Visualizing Protein-Ligand Unpaired Interactions


You want to visualize protein-ligand unpaired and clash interactions. See example in Figure 1.

Figure 1. Example of visualizing protein-ligand unpaired and clash interactions (PDB: 1NQ2))



  • OEChem TK - cheminformatics toolkit (including OEBio TK)

  • OEDepict TK - molecule depiction toolkit

  • Grapheme TK - molecule and property visualization toolkit

Difficulty level

../_images/chilly.png ../_images/chilly.png


Download code

See also Usage subsection.


The depict_unpairedmap illustrates how simple it is to generate these images.

  1. OEInteractionHintContainer object is constructed that stores information about possible interactions between the ligand and the protein.

  2. The interactions are perceived by calling the OEPerceiveInteractionHints function.

  3. The active site is then prepared for 2D depiction by invoking the OEPrepareActiveSiteDepiction function.

  4. When the OE2DActiveSiteDisplay object is constructed, residues are positioned around the ligand close to those atoms which they are interacting with.

  5. The OERenderUnpairedInteractionMap function generates an image that displays the clash and unpaired interactions detected in the ligand and in nearby residues.

  6. The legend associated with the unpaired map is rendered by invoking the OEDrawUnpairedInteractionMapLegend function.

 1def depict_unpairedmap(image, protein, ligand, opts, legend_frame=None):
 2    """
 3    :type image: oedepict.OEImageBase
 4    :type protein: oechem.OEMolBase
 5    :type ligand: oechem.OEMolBase
 6    :type opts: oedepict.OE2DMolDisplayOptions
 7    :type legend_frame: oedepict.OEImageBase
 8    """
10    # perceive interactions
12    asite = oechem.OEInteractionHintContainer(protein, ligand)
13    if not asite.IsValid():
14        oechem.OEThrow.Fatal("Cannot initialize active site!")
15    asite.SetTitle(ligand.GetTitle())
17    oechem.OEPerceiveInteractionHints(asite)
19    # depiction
21    oegrapheme.OEPrepareActiveSiteDepiction(asite)
22    adisp = oegrapheme.OE2DActiveSiteDisplay(asite, opts)
23    oegrapheme.OERenderUnpairedInteractionMap(image, adisp)
25    if legend_frame is not None:
26        lopts = oegrapheme.OE2DActiveSiteLegendDisplayOptions(12, 1)
27        oegrapheme.OEDrawUnpairedInteractionMapLegend(legend_frame, adisp, lopts)



The following commands will generate the image shown in Figure 1.

prompt > wget
prompt > python3 -complex 1nq2.pdb -out 1nq2.svg -interactive-legend

Command Line Parameters

Simple parameter list
    -height : Height of output image
    -width : Width of output image

    SplitMolComplex options :
      -covalentligand : Split covalent ligands
      -ligandname : Ligand name

    molecule display options :
      -aromstyle : Aromatic ring display style

    input/output options :
      -complex : Input filename of the protein-ligand complex
      -protein : Input filename of the protein
      -ligand : Input filename of the ligand
      -out : Output filename of the generated image

    active site display options:
      -interactive-legend : Visualize legend on mouse hover (SVG feature)


Interaction Perception

Currently the OEPerceiveInteractionHints function perceives the following interaction types:

Table 1. Interaction types currently available in OEChem TK


corresponding interaction class

corresponding interaction type namespace
















halogen bond



hydrogen bond






stacking (T and Pi)



The default geometric parameters used by the OEPerceiveInteractionHints function have been set based on literature data ([Kumar-2002], [Cavallo-2016], [Bissantz-2010], and [Marcou-2007] ). The interaction parameters can be customized by using the OEPerceiveInteractionOptions class.

Unpaired Interaction Depiction

The OERenderUnpairedInteractionMap function currently visualizes the following interactions detected by the OEPerceiveInteractionHints function.

  • Atom clash interaction

    When visualizing protein-ligand atom crashes, red outline of a residue circle indicates that there are one or more atoms in that residue which are too close to some ligand atom(s). Clashing ligand atoms are marked with a red arc that is directed towards the corresponding clashing residue. The red shading on the grey line representing the shape of the pocket is used to identify atom clashes easily.

