With a long axis, the twinning test often falsely suggest twinning, as the spots may not be properly resolved. Thus a weak reflection may be overestimated because it is next to a strong one, leading to the same intensity statistics that you get in twinning
Phil
> On 6 Dec 2016, at 14:49, Eleanor Dodson <[log in to unmask]> wrote:
>
> The twinning factors all about 0.25 do point to a higher symmetry than P21.
> As others say - I would be very surprised (and would question result as a referee) if your FreeR was not >> 30% for such low resolution data
>
> Is the data anisotropic - often is with such a long cell edge..
> If so do process to the highest possible resolution in the a b orientation - the quality indicators will be much higher with aniso data and you may be throwing away good measurements .
>
> Eleanor
>
> On 6 December 2016 at 14:37, Andreas Forster <[log in to unmask]> wrote:
> Dear Andre,
>
> I agree with Jacob that P 42 21 2 might be the right space group, and that and R free of 33% isn't so bad. If your electron density is poor, that might just reflect the low resolution. Have you tried refinement with Buster? It has a way with low-resolution data.
>
> All best.
>
>
> Andreas
>
>
>
> On Tue, Dec 6, 2016 at 3:25 PM, Keller, Jacob <[log in to unmask]> wrote:
> What were the twinning tests like in p42212? I suspect that it is really that spacegroup, the low Rfree is artificial (33% is not too bad at 3.9 Ang) and the electron density is bias.
>
>
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> JPK
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>
> From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of Andre Luis Berteli Ambrosio
> Sent: Tuesday, December 06, 2016 9:17 AM
> To: [log in to unmask]
> Subject: [ccp4bb] Choosing test set for twin refinement, with multiple operators
>
>
>
> Dear all,
>
> We are currently refining a low resolution model (3.9 A max), obtained by MR. Dataset was collected from a single crystal with one long cell axis (~620 A) and high solvent content (74%).
>
> Best refinement results (by far!) are obtained in Refmac, by imposing P21 sg (20 multidomain monomers/AU) and allowing for amplitude-based twin refinement.
>
> Accordingly, Refmac identifies four twin operators, which I understand have considerable fractions, as follows:
>
>
>
> **** Twin operators with estimated twin fractions ****
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>
>
> Twin operator: H, K, L: Fraction = 0.249; Equivalent operators: -H, K, -L
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> Twin operator: -H, -K, L: Fraction = 0.260; Equivalent operators: H, -K, -L
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> Twin operator: -K, -H, -L: Fraction = 0.235; Equivalent operators: K, -H, L
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> Twin operator: K, H, -L: Fraction = 0.257; Equivalent operators: -K, H, L
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>
>
> I suspect that the low Rfree (~25%) may be artificial and results from an inappropriate selection of the test due to the multiple operators.
>
> However, I cannot figure out how to properly select the test set when such a high number of operators must be considered.
>
> FYI, space group P42212 resulted in the best processing statistics (5 monomers/AU); however, model never improved from an Rfree of 33%, with the quality of the electron density distribution for some of the domains being heavily compromised.
>
> I honestly apologize if I am missing some obvious points here and will be happy to provide more info, including statistics on data processing.
>
> Since is a multidomain protein, is it also appropriate to include TLS refinement at this low resolution?
>
> I thank you all in advance.
>
> Best,
>
> Andre.
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