AMBER TI Tutorials
A simple TIMD tutorial about T4-lysozyme
Introduction to TIMD
This is a TIMD tutorial based on the tutorial written by Thomas Steinbrecher. But some important changes have been made to suit the current AMBER 10 version according to Miranda's tutorial from Simmerling's lab.
In this tutorial, free energy calculations will be used to calculate the relative binding free energy of two simple ligands, benzene and phenol to the T4-lysozyme mutant L99A. Free energies will be computed by using the thermodynamic integration facilities of the sander program. A modified van-der-Waals equation (softcore potentials) are used to ensure smooth free energy curves.
Thermodynamic cycle and Method
TI calculations compute the free energy difference between two states A and B by coupling them via a parameters λ that serves as an additional, nonspatial coordinate. This λ formalism allows the free energy difference between the states to be computed as:
when the molecules are going through the circle, you can get the perturbation data of the molecules.
Setup of the T4 L99A System
The two ligands were sketched and parametrized with gaff atom types and resp charges were generated using antechamber on gaussian03 output files. (Please refer to a basic AMBER tutorial on how to use the antechamber tools to parametrize a ligand. The benzene and phenol molecules were saved in two OFF-libraries (benz.lib and phen.lib) for further use. The screenshot shows that C6 was selected as the position bearing the hydroxyl group in phenol.
You can get the OFF-libraries from Shun or just download it from the following links:
Generation of ligands files
We will use the X-ray structure of T4-L99A from the pdb (after stripping water molecules and unneeded heteroatoms from it: pdb file) as basis to set up our simulation files. We will use two runs of leap to produce four set of parameter and restart files, containing both ligands in the protein bound and solvated states. The first leap run (input file) will produce pdb files of the solvated und neutralized benzene complex and of the benzene ligand in water (complex.pdb and ligand.pdb). From these two additional pdb files are made by renaming the BNZ molecule to PHN and deleting H6 (t4_phn.pdb and phn.pdb). These four pdb files are then used in a second leap run (input file) to generate the *.prm and *.rst files. This yields 4 parameter and 4 rst files