Talk:Circular dichroism

Application to 3D movies
This article now links from RealD Cinema but some aspects of the technology were better dealt with here than there, because they will be generic to all CD stereoscopy. The actual action of Real D spectacles does not correspond to that of the kind of spiralling waveform described here. Transmitting or blocking of light passed through two filters depends on the direction the light goes through the filters, not just their relative axial orientation, as in conventional polaristion, as if their action is analogous to a rectifying diode on direct current.--Hugh7 (talk) 00:57, 11 June 2012 (UTC)

Untitled
Q: Can anyone tell me if CD spectroscopy has been tested on polymers, and not only on biological materials ? thank you. Guillom 13:59, 4 October 2005 (UTC)

A: Yes you can, provided the polymers you are performing the measurements on are not racemized during the polymerization reaction. Racemization can, for example, occur in concentrated NaOH solutions at high temperatures.

I think the link to ellipticity that is redirected to the flattening article should be removed because this parameter is different than the definition of elliptical polarization. --Bjsamelsonjones 6/28/06

Q: In the 'Experimental limitations' is this 0.1 mm pathlength correct? Or should it be 1 mm? Thanks A: It is correct. Both 1mm and 0.1 mm cuvettes are used. 0.1 mm cuvettes reduce the light scattering, leading to better spectra.

Cells are available with path lengths to .05mm (one cell manufacturer), and shorter paths may sometimes be used; there are practical difficulties, e.g. knowing the true path length, sealing the mating faces so that evaporation does not change the concentration. Non-demountnable cells also exist. They can be very difficult to clean, and are quite expensive. Not only is high-purity syntheic quartz needed, but the cells should be strain-free, and the faces should be parallel. The benefit of short paths is more related to lower solvent absorbtion, as mentioned in the article, than to scattering. It is a challenge to find an appropriate solvent for work below 200nm. Curtis Johnson would be one name to search for in the literarture to learn about appropriate techniques.

--AJim 04:40, 25 March 2007 (UTC)

The article currently reads: "The ultraviolet CD spectrum of proteins can predict important characteristics..." This is misleading. The CD spectrum does not *predict* anything, rather it is a result of the secondary structure present in the protein. --128.227.142.130 21:01, 28 June 2007 (UTC)

CD Spectrometer
How does the CD spectrometer actually operate? What are the components that actually generate, process, and detect the light? It would be great if there was a schematic that explains how measurements are made. M stone 14:37, 2 November 2007 (UTC)


 * I agree. An animation has been added but it doesn't explain much. LostLucidity (talk) 16:38, 12 March 2008 (UTC)

Most of the components in a CD Spectromenter have articles that can be referenced. The most important exception is the photo-elastic modulator (PEM). I believe all current instruments use one to produce the alternating left and right circularly polarized light.

In terms of how measurements are actually made, this is a single-beam technique, so baseline correction needs to be discussed.

The definitions need to be extended to include mean residue elipticity because the protein structure estimation programs work on measurements in those units.

There is quite a bit of trouble in actually setting up an instrument and getting a good baseline. The measurement can be affected by any (inadvertent) poliarizer downstream of the modulator, such as a strained sample cell. --AJim (talk) 08:48, 4 January 2009 (UTC)

I was wrong about the PEM, Photoelastic modulator does exist.

I found it through the Lock-in amplifier article. Although commercial instruments generally do not use commercial lock in amplifiers, they do use phase sensitive detection. The reference signal is derived from the PEM.

The Polarizer, monochromator, and photomultiplier articles look useable.

The Cuvette article might usefully show some of the more exotic cells used in CD. --AJim (talk) 00:49, 5 January 2009 (UTC)

I just discovered that there is a Category:Spectrometers now. Thus, there seems to be a reason to have specific articles on Circular Dichroism Spectrometers, and ORD spectrometers for that matter. The article on Magnetic circular dichroism is in a similar state; it talks about the technique, and describes the instrument, but is not actually about the spectrometer itself.

