On May 9, 2010, at 10:14 PM, Miguel Ortiz Lombardia wrote:
Dear all,
I would like to represent as a volume the space that would be covered by a set of atoms that are bound by "torsionable" bonds when one systematically modifies those torsion angles. An example would be the side-chain of an amino acid; imagine I would like to have a glimpse of what part of the space could be occupied by that side- chain. As a first approximation I wouldn't care about clashes.
I would appreciate if someone can point me to an existing way of obtaining such representation or to an algorithm that could be implemented within chimera (which I would be happy to share with other interested people; if I manage to code it, that's it!)
Hi Miguel, This problem has two parts: adjusting the torsions and visualizing the result. Adjusting the Torsions If you're adjusting the chi angles of an amino acid, then this is easy in Chimera. Each such residue has chi1/chi2/chi3 attributes that you can set, e.g.: for chi1 in range(0, 360, 10): r.chi1 = chi1 for chi2 in range(0, 360, 10): r.chi2 = chi2 for chi3 in range(0, 360, 10): r.chi3 = chi3 do something You'd probably actually want to write the code as a recursive routine, since otherwise it is awkward to handle the fact that some residues have different numbers of chi angles than others. You might also want to change your angle increment based on the recursion depth. If you're changing torsions that aren't the well-known amino acid chi angles, then you'll have to resort to Chimera's more generic torsion- adjustment facilities, the core of which are in the BondRotMgr module. Specifically, if 'b' is the bond you want to adjust, then: from BondRotMgr import bondRotMgr torsion = bondRotMgr.rotationForBond(b) and then torsion.increment(amount) will increment the torsion angle by 'amount' degrees (from its current position). If you want to set the dihedral to a certain value (rather than just increment from the current position) then you will have to measure the dihedral you care about with: chimera.dihedral(a1.xformCoord(), a2.xformCoord(), a3.xformCoord(), a4.xformCoord()) and then use the appropriate increment value, or use chimera.phipsi._setAngle(bond, newAngle, curAngle) [in the latter case you don't have to call rotationForBond yourself]. Visualizing the Result There are a bunch of possibilities of course. I'll touch on two: a bunch of spheres, and an actual volume. 1) As you change the torsion, you can use each atom's .coord() method to get Point instances which can be used to place helium atoms at those positions with BuildStructure.placeHelium(). You could put the heliums in the same model or in a new model (by providing a model name to your first call of placeHelium()). 2) Computing a volume is not too hard conceptually. Basically you decide on a grid spacing and then make a dictionary that maps xyz grid addresses to occupancy counts (omitting counts of zero). Then you find the lower and upper bounds for each axis, create a 3D array with corresponding dimensions, and populate that array with your counts. Then you can open that in Volume Viewer and use all the Volume Viewer facilities to look at the occupancy. There is code that does just this in Movie.gui, it's MovieDialog's computeVolume method. The line: gridData = {} initializes the occupancy-count dictionary and the loop that immediately follows that populates the dictionary. Most of the loop isn't relevant for you (getting the right coordinate set and adjusting for "hold selection steady") but the very end of the loop is, the part that maps coordinates into xyz grid addresses and adjusts the corresponding count. Then after that the loop the "generate volume" and "show volume" sections are largely relevant. --Eric Eric Pettersen UCSF Computer Graphics Lab http://www.cgl.ucsf.edu