    Table 2. Examples of visualizing atom clash(es)
    ../_images/unpairedmap2img-atom-clash.png ../_images/unpairedmap2img-atom-clash-multiple.png

    See also

  • Unpaired types of the hydrogen bonding interaction

    An unpaired hydrogen bond interaction is detected:

    • if there is no ligand/protein acceptor atom that could interact with a protein/ligand donor atom

    • if there is no ligand/protein donor atom that could interact with a protein/ligand acceptor atom

    Different linker types are used to mark unpaired acceptor and donor hydrogen bond interactions. Please note that since these interactions are unpaired, the direction of the linkers has no real spatial meaning. Ligand linkers are directed away from the ligand, while protein linkers are directed towards the ligand.

    Table 3. Examples of visualizing unpaired hydrogen bond interactions
    ../_images/unpairedmap2img-hbond-unpaired-acceptor-ligand.png ../_images/unpairedmap2img-hbond-unpaired-acceptor-protein.png ../_images/unpairedmap2img-hbond-unpaired-donor-ligand.png ../_images/unpairedmap2img-hbond-unpaired-donor-protein.png

    unpaired ligand acceptor

    unpaired protein acceptor

    unpaired ligand donor

    unpaired protein donor

    See also

  • Clash types of the hydrogen bonding interaction

    A hydrogen bond clash interaction is detected:

    • if an acceptor ligand atom interacts with an acceptor protein atom

    • if a donor ligand atom interacts with an donor protein atom

    Table 4. Examples of visualization hydrogen bond clash interactions
    ../_images/unpairedmap2img-hbond-clash-donor-donor.png ../_images/unpairedmap2img-hbond-clash-acceptor-acceptor.png

    donor-donor clash

    acceptor-acceptor clash

    See also

  • Unpaired types of the salt-bridge interaction

    An unpaired salt bridge interaction is detected if there is a positively / negatively charged functional group either in the ligand or in nearby protein without a matching negatively / positively charged functional group, respectively.

    Different linker types are used to mark unpaired positive and negative salt-bridge interactions. Please note that since these interactions are unpaired, the direction of the linkers has no real spatial meaning. Ligand linkers are directed away from the ligand, while protein linkers are directed towards the ligand.

    Table 5. Examples of visualizing unpaired salt-bridge interactions
    ../_images/unpairedmap2img-saltbridge-unpaired-positive-ligand.png ../_images/unpairedmap2img-saltbridge-unpaired-positive-protein.png ../_images/unpairedmap2img-saltbridge-unpaired-negative-ligand.png ../_images/unpairedmap2img-saltbridge-unpaired-negative-protein.png

    unpaired ligand positive

    unpaired protein positive

    unpaired ligand negative

    unpaired protein negative

    See also

Hydrogen Position Optimization

Since the interaction perception dependents on the position of hydrogens, it is highly recommended to optimize those positions prior to perceiving the interactions. The two images below reveal the effect of optimizing the hydrogen bond network in a protein-ligand complex: less atom clash(es) and less unpaired hydrogen bond interaction(s).

Table 6. Example of visualizing protein-ligand unpaired and clash interactions before and after optimizing hydrogen positions
../_images/unpairedmap2img-pdb1d3h.svg ../_images/unpairedmap2img-pdb1d3h-optimized.svg

original complex downloaded from PDB Database (PDB 1D3H)

complex after optimizing hydrogen positions

See also

Unpaired Map vs Active Site Interaction Map

An unpaired interaction map provides a complementary view to the more common active site interaction map. While the interaction map (on the right) depicts interactions between the ligand and protein, the unpaired map (on the left) illustrates interactions that could contribute to binding but are not formed in the complex. Together, these two maps of the protein-ligand binding site provide insights into protein-ligand interactions and communicate complex 3D structural results to medicinal chemists in a directly actionable way.

Table 2. Example of visualizing unpaired and active site interaction maps
../_images/complex2img-pdb1br6-optimized.svg ../_images/unpairedmap2img-pdb1br6-optimized.svg

unpaired interaction map (PDB 1BR6)

active site interaction map (PDB 1BR6)

See also in OEChem TK manual



See also in OEDepict TK manual