P212121

TLS stands for Translation Libration Screw-motion (the dash makes it acronym-ically fine) which is a method of refinement in the program REFMAC5 within the CCP4 suite or in phenix.refine. According to developer, Martyn Winn, TLS refinement can be at almost any resolution.

Why should I use it?
The benefits of using TLS refinement is that it can reduce your Rfree and Rwork values. The implication being that the produced structural model will be a better representation of the collected data.

How Does it Work?
TLS refines ’sequence groups’ that are described using 20 parameters per each group.

How Do I Determine the Groups?
The TLS Motion Determination (TLSMD) is a server that allows for the submission of your amino acid sequence and recommends how to segment your sequence (ref). A number of different TLS groups are possible for the same sequence (ref).
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How do I actually do this?
1) Do a rigid body refinement followed by ~10 rounds of restrained refinement
2) Take this output and submit it to the TLSMD
3) Take the segments that are produced by the TLSMD and fix the B factors to 40 (ref)
Note: the B factor was set to 20 in the literature reference
4) REFMAC needs the following inputs: REFI TLSC 20, TLSIN, BFAC SET 40 (more details and here)
5) Perform TLS refinement
6) Perform restrained refinement followed by the addition of ligands, ions and solvent
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How to do you know if TLS helped?
A decrease in the Rfree value as well as an improvement in the electron density maps.

I have done my best to condense about 100 pages of websites, presentations and literature into 250 words. Please let me know what I need to change/add/remove to make this post more helpful, thanks!