So, with the combined votes of Hendrickson, Blundell, Tickle and Google, can we
safely call it "Multi-wavelength Anomalous Diffraction" from now on and call
all other names wrong?
Mark
On 19 Jan 2012, at 18:50, Ian Tickle wrote:
> Perhaps I could chime in with a bit of history as I understand it.
>
> The term 'dispersion' in optics, as everyone who knows their history
> is aware of, refers to the classic experiment by Sir Isaac Newton at
> Trinity College here in Cambridge where he observed white light being
> split up ('dispersed') into its component colours by a prism. This is
> of course due to the variation in refractive index of glass with
> wavelength, so then we arrive at the usual definition of optical
> dispersion as dn/dlambda, i.e. the first derivative of the refractive
> index with respect to the wavelength.
>
> Now the refractive index of an average crystal at around 1 Ang
> wavelength differs by about 1 part in a million from 1, however it can
> be determined by very careful and precise interferometric experiments.
> It's safe to say therefore that the dispersion of X-rays (anomalous
> or otherwise) has no measurable effect whatsoever as far as the
> average X-ray diffraction experiment (SAD, MAD or otherwise) is
> concerned. The question then is how did the term 'anomalous
> dispersion' get to be applied to X-ray diffraction? The answer is
> that it turns out that the equation ('Kramer-Kronig relationship')
> governing X-ray scattering is completely analogous to that governing
> optical dispersion, so it's legitimate to use the term 'dispersive'
> (meaning 'analogous to dispersion') for the real part of the
> wavelength-dependent component of the X-ray scattering factor, because
> the real part of the refractive index is what describes dispersion
> (the imaginary part in both cases describes absorption).
>
> So then from 'dispersive' to 'dispersion' to describe the wavelength
> dependence of X-ray scattering is only a short step, even though it
> only behaves _like_ dispersion in its dependence on wavelength.
> However having two different meanings for the same word can get
> confusing and clearly should be avoided if at all possible.
>
> So what does this have to do with the MAD acronym? I think it stemmed
> from a visit by Wayne Hendrickson to Birkbeck in London some time
> around 1990: he was invited by Tom Blundell to give a lecture on his
> MAD experiments. At that time Wayne called it multi-wavelength
> anomalous dispersion. Tom pointed out that this was really a misnomer
> for the reasons I've elucidated above. Wayne liked the MAD acronym
> and wanted to keep it so he needed a replacement term starting with D
> and diffraction was the obvious choice, and if you look at the
> literature from then on Wayne at least consistently called it
> multi-wavelength anomalous diffraction.
>
> Cheers
>
> -- Ian
>
> On 18 January 2012 18:23, Phil Jeffrey <[email protected]> wrote:
>> Can I be dogmatic about this ?
>>
>> Multiwavelength anomalous diffraction from Hendrickson (1991) Science Vol.
>> 254 no. 5028 pp. 51-58
>>
>> Multiwavelength anomalous diffraction (MAD) from the CCP4 proceedings
>> http://www.ccp4.ac.uk/courses/proceedings/1997/j_smith/main.html
>>
>> Multi-wavelength anomalous-diffraction (MAD) from Terwilliger Acta Cryst.
>> (1994). D50, 11-16
>>
>> etc.
>>
>>
>> I don't see where the problem lies:
>>
>> a SAD experiment is a single wavelength experiment where you are using the
>> anomalous/dispersive signals for phasing
>>
>> a MAD experiment is a multiple wavelength version of SAD. Hopefully one
>> picks an appropriate range of wavelengths for whatever complex case one has.
>>
>> One can have SAD and MAD datasets that exploit anomalous/dispersive signals
>> from multiple difference sources. This after all is one of the things that
>> SHARP is particularly good at accommodating.
>>
>> If you're not using the anomalous/dispersive signals for phasing, you're
>> collecting native data. After all C,N,O,S etc all have a small anomalous
>> signal at all wavelengths, and metalloproteins usually have even larger
>> signals so the mere presence of a theoretical d" difference does not make it
>> a SAD dataset. ALL datasets contain some anomalous/dispersive signals, most
>> of the time way down in the noise.
>>
>> Phil Jeffrey
>> Princeton
>>
>>
>>
>> On 1/18/12 12:48 PM, Francis E Reyes wrote:
>>>
>>>
>>> Using the terms 'MAD' and 'SAD' have always been confusing to me when
>>> considering more complex phasing cases. What happens if you have intrinsic
>>> Zn's, collect a 3wvl experiment and then derivatize it with SeMet or a heavy
>>> atom? Or the MAD+native scenario (SHARP) ?
>>>
>>> Instead of using MAD/SAD nomenclature I favor explicitly stating whether
>>> dispersive/anomalous/isomorphous differences (and what heavy atoms for each
>>> ) were used in phasing. Aren't analyzing the differences (independent of
>>> source) the important bit anyway?
>>>
>>>
>>> F
>>>
>>>
>>> ---------------------------------------------
>>> Francis E. Reyes M.Sc.
>>> 215 UCB
>>> University of Colorado at Boulder
