Dear colleagues,
we have collected 200 degrees worth of oscillation data on a selenomethionine derivative (1 "non-terminal" methionyl per 120 residues) to 2A resolution.
XDS provides the following output:
<COLLECT.LP>
  LATTICE-  BRAVAIS-   QUALITY  UNIT CELL CONSTANTS (ANGSTROEM & DEGREES)
 CHARACTER  LATTICE     OF FIT      a      b      c   alpha  beta gamma

 *  44        aP          0.0      47.9   47.9  171.2  90.1  90.0 119.9
 *  31        aP          0.4      47.9   47.9  171.2  89.9  90.0  60.1
 *  39        mC          0.8      83.0   47.9  171.2  90.0  90.1  89.9
 *  14        mC          2.2      47.9   82.9  171.2  90.0  90.1  90.0
 *  34        mP          2.3      47.9  171.2   47.9  90.1 119.9  90.0
 *  29        mC          2.6      47.9   82.9  171.2  90.0  90.1  90.0
 *  38        oC          2.8      47.9   83.0  171.2  89.9  90.0  90.1
 *  10        mC          2.8      83.0   47.9  171.2  90.0  90.1  89.9
 *  13        oC          2.9      47.9   82.9  171.2  90.0  90.1  90.0
 *  12        hP          3.4      47.9   47.9  171.2  90.1  90.0 119.9
    35        mP        249.9      47.9   47.9  171.2  90.0  90.1 119.9
    33        mP        250.4      47.9   47.9  171.2  90.0  90.1 119.9
    32        oP        251.8      47.9   47.9  171.2  90.1  90.0 119.9

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 SPACE-GROUP         UNIT CELL CONSTANTS            UNIQUE   Rmeas  COMPARED  LATTICE-

   NUMBER      a      b      c   alpha beta gamma                            CHARACTER

       5      82.9   48.0  171.3  90.0  90.0  90.0   13033    15.3    36401    10 mC

     143      47.9   47.9  171.3  90.0  90.0 120.0    8371    17.4    41063    12 hP

     149      47.9   47.9  171.3  90.0  90.0 120.0    4496    17.0    44938    12 hP

     150      47.9   47.9  171.3  90.0  90.0 120.0    4720    16.8    44714    12 hP

     168      47.9   47.9  171.3  90.0  90.0 120.0    4255    16.9    45179    12 hP

   * 177      47.9   47.9  171.3  90.0  90.0 120.0    2549    16.7    46885    12 hP

      21      48.0   82.9  171.3  90.0  90.0  90.0    6758    15.1    42676    13 oC

       5      48.0   82.9  171.3  90.0  90.1  90.0   12751    15.8    36683    14 mC

       5      48.0   82.9  171.3  90.0  90.1  90.0   12751    15.8    36683    29 mC

       1      47.9   47.9  171.3  89.9  90.0  60.1   24755     9.4    24679    31 aP

      21      47.9   83.0  171.3  90.0  90.0  90.0    6765     9.2    42669    38 oC

       3      47.9  171.3   47.9  90.0 119.9  90.0   12671     9.6    36763    34 mP

       5      83.0   47.9  171.3  90.0  90.1  90.0   13029     8.9    36405    39 mC

       1      47.9   47.9  171.3  90.1  90.0 119.9   24755     9.4    24679    44 aP

</COLLECT.LP>
We scaled reflections, with reasonable residuals, assuming space group P622 and have a reasonable solution in P6122. Refinement does not take us past an Rfree of aprrox. 0.4. The apparent quality of the model-phased map varies across the asymmetric unit.

The L-test does not indicate merohedral twinning. Translational pseudosymmetry was not detected. <Merohedral> twinning has been tentatively ruled out.

Before considering Order-Disorder-type crystal defects (as reviewed by Andrey Lebedev here: http://www.ysbl.york.ac.uk/mxstat/andrey/ecm2010.pdf), we would like to rule out that we might have erroneously applied too many symmetry constraints. To that end, we have scaled data in C2. We observed anomalous difference fourier peaks corresponding to positions of 6 selenium atoms in the current model, but refinement still does not proceed, and maps look similar to P6122 treatment. To "cover all bases" we would like to extend the asymmetric unit to P1. And here is the question:

Does it suffice to pursue any one of the P1-associated lattices in the XDS table or do both need to be tested?

Many thanks as always,

Wolfram Tempel