Hi Boaz, I changed the subject line to try to describe this topic. We aren't the APBS experts but my understanding is that yes, it will generate potential at all points in the rectangular grid, based on the input atoms charges and locations. There will of course be potential values even at grid points where there are no atoms, since electrostatics are fairly long-range interactions. However, the magnitude will depend on what dielectric parameters you have given the calculation. Whether it makes sense: that is somewhat up to your scientific judgement. In my opinion, yes, it makes sense, but you have to think carefully about the "offset" you use to color the cofactor surface. Normally when the surface is on the same atoms that produced the charge, you use a positive offset (default 1.4) to color by values projected that distance outward from the surface, to represent what other outside atoms might "see". If you are coloring a surface of the cofactor atoms to try to understand the potential from the protein that the cofactor "sees," you should consider instead using a negative (say -1.4) or zero offset. The offset is adjustable in the Surface Color dialog options and the "color electrostatic" command: <https://rbvi.ucsf.edu/chimerax/docs/user/tools/surfacecolor.html> <https://rbvi.ucsf.edu/chimerax/docs/user/commands/color.html#map> Note that an analogous procedure could be done in ChimeraX with the Coulombic electrostatic potential. For example, see the MOLE channels tutorial where we calculate the Coulombic potential from the protein but then show a surface on pseudoatoms representing a channel calculated by MOLEonline, and then color that channel surface by the protein potential. <https://www.rbvi.ucsf.edu/chimerax/data/mole-channel/mole-channel.html> I hope this helps, Elaine ----- Elaine C. Meng, Ph.D. UCSF Chimera(X) team Resource for Biocomputing, Visualization, and Informatics Department of Pharmaceutical Chemistry University of California, San Francisco
On Mar 15, 2025, at 10:30 AM, Boaz Shaanan via ChimeraX-users <chimerax-users@cgl.ucsf.edu> wrote:
Hi friends, On a related issue: I'd like to explore the effect of replacing Val by Asp on the electrostatic potential 'seen' by a cofactor which is completely surrounded by the protein (this is relevant for understanding the change in reaction rate following such replacement). I thought of the following steps:
• Omit the cofactor from the pdb2pqr and APBS. • Read into chimerax the complete pdb (i.e. including the cofactor). • Generate the surfaces (protein and cofactor). • Render the electrostatic potential on both surfaces (i.e. Cofactor and protein surrounding it) and probe the potential on the cofactor. • Does it make sense? Does APBS generate potential values in the void where the cofactor resides even though it wasn't present in the input pdb2pqr (i.e. in the void where the cofactor resides). I'd appreciate your input. Boaz