I personally have had problems solving structures with large copy #s in
the asymmetric unit despite a good model (my failure occurred at 14 in
the a/u in primitive orthorhombic) - at the time finding the first
monomer proved to be impossible. This was also a structure in which the
systematic absences proved to me misleading.
However based on some indifferent Zn anomalous data I was able to find
the Zn sites and generate a relatively mediocre quality MAD map - even
though the map was not very interpretable as-is it would have improved
considerably under the influence of 14-fold averaging, I suspect.
However the vicinity of the Zn site was sufficiently well-resolved that
I could manually approximately fit the monomer model at each of the 14
Zn sites that SHELXD found, and follow it with rigid body refinement.
Strikes me that if you can find your Cu sites, even with 6 Angstrom
data, then this may well give you enough information to proceed with at
least some laborious form of molecular replacement (and/or averaging
with the MAD/SAD map).
Phil Jeffrey
Princeton, NJ
Yi Xue wrote:
Dear all:
We already got nice crystals of a drug-protein complex, however, MR
failed due to the huge copies (>12) of protein molecules per asu. Protein
itself is a small one, only ~70 aa.
Later on, we collected MAD data of copper (copper : protein ~ 1: 1),
Rsym of the data was around ~9%, the anomalous signals were weak, and only
good to ~6A, the data also failed to solve the phases.
Thus, basically, the Cu anomallous signal is very weak, and the
crystals are kind of sensitive to radiation, it is dying but not that fast.
I am wondering, Do we stand any chances to solve the phases by Cu MAD
or SAD?
Any suggestions or comments are highly appreciated?
Yi
