[Chimera-users] ITasser model looks different once PDB opened in UCSF Chimera

Elaine Meng meng at cgl.ucsf.edu
Wed Aug 25 17:36:43 PDT 2021

Hi Athena,
You are not doing anything wrong.

Although the atomic coordinates are identical, it is simply that different programs have different methods for deciding what is helix and strand.  When you get an experimental PDB file from the databank, the file contains information on where these were assigned, and then Chimera uses that.  However, when the PDB file does not contain such information, Chimera runs its own calculation to figure out what is strand and helix.  Again, the model atomic positions are not affected at all.  It is only how the ribbon is drawn.

Presumably the iTasser website uses some different method to figure out what is strand and helix, but the iTasser output download file only contains the atomic coordinates, not this extra information.

You can try re-running the calculation in Chimera with the "ksdssp" command using different (nondefault) parameter settings, or you can manually reassign secondary structure to specific ranges of residue numbers to control how the ribbon is drawn.

ksdssp command:

manual reassignment examples are given in previous posts, e.g.
... and in this tutorial

I hope this helps,
Elaine C. Meng, Ph.D.                       
UCSF Chimera(X) team
Department of Pharmaceutical Chemistry
University of California, San Francisco

> On Aug 25, 2021, at 5:03 PM, Athena Andreosso via Chimera-users <chimera-users at cgl.ucsf.edu> wrote:
> Hi,  
> I recently submitted a sequence to get a model in PDB format. When I opened the PDB file of model 1 in Chimera it looked completely different than the model on the ITasser results page – it had less beta strands.  When I clicked on model 2 it was better but there’s still a visible discrepancy. 
> Am I doing something wrong? Am I supposed to give chimera more info than just the PDB file ?
> Any help is appreciated.  
> Cheers
> Athena Andreosso, D.Phil.  | Research Officer 
> Mucosal Immunology  
> QIMR Berghofer Medical Research Institute
> e AthenaAndreosso at qimrberghofer.edu.au  |  www.qimr.edu.au
> 300 Herston Road, Herston QLD 4006

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