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anomalous dispersion of Zn?

Ethan A Merritt merritt at provolone.bchem.washington.edu
Wed Sep 22 18:28:32 EST 1993

In article <01H39317NE2Q001KLO at INDYVAX.IUPUI.EDU> ILCN500 at INDYVAX.IUPUI.EDU writes:
>I wonder whether Zn is heavy enough for anomalous dispersion, to phase 
>the Zn-containing proteins.  Or someone even has experience of it. Love
>to have your advice.  Thank you.
>Zhongning Yang

Yes, but the K-edge for Zn is at 9.55 keV (1.28A), which is above the 
energy put out by a copper x-ray tube or anode.  
For this experiment, then, you would want synchrotron radiation.

The anomalous signal available from the Zn K-edge is comparable to other
transition element K-edges; i.e.  f" <= 10 electrons just above the K edge,
f' ~~ -10 electrons at the K-edge.  In practice you get somewhat less signal
than this due to finite energy bandwidth.  Say f" about 6-7 electrons and
f' about -8 electrons with a Si monochromator crystal.
As a very, very, rough rule of thumb you might figure that in a careful
MAD phasing experiment the signal from one Zn atom might allow you to phase
a structure of 10kD.  Equations for calculating the phasing power may be
found in, for example, Hendrickson (1985) Transactions of the ACA Vol 21,
or (perhaps easier to find) table 1 of Krishna Murthy et al (1988)
J. Biol. Chem. 263:18430.

					Ethan A Merritt
Dept of Biological Structure            H510 Health Sciences
University of Washington SM-20          
Seattle, WA 98195                       merritt at u.washington.edu

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