Just to add to what has been "said" (written) before:
coiled coiled or simply helices can be very problematic for M.R.. Human
sacrifice has never given any positive result (as reported in the
literature as far as I know), but "heavy atom sacrifice" could be
attempted ("heavy atom" includes in my view atoms that are used for SAD,
MAD - such as S, Se... - in addition to Au, Pt...) in parallel to your
M.R. searches which may never provide you with an acceptable solution.
Fred.
Napolećo Valadares wrote:
> Hi there!
> I got crystals from some synthetic peptides I bought, they are 30
> residues long and are supposed to form a coiled coil. I collected
> various data sets (home source, Brookhaven and Diamond), including
> some at the resolution of 1.65 A, for which the space group appears to
> be C222 or C2221. The unit cell is small, 22.67, 88.06, 26.13, and the
> Matheus Coefficient indicates that's there's only one helix in the
> asymmetric unit and a 25% solvent content.
>
> I have tried A LOT of Molecular Replacement using Phaser and
> Phenix AutoMR. I'm using a 80% identity coiled coil helix as search
> model. The programs give me solutions with "reasonable" maps, but it
> is never possible to refine to achieve Rvalues below 0.40.
> Additionally, maps from different solutions look reasonable, so I'm
> thinking these are all bias.
>
> I have 5 other synthetic 30 residues peptides (that crystallize in
> different space groups and diffract to lower resolutions), including a
> SelenoMethionine (SM) derivative (but it does not have enough
> anomalous signal, ASU is too big, it is possible that the SM are
> disordered). I'm stuck on this since March.
>
> Regarding the search model, I already tried trimming some or all
> side chains and removing 2, 3 or 5 residues on each/both sides. I also
> tried other search models. Maybe some "magic" combination of
> parameters on Phaser or other programs can help me.
>
> What is your advice regarding how to proceed with MR? Is there
> some program, procedure, parameter, pray or human sacrifice that could
> help me?
> Thank you.
> Regards,
> Napo
>
>
