[phenixbb] Reliable twinning fraction estimation

[Xiao Lei]

Dear All,

I have a x-ray dataset of a protein-DNA complex to 2.8 A resolution with
space group P312 checked by phenix xtriage for twinning. The estimated twin
fraction from the output of xtriage is: 0.115 (Britton analysis); 0.119 (H
test) and 0.022 (maximum likelihood method). However, the L-test graph in
xtriage shows my observed data almost perfectly overlay with theoretical
perfect twin data. In addition, when I tried to use phenix to do refinement
with twin law -h,-k,l, the log file shows my twin fraction estimation is
0.49, which is very high and much bigger than Britton analysis and H test
estimation.

As far as my understanding is that if a twin fraction is lower than 15%, I
still have hope to solve the structure (molecular replacement in this case)
with reasonable R value, but if the twin fraction is 0.49, which is almost
a perfect twin, which makes detwin impossible and refinement will stall at
high R values (in my case, R free start: 0.4199; R work start: 0.4121;  and
R free final:  0.4038 and R work final: 0.3640 after running phenix
refinement with twin law -h, -k, l).

My question:  which twin fraction estimation is more reliable? is my data
almost perfectly twined?

I attached the graphs of L test, Britton analysis and twin estimation from
phenix xtriage and part of log file from phenix refine here.

Many thanks in advance.

Xiao
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