A particularly useful command line option in SHELXT for such cases is
-L15 to try all trigonal and hexagonal space groups. SHELXT is included
in recent CCP4 distributions and is also available with documentation
from shelx.uni-goettingen.de
George
On 02.08.2018 15:53, Aaron Finke wrote:
Hi Kristof,
Direct methods/charge flipping won’t work if your data quality is poor
below ~1Å. Take a look at your Rmerge, as Jeffrey mentioned. Small
molecule crystals have more stringent standards for data quality. If
your Rmerge is 50% or more, it’s probably just noise and not useful
for direct methods.
If SHELXT doesn’t work on the full data set, try cutting the data down
to 0.9 Å or even 1 Å. I have had limited success doing that on some
particularly bad data sets. Alternatively, run SHELXD at full
resolution indefinitely (NTRY 0) until it finds a solution. It could
take days, but you may get something.
As a last resort, if you think this hexapeptide may have any secondary
structure, you may want to try ab initio MR with ARCIMBOLDO.
Aaron
------------------------------------------
Aaron Finke
Staff Scientist, MacCHESS
Cornell University
e-mail: af...@cornell.edu <mailto:af...@cornell.edu>
On Aug 2, 2018, at 8:53 AM, Kristof Van Hecke
<kristofrg.vanhe...@gmail.com <mailto:kristofrg.vanhe...@gmail.com>>
wrote:
Dear all,
I’m trying to solve a structure of a (modified) hexapeptide:
- inhouse (very decent) data up to 0.8 Angstrom
- average redundancy = 10
- according to the Matthews coefficient of 1.88 with 34.77 %solvent,
there should be 3 Nmol/asym
- ‘large’ unit cell of about a=54, b=54, c=12
- SG = P3(1)12 or P3(2)12
As there’s (presumably) only C, H, N and O in the structure, I’m not
able to solve this via Direct Methods, Charge Flipping etc,.
Trying MR (with Phaser) doesn’t give any results either, as there’s
hardly any homologous models
Has anyone encountered a similar problem please, and could provide
any possible solutions?
(building in heavy atoms isn’t my first option at the moment,. )
Thank you very much
Regards
Kristof
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Prof. George M. Sheldrick FRS
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University of Goettingen
Tammannstr. 4
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Germany
Tel: +49 551 3933021 or +49 5594 227312
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