The short answer is: no. Wavelength does not matter. Not for native
data anyway.
I wrote a paper about this recently. It is open access:
http://dx.doi.org/10.1107/S0907444910007262
In particular, check out Figure 2. The two solid lines are pretty darn
flat, and that means the wavelength dependence of damage and scattering
power almost exactly cancel. More on the dotted lines in a bit...
It is easy to screw up the "scattering per damage" calculation as there
are many mathematical pitfalls. Perhaps the trickiest one is thinking
that the longer detector distances that would be used at shorter
wavelengths (keeping the resolution on the edge of the detector fixed)
leads to a net reduction of background/spot. However, if you carefully
calculate the area occupied by a spot, you again find that the noise due
to background balances out, and there is again no wavelength
dependence. Lots of people have made that mistake. Including me! But
eventually I found the error. Some assurance can be hand that the "no
wavelength dependence" conclusion is correct because experimental
studies (http://dx.doi.org/10.1107/S0907444993013101), also found no
significant wavelength dependence to "signal/noise/dose", as expected.
This is not to say that moving the detector back at constant wavelength
is not a good idea. It is! You will generally get a signal/noise
increase proportional to the distance (for weak spots). And yes, this
is why we spend so much money on large-area detectors!
Of course, the wavelength dependence of detector sensitivity is a
completely different story. For most theoretical calculations you
assume a perfect detector system where the only noise is photon counting
(also called shot noise). It is important to remember that no such
detectors actually exist. Even Pilatus has some calibration error,
pile-up error, etc. as well as a finite "capture fraction". In fact,
pretty much any modern detector is designed to capture only 80-90% of
the incident photons at most wavelengths. I could go on and on, but
since the OP was only asking about "1.0 A vs 0.9 A", the change in
detector performance over such a narrow range will be negligible when
compared to things like crystal-to-crystal variation. Did you know that
a 110 micron crystal is twice the volume of a 90 micron crystal? And
therefore can absorb twice as much energy before enduring the same dose?
The only other "wavelength dependence" that could be of practical
importance is the escape of photoelectrons from the illuminated volume
because these can carry away some energy that would otherwise cause
damage. This "build up region" effect has long been a trick of medical
dosimetry using MeV-class photons (Johns & Cunningham, 1974). It was
only recently demonstrated experimentally on an MX beamline
10.1073/pnas.1017701108. It may be possible to take advantage of this
effect in a "real-world" data collection, but any real gain will require
crystal volumes so small that you cannot get a complete dataset from
just one. That is, unless you are working with VERY small molecules,
you will need to be in the "multi-crystal dataset regime" before you can
take advantage of photoelectron escape.
So, for any "regular" native data collection, I'd say: no, wavelength
doesn't matter.
-James Holton
MAD Scientist
On 2/15/2012 3:55 PM, Bart Hazes wrote:
Diffracted intensity goes up by the cube of the wavelength, but so
does absorption and I don't know exactly about radiation damage. One
interesting point is that on image plate and CCD detectors the signal
is also proportional to photon energy, so doubling the wavelength
gives 8 times diffraction intensity, but only 4 times the signal on
integrating detectors (assuming the full photon energy is captured).
So it would be interesting to see how the equation works out on the
new counting detectors where the signal does not depend on photon
energy. Another point to take into account is that beamlines can have
different optimal wavelength ranges. Typically, your beamline guy/gal
should be the one to ask. Maybe James Holton will chime in on this.
Bart
On 12-02-15 04:21 PM, Jacob Keller wrote:
Well, but there is more scattering with lower energy as well. The
salient parameter should probably be scattering per damage. I remember
reading some systematic studies a while back in which wavelength
choice ended up being insignificant, but perhaps there is more info
now, or perhaps I am remembering wrong?
Jacob
On Wed, Feb 15, 2012 at 5:14 PM, Bosch, Juergen<jubo...@jhsph.edu>
wrote:
No impact ? Longer wavelength more absorption more damage. But
between the choices given no problem.
Spread of spots might be better with 1.0 versus 0.9 but that depends
on your cell and also how big your detector is. Given your current
resolution none of the mentioned issues are deal breakers.
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 Feb 15, 2012, at 18:08, "Jacob
Keller"<j-kell...@fsm.northwestern.edu> wrote:
I would say the better practice would be to collect higher
multiplicity/completeness, which should have a great impact on maps.
Just watch out for radiation damage though. I think the wavelength
will have no impact whatsoever.
JPK
On Wed, Feb 15, 2012 at 4:23 PM, Seungil Han<shan06...@gmail.com>
wrote:
All,
I am curious to hear what our CCP4 community thoughts are....
I have a marginally diffracting protein crystal (3-3.5 Angstrom
resolution)
and would like to squeeze in a few tenth of angstrom.
Given that I am working on crystal quality improvement, would
different
wavelengths make any difference in resolution, for example 0.9 vs.
1.0
Angstrom at synchrotron?
Thanks.
Seungil
--------------------------------------------
Seungil Han, Ph.D.
Pfizer Inc.
Eastern Point Road, MS8118W-228
Groton, CT 06340
Tel: 860-686-1788, Fax: 860-686-2095
Email: seungil....@pfizer.com
--
*******************************************
Jacob Pearson Keller
Northwestern University
Medical Scientist Training Program
email: j-kell...@northwestern.edu
*******************************************
