FAST INDIVIDUAL ATOMIC ANISOTROPIC REFINEMENT OF MACROMOLECULES USING FFT
Murshudov, G.N.1,2,*, Lebedev, A.3, Vagin, A.A.1, Wilson, K.S.1 and Dodson, E.J.1
1 Chemistry Department, University of
York,
2 CLRC, Daresbury Laboratory,
3 MPG-ASMB c/o DESY, Gebaude 25 B,
Notkestrasse 85, 22603 Hamburg,
*e-mail: garib@yorvic.york.ac.uk
Parameterisation of the model in a manner appropriate to the resolution is one of the essential tasks of a refinement program. Atoms at high and/or atomic resolution (higher than 1.8 A resolution) can be best described using individual anisotropic atomic displacement parameters (ADPs), often referred to as anisotropic temperature factors. So far this parameterisation has only been available in programs using classical least-squares equations. These are much slower to set up and solve than the fast Fourier approximations used to generate both structure factors and shifts in most macromolecular refinement packages. The necessary equations to incorporate anisotropic displacement parameters into the FFT formulation for all non-centric space groups have been derived and implemented in the program REFMAC[1] available from CCP4[2]. Similarity restraints on bonded atoms and sphericity restraints on individual atoms also will be discussed.
REFMAC uses a maximum likelihood residual which means that all parameters are "weighted" automatically according to the state of refinement, the amount of experimental data and its reliability, and the restraints imposed. This means that anisotropic displacement parameters can be used safely at lower resolutions.
Tests show that refinement of individual ADPs using FFT is very fast (refinement of a structure with 200 000 reflections and 12000 (x9=108000 parameters) atoms take approximately 10 minutes per cycle on a O2 SGI with R5000 processor). Refining anisotropic ADPs at high resolution (higher than 1.8A) improves not only the R-value and free R-value but also the geometric properties. This shows that anisotropic ADPs are important for the derivation of a good quality model. Test and real cases at different resolutions and levels of difficulty will be presented. Refinement of user-defined parts of structures (for example heavy atoms) in different modes is allowed.