (  65-)  A      1
  63 LEU   (  67-)  A      1
  66 ASP   (  70-)  A      1
  38 ASN   (  42-)  A      2
  40 GLU   (  44-)  A      2

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 : 2.102

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].

   1 PRO   (   5-)  A    -4.7 envelop N (0 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.

   6 ARG   (  10-)  A      NH2 <->    7 CYS   (  11-)  A      SG     0.39    2.91  INTRA BF
   3 GLU   (   7-)  A      OE1 <->    6 ARG   (  10-)  A      NH2    0.21    2.49  INTRA BF
  62 ASP   (  66-)  A      OD2 <->   67 GLY   (  71-)  A      N      0.21    2.49  INTRA BL
  30 LYS   (  34-)  A      NZ  <->   55 ASP   (  59-)  A      OD2    0.17    2.53  INTRA BF
  28 ILE   (  32-)  A      N   <->   69 LEU   (  73-)  A      O      0.14    2.56  INTRA BL
  47 ASN   (  51-)  A      ND2 <->   48 GLN   (  52-)  A      OE1    0.14    2.56  INTRA BF
  15 PHE   (  19-)  A      O   <->   19 ALA   (  23-)  A      N      0.14    2.56  INTRA BL
   2 THR   (   6-)  A      N   <->    5 GLU   (   9-)  A      OE1    0.11    2.59  INTRA BL
  29 SER   (  33-)  A      N   <->   32 GLU   (  36-)  A      OE1    0.11    2.59  INTRA BL
  47 ASN   (  51-)  A      CG  <->   48 GLN   (  52-)  A      N      0.09    2.91  INTRA BF
  73 GLU   (  77-)  A      CD  <->  111  CA   ( 102-)  A     CA      0.08    3.12  INTRA BL
  59 LYS   (  63-)  A      NZ  <->   62 ASP   (  66-)  A      OD2    0.08    2.62  INTRA BF
  30 LYS   (  34-)  A      NZ  <->  112 HOH   (2008 )  A      O      0.08    2.62  INTRA BF
  42 ALA   (  46-)  A      O   <->   46 GLN   (  50-)  A      N      0.06    2.64  INTRA BF
  26 THR   (  30-)  A      N   <->   27 LYS   (  31-)  A      N      0.03    2.57  INTRA BL
  90 ILE   (  94-)  A      O   <->   94 GLN   (  98-)  A      N      0.03    2.67  INTRA BL
  53 VAL   (  57-)  A      CG1 <->   54 LEU   (  58-)  A      N      0.03    2.97  INTRA BF
  47 ASN   (  51-)  A      ND2 <->  106 TRP   (  11-)  D      O      0.01    2.69  INTRA BF
  64 ASP   (  68-)  A      N   <->   65 SER   (  69-)  A      N      0.01    2.59  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.

  21 ARG   (  25-)  A      -6.33
  49 LYS   (  53-)  A      -6.10
  94 GLN   (  98-)  A      -5.47
  47 ASN   (  51-)  A      -5.08

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

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.

  25 ASN   (  29-)  A

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.

   4 THR   (   8-)  A      OG1
  54 LEU   (  58-)  A      N
  55 ASP   (  59-)  A      N
  99 SER   (   4-)  D      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.

  16 GLN   (  20-)  A    A OE1

Warning: Unusual ion packing

We implemented the ion valence determination method of Brown and Wu [REF] similar to Nayal and Di Cera [REF]. See also Mueller, Koepke and Sheldrick [REF]. It must be stated that the validation of ions in PDB files is very difficult. Ideal ion-ligand distances often differ no more than 0.1 Angstrom, and in a 2.0 Angstrom resolution structure 0.1 Angstrom is not very much. Nayal and Di Cera showed that this method has great potential, but the method has not been validated. Part of our implementation (comparing ion types) is even fully new and despite that we see it work well in the few cases that are trivial, we must emphasize that this validation method is untested. See: swift.cmbi.ru.nl/teach/theory/ for a detailed explanation.

The output gives the ion, the valency score for the ion itself, the valency score for the suggested alternative ion, and a series of possible comments *1 indicates that the suggested alternate atom type has been observed in the PDB file at another location in space. *2 indicates that WHAT IF thinks to have found this ion type in the crystallisation conditions as described in the REMARK 280 cards of the PDB file. *S Indicates that this ions is located at a special position (i.e. at a symmetry axis). N4 stands for NH4+.

 110  CA   ( 101-)  A     0.53   0.74 Scores about as good as NA
 111  CA   ( 102-)  A     0.50   1.48 Is perhaps  K

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.

  55 ASP   (  59-)  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 :   0.702
  2nd generation packing quality :   0.651
  Ramachandran plot appearance   :   0.258
  chi-1/chi-2 rotamer normality  :  -2.420
  Backbone conformation          :   1.027

RMS Z-scores, should be close to 1.0:
  Bond lengths                   :   0.789
  Bond angles                    :   1.588
  Omega angle restraints         :   0.382 (tight)
  Side chain planarity           :   0.445 (tight)
  Improper dihedral distribution :   1.390
  B-factor distribution          :   0.806
  Inside/Outside distribution    :   1.082

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 : 2.30

Structure Z-scores, positive is better than average:

  1st generation packing quality :   1.5
  2nd generation packing quality :   1.1
  Ramachandran plot appearance   :   1.4
  chi-1/chi-2 rotamer normality  :  -0.9
  Backbone conformation          :   1.1

RMS Z-scores, should be close to 1.0:
  Bond lengths                   :   0.789
  Bond angles                    :   1.588
  Omega angle restraints         :   0.382 (tight)
  Side chain planarity           :   0.445 (tight)
  Improper dihedral distribution :   1.390
  B-factor distribution          :   0.806
  Inside/Outside distribution    :   1.082

      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
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    M.Nayal and E.Di Cera,
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    P.Mueller, S.Koepke and G.M.Sheldrick,
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Checking checks
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