dssr did not recognize some canonical nucleotide in 6nd4 chain 2

[zcx]

I am using dssr version v1.9.10-2020apr23 to analyze PDB 6nd4 chain 2. There are 146 standard nucleotides in this chain. However, when I run

x3dna-dssr -i=6nd42.pdb  -o=output.dssr

It reports
    total number of nucleotides: 144

This also causes the dssr-2ndstrs.dbn files to have 144 rather than 146 positions. It seems dssr disregard residue G248 for unknown reason.

[xiangjun]

DSSR is behaving as designed. Please see the section "Identification of nucleotides" of  the 2015 DSSR paper:

A nucleotide is identified if a residue contains at least three base ring atoms and the root-mean-square deviation (rmsd) of the fit falls below a user-definable cutoff. Since base rings are rigid, the rmsd is normally <0.1 Å. To account for experimental error and special non-planar cases, such as 5,6-dihydrouridine (H2U) in yeast tRNA^Phe (Figure 2), the default rmsd cutoff is set to 0.28 Å.

The default DSSR cutoff values are based on extensive tests in real-world applications. Any unidentified nucleotide is almost always due to heavy distortions in its base geometry that is 'beyond recognition'. For example, G248 in your attached 6nd42.pdb file has the PyMOL rendered image as attached. Note the N1-C2 distance is 2.2 Å, far larger than ~1.5 Å (the normal covalent C-N bond length).

DSSR Pro has provisions to handle extreme cases like yours.

[zcx]

This is probably misleading. For the sake of calculating base pairs, I completely agree that we should not consider nucleobases with highly distorted conformation. However, when counting how many nucleotides are there in the sequence (e.g., when generating ct or dbn files), nucleotides with distorted base conformation (or even nucleotides without their base groups) are still nucleotides. It may not be a good idea to just delete them from the sequence without at least printing out a warning message.

[xiangjun]

As mentioned in my previous response, DSSR Pro has options to handle such cases, among other features.

DSSR Pro's default output reports 146 nucleotides, along with a diagnostic note for the two deformed bases. Such deformed bases can participate in a variety of loops but not in pairing interactions.

Processing file '6nd42.pdb'
  2.G.248 0.808 -- distorted, without fitted base frame
  2.G.323 0.319 -- distorted, without fitted base frame
    total number of nucleotides: 146

DSSR Pro also has an option that treats those distorted bases as normal for base-pairing interactions.