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 <pjeff...@princeton.edu> 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