--AJim (talk) 18:34, 20 June 2011 (UTC)

Inaccurate summary / article title: move, rewrite, merge?
While some of the article is about circular dichroism, the article summary inaccurately states "Circular dichroism [...] is a form of spectroscopy". No, circular dichroism spectroscopy is a form of spectroscopy, employing circular dichroism. Circular dichroism is an optical effect, in which a material absorbs light of one circular polarization differently from the other. Given that basically the whole article is "applied" (even the more theoretical first section is mostly about samples and experiments and quantities), I'd suggest that someone a bit more knowledgeable on the subject than I might do something like this:


 * 1) Move this article to a new Circular dichroism spectroscopy article, excising theoretical stuff and replacing it with links here and to Dichroism.
 * 2) Rewrite the existing theoretical/optical content into a new Circular dichroism, probably a stub or start-class article.
 * 3) Evaluate merging Circular dichroism into dichroism.
 * 4) Evaluate whether Circular dichroism spectroscopy needs to live.

Of course, if there's strong consensus on the third or fourth points, they may override the need to go through with the first or the second.

Andrew Rodland (talk) 21:10, 31 December 2007 (UTC)

I vote for the second option. Circular dichroism spectroscopy definitely needs to live and should focus on the applications of CD, as discussed in the NMR spectroscopy talk page. A good source for the Circular Dichroism article is chapter 8 of Optics by Hecht (4 ed.) --Biophysik (talk) 17:57, 1 November 2008 (UTC)

Having looked at the page on polarization, it seems more sensible that the concept of circularly polarized light be dealt with there, and not in this article. This article should reference that one. A brief and mathematically light introduction would probably be appropriate to introduce the principle of the technique and the units, similar to what is currently in place. ravenacious (talk) 15:22, 4 August 2008 (UTC)

Polarization is a big topic, so it is probably best to instead split the article into Circular dichroism and Circular dichorsim spectroscopy. --Biophysik (talk) 18:05, 1 November 2008 (UTC)

Missing Images
This article lacks foremost CD-spectra of alpha-helix/beta-sheet/random coil... —Preceding unsigned comment added by 130.60.169.215 (talk) 14:09, 1 July 2008 (UTC)

I can probably contribute quite a bit to this page, especially in the diagram and illustration department. I have never edited a Wikipedia article before, so I don't really know what the etiquette is... do I just dive in and start editing things, or is there an agreed editor for the page or something? Firstly, I think that the diagram of electric field vectors should be replaced, as it is slightly mis-leading, secondly I have diagrams of representative spectra for protein secondary structure that was requested above. I also have made diagrams of the workings of a CD spectrometer and could post those. Any input on this would be appreciated, many thanks. Kevinchannon (talk) 13:19, 4 August 2008 (UTC)

1. Dive right in and start editing 2. The main issue here is copyright. Pictures from a website, journal article, or instructional manual are generally not useable unless permission is given. The exception I believe is NIH and other government websites, which I believe are in the puble domain (does someone know this for sure?)

It's OK, I can make my own images, thus avoiding the copyright issue :-)kevinchannon (talk) 23:51, 2 April 2009 (UTC)

Woody Citation
Woody is cited in over 30 papers in 1994. Could the reference be a little be more specific as to which paper/journal co-authors it was. 128.243.253.104 (talk) 21:56, 28 April 2011 (UTC)
 * Should be better now (not sure about the page number though). Materialscientist (talk) 04:30, 29 April 2011 (UTC)

Units of CD
There may be confusion in the article (and maybe also in the field) about the units of CD. It is stated: "Circular dichroism is the difference Δε ≡ εL- εR", but later it's said: "Then Δε ≡ εL- εR is the molar circular dichroism". So here's no difference between circular dichroism and molar circular dichroism. I guess the first statement should read: "Circular dichroism is the difference ΔA ≡ AL- AR"? — Preceding unsigned comment added by 130.238.59.124 (talk) 15:50, 29 August 2012 (UTC)

Sign of rotational strength
I believe the formula for the theoretical rotational strength should be negative, i.e. Rtheo = -Im ..., alternatively <Ψe|Mmagn.dip.|Ψg> should be used. The magnetic dipole operator is non-hermetian and exchanging wavefunctions leads to a minus. Ref.: William W. Parson: Modern Optical Spectroscopy, Springer 2007 — Preceding unsigned comment added by 130.238.59.124 (talk) 16:22, 31 August 2012 (UTC)

point gruops
According to the article: Symmetry restrictions for natural and induced circular dichroism and optical activity for all point groups, not only Dn and Cn groups show circular dichroism, but also T, O and K groups. — Preceding unsigned comment added by 94.255.155.158 (talk) 10:02, 31 May 2020 (UTC)