[Chimera-users] Calculating distances in and between sheets

Elaine Meng meng at cgl.ucsf.edu
Sat Jan 27 09:27:45 PST 2018

Dear Einav,
There may not be a way to do exactly what you want, since for distance measurements the planes are treated as infinite.  However, axes are treated as finite line segments for distance measurements (although infinite for angle measurements), so I would have guessed it would work, but maybe not with the specific geometry of your system.

One idea is that you can use command “measure distance” to report the minimum distance between one set of atoms and another, and in that case I'd use only CA atoms or N,CA,C,O to avoid the variability from sidechains.  I would do it several times (say for different regions of the sheets) to see the spread of the resulting values. 

Oooh, I just thought of a trick: use this command to measure the minimum distance between the discs of defined planes (but treating them as finite) and to my surprise, it actually worked!!

(1) define two planes
(2) Ctrl-click one to select it, command: namesel sel1
(3) Ctrl-click the other plane to select it, command: namesel sel2
(4) command: measure distance sel1 sel2

(I thought it wouldn’t work because these plane surfaces are hidden from the model panel, but as far as I can tell, the distance reported in the status line and Reply Log is correct for my two planes)

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

> On Jan 27, 2018, at 1:56 AM, Einav Tayeb-Fligelman <einavt at gmail.com> wrote:
> Dear all,
> I am trying to calculate distances in a unique structure in which alpha-helices stack into sheets, as in beta-sheets. I wish to calculate the distance between two facing sheets and between helices along each sheet. The structure theoretically contains tens of thousands of helices in each sheet (the content of the asymmetric unit is only one helix and the rest are a result of symmetry operations). 
> I tried creating planes, one for each sheet, and calculate their distances, but I get zero since the planes are not exactly parallel. 
> I also tried calculating centroid to centroid distance (I used 4 helices for each sheet), but there is an angle between the centroids and therefore I get a longer value then expected. Same when calculating axis to axis distance. 
> Is there a good way to calculate such things with chimera? and if so, I would appreciate an explanation for an "alpha-helical sheet" containing structure, as well as for facing beta-sheets containing structure (in case there is a difference in the explanation).
> Thank you in advance,
> Einav 
> -- 
> PhD student at the Landau lab 
> Faculty of Biology 
> Technion, Israel Institute of Technology
> Haifa, Israel
> E-mail :einavtf at campus.technion.ac.il
> Tel at the lab:+972-77-8871964
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