Re: [ccp4bb]: physical meanings of positive and negative peaks in DANO Fourier map

[<wgscott@chemistry.ucsc.edu>]

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F(+) > F(-) because F"(+) and F"(-) are both always 90 degrees in front 
of the scattered wave (this is absorption, which breaks the Friedel 
symmetry; the other way around violates conservation of energy).  The 
negative peaks presumably are noise.  For centrosymmetric reflections 
these should be zero.  I find it easiest to think of this in terms of 
refractive index.

n = 1 - (alpha) - i(beta)

(alpha) = real component of the correction for light traveling through 
a refracting (optically dense) medium = dispersion =  (constant) * F'
(beta) = imm. component of the correction  = absorption = (constant) * 
F"

Changing the sign would make (beta) correspond to emission, which would 
mean creation of light energy, so it is unphysical.

On Saturday, November 2, 2002, at 01:41 PM, Q. Steven Xu wrote:

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>
> I have two questions regarding the DANO fourier map.
>
> 1) What's the difference between the maps calculated with
> coeff. F(+)-F(-) and with coeff. F(-)-F(+)? the postive and
> negtive peaks would be reversed?
>
> 2) how to explain the positive and negative peaks in such a
> map? for instance, if I am searching a sulfur atom which produces the
> anomalous signal, I guess I need to search both the positive and 
> negative
> peaks, is that right?
>
> Thanks in advance.
>
> regards,
> Q. Steven Xu
>
>
>
William G. Scott

Associate Professor
Department of Chemistry and Biochemistry
and The Center for the Molecular Biology of RNA
Sinsheimer Laboratories
University of California at Santa Cruz
Santa Cruz, California 95064
USA

phone:  +1-831-459-5367 (office)
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