Hi everyone,

I have a tricky structure that has pseudo-symmetry. Before we publish the structure, we would really appreciate some valuable feedback from experts like yourself.

I have recently solved a crystal structure (2.2 Angstrom) made up of a peptide bound to a dimer.

If I solve the structure in C2221 (Rfree = 25%), the asu consists of:

1) Dimer A-A' + peptide 1 (on symmetry axis)
2) Dimer B-B' + peptide 2 (on symmetry axis)
3) Dimer C-D + peptide 3

However, if I solve the structure in P21 (Rfree = 28%), the asu consists of:

1) Dimer A-B + peptide 1
2) Dimer C-D + peptide 2
3) Dimer E-F + peptide 3
4) Dimer G-H + peptide 4

Our peptide sequence is almost symmetrical (e.g. LLLGGGRGGGIII) , thus it is possible for it to be on the symmetry axis. However, in order to determine if the peptide really binds in both directions, we decided to synthesise a Se-Met labelled peptide. (We mutated one of the residues that does not interfere with binding into Met.)

Recently, we collected 3 SAD data sets (3.0, 3.5, 3.7 Angstrom, 360 degrees each) of the Se-Met labelled peptide bound to the dimer.

We processed the data in both P21 and C2221, then generated the anomalous difference map, using the above 2.2 Angstrom solved structure.

The results are as follows:

P21, 3.0 Angstrom
Peptide 1: 2 anomalous signals (sigma level set to 3.0) --> Forward and reverse direction
Peptide 2: 2 anomalous signals
Peptide 3: 1 anomalous signal --> Forward direction only
Peptide 4: 1 anomalous signal --> Forward direction only

P21, 3.5 Angstrom
Peptide 1: 2 anomalous signals
Peptide 2: No anomalous signals
Peptide 3: 1 anomalous signal --> Forward direction only
Peptide 4: 1 anomalous signal --> Forward direction only

P21, 3.7 Angstrom
Peptide 1: 2 anomalous signals
Peptide 2: 1 anomalous signals --> Forward direction only
Peptide 3: 1 anomalous signal --> Forward direction only
Peptide 4: 2 anomalous signals

C2221, 3.0 Angstrom
Peptide 1:  1 anomalous signal --> Forward direction only
Peptide 2 & 3: (on symmetry axis)

C2221, 3.5 Angstrom
Peptide 1:  1 anomalous signal --> Forward direction only
Peptide 2 & 3: (on symmetry axis)

C2221, 3.7 Angstrom
Peptide 1:  2 anomalous signals
Peptide 2 & 3: (on symmetry axis)

As the results do not consistently show that the peptide binds in both directions, we are now in a dilemma. How should we publish this structure and in which space group, since the structures in both space group refine very well with good statistics?

Thank you in advance for all your suggestions and advice.

Yours sincerely,
Kris