A structure can look plausible in the viewer and still contain a segment that deserves a closer look. Perhaps a chain has far more residues than you expected, or a segment contains an unusual number of structural groups. Manually inspecting a large model node by node can be slow. SAMSON’s Node Specification Language (NSL) offers a practical first check: query segments by their counts, then inspect the matches in context.
In NSL, the segment attribute space uses the prefix s. Two attributes are especially useful for a first pass:
s.nris the number of residues in a segment.s.nsgis the number of structural groups in a segment.
For example, s.nr > 100 matches segments with more than 100 residues. If you have a rough expected size, a range can help narrow the search: s.nr 100:120 matches segments with a number of residues between 100 and 120. Similarly, s.nsg > 10 finds segments with more than 10 structural groups, while s.nsg 10:12 searches for a count between 10 and 12.
These queries are useful as triage, not as verdicts. A high residue count is not automatically an error: a large protein segment may be expected. The value is that a count gives you a concrete way to locate candidates for review instead of relying only on visual impression. Start with a threshold that makes sense for the structure you are examining, and adjust it when you know more about the model.
Use more than one kind of count
Residues and structural groups describe different aspects of a segment, so they can help answer different questions. If the concern is segment length in residue terms, start with s.nr. If you want to find segments with many structural groups, use s.nsg. You can also compare these with inherited structural-group attributes such as s.nat, the number of atoms. For example, s.nat > 1000 matches segments with more than 1,000 atoms, and s.nat 100:200 matches those with a count between 100 and 200.
Element counts can add another useful piece of context. The segment attribute space inherits counts such as s.nC for carbons, s.nN for nitrogens, and s.nO for oxygens. For instance, s.nO > 10 searches for segments with more than 10 oxygen atoms. These counts do not explain why a segment has its composition or size, but they can help you decide what to inspect next.
A simple review routine
- Choose the unit that best fits your question: residues with
s.nr, structural groups withs.nsg, or atoms withs.nat. - Run a broad threshold query, such as
s.nr > 100, to identify candidate segments. - If you already know an expected range, use a range expression such as
s.nr 100:120to focus the search. - Review the matching segments in the model and use your knowledge of the structure to judge whether the counts are expected.
The key benefit is a repeatable first check: instead of guessing from the view, you can express a size question directly and use the resulting matches to guide inspection. Segment counts are simple, but they can make a large structure easier to navigate and help bring unexpected cases to your attention.
For the complete list of segment attributes and examples, see the SAMSON segment attributes reference.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.