Dear all,
we have encountered a problem in solving one mutant structure. The
mutant protein crystallizes in the same space group as the native (C2)
but the unit cell dimensions are different. These for the native
structure are 101.2 33.1 73.4 90 90.3 90 and for the mutant 160.4 32.3
107.0 90 125.7 90. As a result the mutant structure has four molecules
in the asymmetric unit while the native had two. When we run molecular
replacement all programs (CNS, molrep, and amore) find only two
molecules. Phaser finds four but when we try to refine the Rfree does
not drop below 0.44 if we use four molecules and 0.53 if we only use two
no matter how well we built the molecule and regardless of any addition
of water molecules (the resolution of the data is 2.1). The interesting
thing is that in the electron density map we can clearly see density for
a substrate analog that was included in the crystallization media. Do
you thing that we have a case of twinning here ? We have to mention
that Tod Yates served did not indicate any perfect merohedral twinning
(partial merohedral twinning for this space group is not possible).
We would appreciate any comments
Many thanks
Demetres
--
Demetres D. Leonidas, Ph.D.
Structural Biology & Chemistry Group
Institute of Organic and Pharmaceutical Chemistry
The National Hellenic Research Foundation
48, Vassileos Constantinou Avenue
Athens 116 35, Greece
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