[phenixbb] Tantalum anomalous signal of a low-resolution SAD dataset with lattice-translocation disorder

[Shun Liu]

Dear Phenix colleagues,

We are working on a Ta6Br14 cluster-SAD dataset (4.3 angstroms) with lattice-translocation disorder (with a total Rmerge of 0.16). Both SHELXC and Xtriage gave the similar positive result about the anomalous signal (See below). 

-------------------------------------------------------------------------------------------------
 Resl.   Inf.  9.58  7.61  6.65  6.04  5.61  5.27  5.01  4.79  4.61  4.45  4.31
 N(data)    1427  1410  1410  1409  1385  1452  1367  1399  1367  1395  1385
 <I/sig>    76.3  52.1  26.7  13.7   8.7   6.1   5.1   5.0   3.9   2.7   1.8
 %Complete  98.8  99.9 100.0  99.9 100.0 100.0 100.0  99.9  99.7  99.6  95.8
 <d"/sig>  11.67  9.67  6.12  3.73  2.50  1.59  1.21  0.99  0.84  0.79  0.68
------------------------------------------------------------------------------------------------ 
unused:         - 49.7669 [   0/29  ]
bin  1: 49.7669 -  9.2629 [3086/3100]  0.8212
bin  2:  9.2629 -  7.3599 [3031/3031]  0.7689
bin  3:  7.3599 -  6.4318 [3073/3073]  0.5380
bin  4:  6.4318 -  5.8447 [3071/3075]  0.2933
bin  5:  5.8447 -  5.4263 [3054/3054]  0.1230
bin  6:  5.4263 -  5.1067 [3070/3071]  0.0363
bin  7:  5.1067 -  4.8512 [3038/3039]  0.0126
bin  8:  4.8512 -  4.6402 [3028/3048]  0.0074
bin  9:  4.6402 -  4.4617 [2955/3037]  0.0021
bin 10:  4.4617 -  4.3078 [2730/3059]  0.0000
unused:  4.3078 -         [   0/0   ]
--------------------------------------------------------------------------------------------------
However, the tantalum sites we found and the initial map seem ambiguous, with which automatic model-building failed. 
I am wondering whether the lattice-translocation disorder of the dataset impacts the reliability of the anomalous signal, tantalum sites and the initial map. If it does, how can we decrease its impact? If it doesn't, is it possible to find the accurate Ta sites and generate an interpretable map suitable for model-building with this dataset? (After all, it has been reported that Ta sites can be found at 6A resolution.)

Any suggestion and comment will be highly appreciated! 

Best,
Shun