Dear CCP4 experts,
I'd like to ask your opinion about using anomalous signal in refinement of
crystal structures in addition to using high resolution native data.
I am working on a series of structures for which I have collected two data
sets (from the same crystal):
1 - native with higher resolution
2 - anomalous at MN absorption edge peak.
The structures were solved with MR and preliminary refinement using the
native data only yields decent statistics, but I also use anomalous data to
verify presence and position of manganese ions. For this I used ANODE which
lists four strong peaks (~30 sigma) as expected for manganese ions and
around 45 weaker peaks (~9-5 sigma) for sulfur atoms in Cys and Met. I am
happy to use this information in model building but I was also wondering if
(and how) beneficial would it be to use both high resolution structure
factors and somehow lower resolution yet highly specific anomalous signal
in the same round of refinement?
In Refmac5 I can use either refinement with "no prior phase information"
taking FP and SIGFP or "SAD data directly" with SIGFP F(+) SIGF(+) F(-)
SIGF(-), but I didn't find any "MAD" option to use both.
I have the following columns in my mtz files:
for native data: H K L FP SIGFP FreeRflag
for anomalous data : H K L FP SIGFP F(+) SIGF(+) F(-) SIGF(-) FreeRflag
or : H K L FP SIGFP DANO SIGDANO ISYM FreeRflag
I could use CAD to merge the interesting columns into a single mtz file
containing:
H K L FP SIGFP FreeRflag F(+) SIGF(+) F(-) SIGF(-) DANO SIGDANO
I'd appreciate any comments or advise how to use both sources of
information in the refinement.
I wish you all a Happy New Year.
Kind regards,
Piotrek
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