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twinning overview

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Good, overview of reaction on twinning; many thanks for all replies!

a) Conditions to prevent twinning 

Many people mentioned the use of organics in small quantities (0.1-2.0%),
	particularly dioxane (0.5 +/- 0.4% ; 0.1-0.5% )
	also ran: aceton, dmso, PEG200, ethanol, n-butanol,
		DMF or b-octylglucoside (5-15mM), MPD (1-3%) 

Otherwise change anything:
	pH (~0.5 unit, or less cf Wim Hol's subtilisin :0.1 pH unit),
	temp (Frazao, Acta cryst. 1999, D55, p.1465)
	crystallization method etc. 

And improve the protein!

b) Twinned MIR structures 

best answer: search for low twin fraction. However, some structures were
solved with proper detwinning:

Goldman et al, muconate lactonising enzyme, JMB, 1985

Hillig, R.C., Renault, L., Vetter, I.R., Drell, T., Wittinghofer, A., and
Becker, J. (1999). The crystal structure of rna1p: A new fold for a
GTPase-activating protein.  Molecular Cell. 3, 781-791.

Forst, D., Welte, W., Wacker, T., and Diederichs, K. (1998). Structure of
the sucrose-specific porin scry from salmonella typhimurium and its 
complex with sucrose.  Nature Structural Biology 5, 37-46

Wang et al, in progress (mit), using detwin and Solve 

c) Detwinning for MAD has not yet been implemented in most software, is
not exactly trivial; sometimes succesfull MAD by ignoring the twin, when
this is only a minor fraction (3-10%) 

d) nice but very complicated example from Eleanor Dodson

Finally lots of referrals to general things: 
$CHMTL/ pages on twinning in ccp4
http://www.doe-mbi.ucla.edu/Services/Twinning/ Todd Yeates twinning server 
SHELXL refinement against twinned data 
DETWIN in ccp4, which will apparently change a lot in the upcoming release
detwinning program of Rams (S. Ramaswamy) in Uppsala



Titia K. Sixma                    E-mail: Sixma@nki.nl
Netherlands Cancer Institute      Tel   : +31-20-5121959/1964
Department H2                     Fax   : +31-20-5121954
Plesmanlaan 121
1066 CX Amsterdam
The Netherlands