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On Tuesday 25 April 2006 10:48 am, James Holton wrote:
>
> Ahh yes, the ol' "beam heating hypothesis". I used to believe in it too...
>
> James Murray just cited studies that cast a great deal of doubt on the
> idea that there is any significant heating of protein crystals by x-ray
> beams.
Due to the asynchronous nature of the web, I have only received
James Murray's message just now, followed soon after by Eddie's.
Looks like interesting reading.
> If you think about it, a 30-degree gradient over 100 microns is 3000
> degrees/cm. That's like poking a red-hot nail into ice and expecting it
> to keep glowing.
Well, more like expecting it to melt the ice.
> Even the most powerful
> x-ray beams in the world are only milliwatt-class sources of energy and
> typically, only about 2% of the energy from the beam is deposited in the
> sample.
Back of the lunch-time napkin thought experiment:
100 watt light bulb is exposed to air (essentially nitrogen), losing
heat through radiation and conduction over a surface of 4piR^2 for R
approximately 30 mm. ===> ~ .01 watt per square mm.
The light bulb is hot, according to everyday experience
100 micron cube protein crystal heated by 2% of a 30 milliwatt
X-ray beam losing heat through conduction over a surface
of 6 x 0.1^2mm ===> ~ .01 watt per square mm.
Crystal is now, one would think, light-bulb temperature.
> Then again, it is always easier to rationalize things once you know the
> right answer...
Measurement trumps thought experiment, but I am curious to learn
if I dropped a decimal point into my soup or if I am overlooking
something fundamental :-)
Ethan
--
Ethan A Merritt
Biomolecular Structure Center
University of Washington, Seattle WA
