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[ccp4bb] TLS, B factors, phenix and refmac |
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CCP4bb navigationCCP4bb <-- 2008 <-- April 2008 <-- 01 April 2008Subject: TLS, B factors, phenix and refmac From: Mischa Machius Mischa {- dot -} Machius {- at -} UTSOUTHWESTERN {- dot -} EDU Date: 2008-04-01 and differences between Phenix and refmac, we looked into these matters in more detail. We found that the crux of the problem lies in the fact that TLS and B value refinements are usually decoupled. We have developed a formalism that rolls both TLS and B value refinement into one. Phenix and refmac were modified to carry out the calculations, and the outputs from both programs were made compatible to allow proper comparison of the results. We found that the stability of the refinements is now vastly improved. More importantly, however, due to the reduced number of parameters, these calculations can be carried out to resolutions of 7 Å with meaningful representations of indiviual, anisotropic atomic displacement parameters. This low-resolution limit required reformulating the calculation of Wilson B values, but that is only a minor aspect of our treatment that can be neglected. The new, combined procedure for the simultaneous refinement of TLS/B is called 'TBS' refinement, reflecting all required components: Translation, Bibation, Screw. Interestingly, the ‘T’ component is fairly insensitive to input parameters, whereas the overall quality of the refinement is greatly dependent on the ‘B’ component. The more emphasis is put on ‘B’, the more convincing the results. There is a limit, though. At very high levels of ‘B’, the so-called ‘bibacity limit’, the refinement becomes very unstable, leading to inversion in severe cases. Seasoned crystallographers familiar with the concepts can successfully push the procedure to quite high 'B limits', whereas less experienced practitioners should follow the protocols very carefully. Please contact us for any details. Best - MM -------------------------------------------------------------------------------- Mischa Machius, PhD Associate Professor UT Southwestern Medical Center at Dallas 5323 Harry Hines Blvd.; ND10.214A Dallas, TX 75390-8816; U.S.A. Tel: +1 214 645 6381 Fax: +1 214 645 6353 CCP4bb navigationCCP4bb <-- 2008 <-- April 2008 <-- 01 April 2008 |
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