Hi Edward

There's no difference between any of the space groups you mention in
terms of which is right or wrong, they are all equivalent: for the
purposes of solving & refining the structure it makes absolutely no
difference which one is chosen.  A2, C2 and I2 differ only in the unit
cell and the indexing.  The convention is to choose the one that gives
beta as close to 90 as possible (but >=90).  In all cases b is chosen
as the unique axis and c >= a.  But that is only a convention which
ensures that if you determine the structure of an isomorphous crystal
you will always get the same setting, which just makes it easier to
determine that they are isomorphous.  'C121' (or 'C 2' or 'C 1 2 1')
and 'I121' (or 'I 2' or  'I 1 2 1') are simply alternative names for
'C2' and 'I2'; if the program doesn't accept them it just means the
other names haven't been programmed.  I've never understood the
rationale for the longer names, I think it was supposed to make the
names unique, but provided you stick with the conventional settings
(and in the vast majority of cases there's no reason not to), the
short names are already unique without having the extra '1's and
spaces.  It seems to me that having the option of adding '1' & spaces
just adds to the confusion and leads to the exact problem you have
encountered!  Finally, 'C21' or 'C1211' or 'I21' or 'I1211' (or the
versions with embedded spaces) are just origin-shifted
non-conventional settings of C2 and I2; again it's very unlikely you
would have a good reason to use them.

Personally I would just stick with C2 or I2!

Cheers

-- Ian

On Thu, Mar 11, 2010 at 6:07 PM, Edward Snell <[log in to unmask]> wrote:
> Dear All,
>
>
>
> I may be asking a dumb question and if so I apologize. I have a ~200 amino
> acid N-terminal ‘arm’ of a full protein (C-term already solved) that
> diffracts to ~2.1A. It integrates nicely in C2 and gives good molecular
> replacement models from Balbes in C121, 1I21, A121, C1211 and I1211 (>90%
> certain). Phaser gives nice hits in C2 and I2 but I have not been able to
> get it to accept the C1211 or I1211.  It appears from refining the data that
> my solution is either C1211 or more probably I1211 unfortunately I am having
> problems using these two groups. In addition to this I also have some
> evidence of twinning L Molrep and Refmac will accept the I1211 and C1211
> groups but the freer and uniquify components after Scala will not. I’ve used
> reindex with the space group as is, in brackets, and in the ‘I 1 21 1’
> notation all to no avail. My refinements have been forced to use the mtz
> files output by the Balbes server which loose 50% completeness in these two
> cases. My current R and Rfree with this data are 27% and 38% and the overall
> chain that I have built is in good agreement with one of the SAXS envelopes
> I have, but the data completeness issue is causing me problems. Has anyone
> come across this, do I need to add a symm group to the existing groups? I
> seem to remember some issues with these a few years back but that part of my
> brain is too dusty at present!
>
>
>
> Thanks for any help and hopefully I will not be hitting my head against the
> wall with the obvious.
>
>
>
> Cheers,
>
> Eddie
>
>
>
> Edward Snell Ph.D.
> Assistant Prof. Department of Structural Biology, SUNY Buffalo,
> Hauptman-Woodward Medical Research Institute
> 700 Ellicott Street, Buffalo, NY 14203-1102
> Phone:     (716) 898 8631         Fax: (716) 898 8660
>
> Skype:      eddie.snell                 Email: [log in to unmask]
>
> Telepathy: 42.2 GHz
>
> Heisenberg was probably here!
>
>