Nat and Misha,
Thank you for the suggestions.
Xtriage does indeed detect twinning in P1, reporting similar values
for <|L|>, <L^2>, and twin fraction as in P212121.
The unit cell dimensions for the 2.0-A structure (P1) are:
72.050 105.987 201.142 89.97 89.98 89.94 P 1
The unit cell dimensions for the 2.8-A structure (P212121) are:
75.456 115.154 202.022 90.00 90.00 90.00 P 21 21 21
I have been processing in HKL2000, which only recognizes one set of
unit cell parameters for each Bravais lattice (does anyone know how to
change this?). Specifically, for a primitive monoclinic unit cell it estimates:
104.53 71.82 200.99 89.86 91.80 91.16
This is the unit cell which refined to Rwork/Rfree ~ 27%/34%.
Indexing in mosflm gives three options for primitive monoclinic:
105.6 71.7 200.9 90.0 90.1 90.0
71.7 105.6 201.0 90.0 89.9 90.0
71.7 200.9 105.6 90.0 90.3 90.0
Attempting to integrate in any of these space groups leads to a fatal
error in subroutine "MASKIT". I can also use the "index multiple
lattices" feature to get a
whole slew of potential space group; however, integrating reflections
leads to the same fatal error.
Finally, Zanuda tells me that P212121 is the best space group,
according to R-factors. However, I do not believe P212121 is the
correct assignment.
Best,
Chris
On 7/10/14, Isupov, Michail <[log in to unmask]> wrote:
> I would recommend to run ZANUDA in the default mode from ccp4i or on CCP4
> web server.
> ZANUDA has resolved several similar cases for me.
>
> Misha
>
> ________________________________________
> From: CCP4 bulletin board [[log in to unmask]] on behalf of Chris Fage
> [[log in to unmask]]
> Sent: 10 July 2014 01:14
> To: [log in to unmask]
> Subject: [ccp4bb] Proper detwinning?
>
> Hi Everyone,
>
> Despite modelling completely into great electron density, Rwork/Rfree
> stalled at ~38%/44% during refinement of my 2.0-angstrom structure
> (P212121, 4 monomers per asymmetric unit). Xtriage suggested twinning,
> with <|L|> = 0.419, <L^2> = 0.245, and twin fraction = 0.415-0.447.
> However, there are no twin laws in this space group. I reprocessed the
> dataset in P21 (8 monomers/AU), which did not alter Rwork/Rfree, and
> in P1 (16 monomers/AU), which dropped Rwork/Rfree to ~27%/32%. Xtriage
> reported the pseudo-merohedral twin laws below.
>
> P21:
> h, -k, -l
>
> P1:
> h, -k, -l;
> -h, k, -l;
> -h, -k, l
>
> Performing intensity-based twin refinement in Refmac5 dropped
> Rwork/Rfree to ~27%/34% (P21) and ~18%/22% (P1). Would it be
> appropriate to continue with twin refinement in space group P1? How do
> I know I'm taking the right approach?
>
> Interestingly, I solved the structure of the same protein in P212121
> at 2.8 angstroms from a different crystal. Rwork/Rfree bottomed out at
> ~21%/26%. One unit cell dimension is 9 angstroms greater in the
> "twinned" dataset than in the "untwinned".
>
> Thank you for any suggestions!
>
> Regards,
> Chris
>
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