Hi everyone,
I thought I start a new thread while it is unusually quiet on the bb. I
am pondering over the practical limitations to MAD and SAD phasing with
Se-Met at low resolution. What is the lowest resolution at which people
have solved structures "only" using phases from selenium in a
"realistic" case? Let me further qualify my question: My *realistic*
*low* resolution case is where
1. Rmerge over all resolution bins is 6-10% (i.e. your crystals are lousy).
2. Resolution limit is worse than 3.5 Angstroms, where <I>/<sigma> in
the last resolution bin is between 1 and 3 (i.e. your crystals are
really lousy).
3. Assuming good selenium occupancy (~85%; I work with eukaryotic
expression systems, so 100% is not usually achieavable),
4. The number of selenium atoms are enough many that the Crick-Magdoff
equation would give you *at least* an average 5% change in intensities
(assuming 6 electrons contributed per selenium, based on both absorptive
and dispersive differences being at about 6 e- at the absorption edge).
5. and specifically, no other phases and molecular replacement
solutions are available.
Obviously, I have a case very similar to what's described above, and
three years of failure with heavy atom derivatization (I am still
trying). I would be happy to hear about Se-Met cases, and data
collection strategies (2wl vs. 3wl MAD vs. SAD, etc.) and phasing
methods used in these cases, or references of them. Again, no other
partial phases, and no data cut off at 3.6 A with an I/s of 15 in the
last resolution bin. Are there any examples out there? Searching the
RCSB and PubMed did not point out to me many successful cases.
Thanks,
Engin
P.S. I would also appreciate the specific query type for searching the
PDB on the web for phasing method (MR, MAD, SAD, MIR, etc.). They seem
to have everything under the sun searchable, but I cannot find this one.
