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RE: [ccp4bb]: smallest diffracting crystals?
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Do we believe that in a typical crystal the whole irradiated volume scatters
in phase. Mosaicity is often explained by assuming a crystal is made up of
small slightly misaligned crystallites. If that assumption is correct wouldn't
a single crystallite then be the relevant volume that determines line
broadening.
I don't know my quantum stuff but even in a perfectly aligned crystal I don't
think a photon interacts with the whole crystal volume. I don't know what
volume of a crystal affects the diffraction probability of a photon but I
expect that it is rather small (and possibly wavelength dependent?).
The point I am getting at is that we only have to start worrying about line
broadening once the crystal volume gets significant smaller than the
"diffraction volume" experienced by a photon in a typical larger crystal. One
should also remember that spot size depends on the irradiated crystal volume,
mosaicity, beam divergence, and point spread function of the detector.
Broadening due to too few unit cells only becomes relevant when it starts to
dominate the other contributing factors.
To make this a useful discussion we should have some idea about the magnitudes
of the effects we are talking about. Again, as a biologist I don't dare to
venture out in the quantum world, but I would very much like to see input from
others that are more knowledgeable in these areas.
Bart
PS As a side topic I think that instead of just trying to boost the signal by
increasing brilliance we should pay more attention at getting the maximum
information out of each diffracted photon because radiation damage is the real
enemy.
On Thu, 23 Jan 2003, Bernhard Rupp wrote:
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>
> One thing to consider is following:
>
> A small number of repeating units afffects
> the sampling of the lattice (see convolution
> of molecular envelope with delta lattice function).
>
> In practice this is used in the Scherrer formula for
> determining particle sizes via line broadening
> in powder diffraction.
>
> With a 100 A molecule, the 1000 unit per period
> limit is reached at 10 um. With very large
> molecules and at very small dimensions, additional line
> broadening contributing to lower S/N and thus
> resolution must occur.
>
> There is probably a specific optimum between
> competing effects like minimum strain (mosaicity),
> low absorption, line broadening and signal loss
> for each crystal. I wager that 1 um is not going to
> give hi res with large molecules/cells...
>
> The S/N issue is, btw, often ignored in the single
> molecule scattering presentations..
>
> best regards, br
>
> > -----Original Message-----
> > From: owner-ccp4bb@dl.ac.uk [mailto:owner-ccp4bb@dl.ac.uk] On
> > Behalf Of Anastassis Perrakis
> > Sent: Wednesday, January 22, 2003 11:47 AM
> > To: Richard Gillilan
> > Cc: ccp4bb@dl.ac.uk
> > Subject: Re: [ccp4bb]: smallest diffracting crystals?
> >
> >
> > *** For details on how to be removed from this list visit the ***
> > *** CCP4 home page http://www.ccp4.ac.uk ***
> >
> > > I am interested to know:
> > >
> > > What size is the smallest dimension protein crystal for which you
> > > have obtained usable diffraction data on any synchrotron beamline.
> > >
> > > I will be happy to summarize in the form of a histogram
> > > if I get enough response.
> > >
> >
> > from the microfocus (ID13) experience between 1998-2000:
> >
> > ±3.2 A from ±5 micron thick plates (? not 100% sure of size )
> > from Rod
> > Mackinon's pumps ... - fairly anisotropic -
> > a few datasets collected, some of which are used in Rod's nature
> > paper(s).. not sure which one(s) .. he gets too many ;-))
> >
> > ±3.5 A from ±1 micron thick plates for Werner Kuehlbrant's
> > photosystem
> > - VERY anisotropic, I don't know what
> > happened to these data
> >
> > 3.2 A from one ±30x5x2 micron xtal from O. Weichenrieder's/S.Cusack's
> > Alu RNP isotropic (c221, complete data collected, 60Kd in AU)
> >
> > A funny cubic thingy from Stefania diMarco. M. Walsh et al, was
> > 30x30x30 xtals and diffracted at 3.0 but it was 'nightmare in space
> > group street'
> >
> > Belgian group for an iron containing protein got data on needles of
> > 5-10 micron thickness 9veeeery long ones) up to 1.4 A !!!
> >
> > The Dortmund group had several pretty small xtals of GTPases
> > that gave
> > usuable data ... I am sure (?) Renaud and Klaus can
> > enlighten us on the details of these
> >
> > 30x10x10 crystals (hey, these were mine !) were measured at
> > 2.0, never
> > reproduced and I wish I knew what was inside
> > (45 kD protein- 22mer DNA complex but not both would fit - or the
> > structure would be really weird .. structure pending)
> >
> > OF COURSE
> > I am carefully avoiding all the things of similar size or bigger that
> > never diffracted beyond 15-20 A !!!
> > but the question was 'what WORKED ?' ;-))
> >
> > .... maybe I forget a few other small xtals but these are the
> > ones that
> > surfaced up in my memory now ...
> >
> > A.
> >
>
>
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