dengzq1987,

I am also working on a data set that has this exact alignment/mosaicity problem noted by others. If you make a movie of your dataset that displays your images in series, or if you just click through them quickly in adxv or your program of choice, you will see that very few of the spots ever leave their positions on the detector as you rotate the crystal. This demonstrates the mosaicity issue described by Bert and Ed. In my case, Denzo seems to be able to properly measure the a and b axes of my hexagonal cell (I know this based on a comparison of these values to structures of homologs in the same SG), but tells me that my c axis is only 0.21A. It seems to me that in the case of overlaps, the dimensions of the corresponding axis would be overestimated rather than seriously underestimated (0.21A!), so I'm not really sure of what is going on here. Maybe someone who knows more about the software could shed some light on this crazy measurement? I've been told by a few people that XDS may be able to handle this dataset, but I haven't gotten to trying it yet. I'll work on it today or tomorrow and follow up if I have any success. 

Best,

Mike






----- Original Message -----
From: "Edward A. Berry" <[log in to unmask]>
To: [log in to unmask]
Sent: Tuesday, April 5, 2011 8:38:17 AM GMT -08:00 US/Canada Pacific
Subject: Re: [ccp4bb] how to Collecting Data from Long Unit Cell Axes ?

In the first image (spot 2) the long unit cell is obvious from the closely spaced
spots, but this kind of image can be handled easily by Denzo. The second one
(spot 1), although at first glance the spots seem well separated, is the problem.
As Bert pointed out the mosaicity is high, so the lunes are overlapping
(you don't see separate rings or crescents where Ewalds sphere intersects
different layers of the lattice). That means the apparently individual
spots are multiple spots one on top of the other, with the long axis
nearly perpendicular to the plane of the picture. This means there is
no information about the spacing of the long axis in this picture,and
the picture is rather insensitive to rotation, so integration is
likely to get lost unless you fix these parameters. If you fix the
parameters based on the good orientation and integrate on through,
you will be sampling the lattice rods at intervals (as the 2D electron
crystallographers say) rather than measuring discreet spots.
However if you collect while rotating around the long axis, you will
never see this orientation and should have no serious problem.
eab

dengzq1987 wrote:
> hello Jürgen_Bosch,
> because i have collected the data,but can't index.in some direction,the
> spot is separated .but the others are set close together(picture spot1
> and spot 2).so we think there is one long unit cell axes.
> 2011-04-05
> ------------------------------------------------------------------------
> dengzq1987
> ------------------------------------------------------------------------
> *发件人：* Jürgen_Bosch
> *发送时间：* 2011-04-05 20:29:18
> *收件人：* dengzq1987
> *抄送：* [log in to unmask]
> *主题：* Re: [ccp4bb] how to Collecting Data from Long Unit Cell Axes ?
> What do you consider long ? 200, 300 ? 600 A ? Before shooting try to
> run strategy or xplan. Move the detector back to first reliably be able
> to determine your cell. Then double your estimated mosaicity and see
> what strategy suggests. If you don't get many overlaps (<5%) then try a
> closer distance. Don't rotate 1degrees but take 1/2 of the mosaicity.
> Obviously you want to make good use of the detector area so adjust the
> edges to where your crystal really diffracts. And if that resolution
> leads to too many overlaps then limit your resolution and get first a
> good datasets home. You then can play with 2theta for a higher
> resolution dataset.
> Another obvious thing to do and you don't mention what reduction program
> you use is to let XDS sort your problem out. Unless you collected to
> high resolution without being cautious XDS could help. If not, well then
> you had your experience and now should know better.
> SSRL has options to collect 450 A cells to 3A without much hassle. That
> was my largest cell so far.
> Jürgen
>
> ......................
> Jürgen Bosch
> Johns Hopkins Bloomberg School of Public Health
> Department of Biochemistry & Molecular Biology
> Johns Hopkins Malaria Research Institute
> 615 North Wolfe Street, W8708
> Baltimore, MD 21205
> Phone: +1-410-614-4742
> Lab: +1-410-614-4894
> Fax: +1-410-955-3655
> http://web.mac.com/bosch_lab/
>
> On Apr 5, 2011, at 1:05, dengzq1987 <[log in to unmask]
> <mailto:[log in to unmask]>> wrote:
>
>> hello all,
>> does anyone have the experience of Collecting Data from Long Unit Cell
>> Axes ? I have a crystal that diffracts to about 4 A. in some
>> /direction/ the spots overlap. we can't use the data to index .we
>> think it is because that there is a long unit cell axes. so is there
>> any method to solve this problem?
>> best wishes.
>> 2011-04-05
>> ------------------------------------------------------------------------
>> dengzq1987

-- 
Michael C. Thompson

Graduate Student

Biochemistry & Molecular Biology Division

Department of Chemistry & Biochemistry

University of California, Los Angeles

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