Hi T.M., To find the surface area with positive electrostatic potential would require computing the potential at surface points. Chimera does not compute solvent accessible surface (SAS) points (although it does compute total area). So there is no simple way to get the SAS area for positive potential points. Chimera does compute the solvent excluded surface (SES) using menu Actions / Surface or command "surface". For an SES surface a little Python code can give the fraction of the surface area colored red. I've attached that Python code red_area.py. You color the surface using the normal red-blue electrostatic coloring. The red_area.py code adds a command called "redarea" that computes the area of the surface that is more red than blue. Here's an example from the header of the red_area.py file. # Example use. # # menu File / Open..., red_area.py, Creates the new redarea command. # # open 1a0m # surface # coulombic -10 red 0 white 10 blue #0 # redarea #0 # # menu Favorites / Reply Log # -> "Surface MSMS main surface of 1a0m, red area 1400, total area 1888, ratio 0.7414" This script and other odd Python code for doing computations is on the Chimera developer web site: http://plato.cgl.ucsf.edu/trac/chimera/wiki/Scripts Tom On Dec 3, 2013, at 9:05 PM, Tatsuhito Matsuo wrote:

Dear Tom,

Thank you very much for the reply and I'm sorry for inadequate information on what I've done (your guess is completely right). What I'd like to know is the ratio of the solvent accessible surface (SAS) with positive (or negative) potential values to the total SAS. The total SAS value is automatically calculated by the surface plot, and I need the value of SAS having positive/negative potential values. At first, I thought the ESP value of "Attributes of Residues" is relevant to the one at the corresponding protein surface area. I now understand that this was wrong. Could you teach me how to get the SAS having positive/negative potential values?

Best regards,

T.M.

On 2013/12/04, at 12:19, Tom Goddard wrote:

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Hi T.M.,

I think you've left out details about what you have done -- how did you get this value_Coulombic_ESP attribute? Did you use values at atom positions? Did you use the "compute grid" option in the Coulombic Surface Coloring dialog? I guess you did that. And you say you use Attributes of Residues -- is that an average of the atom attribute obtained from the values at atom positions dialog? I can't be sure what you've done, but here's a guess at what your problem is. The Chimera Coulombic Surface Coloring dialog computes the potential at points 1.4 Angstroms out from the solvent excluded molecular surface and colors the surface points using that value. If you instead use a potential value interpolated at the center of an atom, that will give a much different value -- in fact it should tend to give an extremely large value because if the atom is charged the potential at the atom center is infinite using the simple Coulomb calculation. But because you are (I think) using a value interpolated from a grid you don't get an infinite value, just a very large value. So I think your problem is you are not considering that the electrostatic potential varies rapidly in space and you are comparing colors using potential values at different positions in space.

Tom

On Dec 2, 2013, at 6:31 PM, Tatsuhito Matsuo wrote:

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Dear All,

I'd like to get the electrostatic potential (ESP) values at each residue in a protein. I implemented the "Coulombic Surface Coloring", then get the distribution of ESP values at each residue by "Render/Select by Attribute" (Attributes of [residues], Select -> Attribute [value_Coulombic_ESP]). However, The surface plot and the ESP values obtained by "Render/Select by Attribute" do not coincide with each other. For example, there are some residues where the surface plot reports positive ESP values, while "Render/Select by Attribute" gives negative ones (e.g. VAL70 CG2 in the Chimera session file attached.). Could you teach me how to solve this discrepancy?

Thank you very much in advance,

T.M.

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