Re: [ccp4bb]: molecular replacement woes!

["Fred. Vellieux" <vellieux@ibs.fr>]

***  For details on how to be removed from this list visit the  ***
***          CCP4 home page http://www.ccp4.ac.uk         ***

Just to make one point clear: the space group of the search model does not
influence the outcome of molecular replacement calculations. This is
because the model is placed in an artificial P1 cell (triclinic, only
symmetry is "1-fold").

So it does not matter if the crystal structure of the model was
P2(1)2(1)2(1), it could have been C2(1)2(1)2(1) (which does not
exist!) for that matter.

However, the space group of the crystal structure to be solved does
matter. I am assuming that the crystal system (orthorhombic) is
correct, and also that it is primitive. Then there will be no differences
in the outcome of the rotation function. Only the translation function
will differ (and possibly show the correct space group).

Hence: 1- do check carefully the systematic absences along a*, b*, c* to
confirm that you have the correct space group;
2- you may have to carry out translational searches in all primitive
orthorhombic space groups (there are few so you're lucky).

Finally, beware of rows of reflections which are extremely weak in a
systematic manner: the initial frame processing discards these, you end up
with a wrong assignment and the molecular replacement calculations and/or
refinement fail. Then one should reprocess to make sure that the weak
reflections are properly processed as well, and "do it again".

Fred.

PS: the choice of the search model, however, is clearly important. There
are numerous examples in the literature about this!

-- 
s-mail: F.M.D. Vellieux (B.Sc., Ph.D.)
        Institut de Biologie Structurale J.-P. Ebel CEA CNRS UJF
        41 rue Jules Horowitz
        38027 Grenoble Cedex 01
        France
Tel:    (+33) (0) 438789605
Fax:    (+33) (0) 438785494
e-mail: vellieux@ibs.fr
 ==========================================================================
 Don't even think of asking me: "I would              Het leven is genieten
  if I could, but I can't" (L.R., 1998)               (to live is to enjoy)
 ==========================================================================
BEGIN_ELECTRONIC_SIGNATURE
M/R-:@ZF:!H)4!":,+WW$O;CZMD.9)UDE'9=GB8HB/]>X><9JL0+B,:..\_@J
MP/H/0?*ZM_GS&C'30I,>S1G3UI>T-H;`!!>N'*JT1?[?;#OW[]UR'@YY2X`Z
MN)5"J6`$X-<O3Q/),4@]C=2$")KC"37!"Z1L]XW(-8E&KIE9/0[=K]@C3[-#
?79H.\3W6F8,D<4WH4D.]E.>U+FCGZ3L6+P0_(.@T$```
END_ELECTRONIC_SIGNATURE