Circular dichroism instrumentation

Spectrometry, circular dichroism instrumentation, selection and calibration of, 226-227... 

G. A. Osborne, J. C. Cheng and P. J. Stephens, A near-infrared circular dichroism and magnetic circular dichroism instrument, Rev. Sci. Instrum., 44 (1973) 10-15. 

As seen in Table 7, the halogen atoms vary widely in atomic weight and this is reflected in the location of their infrared stretching bands. These bands were outside the range of the first generation if vibrational circular dichroism instruments and only the C—F bands are... 

However, the improved sensitivity of FT-IR allows one to obtain better sensitivity using the conventional sampling accessories and expand the range of sampling techniques. Emission, diffuse reflectance and photoacoustic spectroscopy represent new areas where FT-IR reduces the difficulty of the techniques considerably. Greatly improved results are also achievable from reflection spectroscopy. Special effects such as vibrational circular dichroism can be observed using FT-IR instrumentation. 

Instrumentation for the measurement of circular dichroism past, present and future developments... 

We present the basic concepts and methods for the measurement of infrared and Raman vibrational optical activity (VOA). These two forms of VOA are referred to as infrared vibrational circular dichroism (VCD) and Raman optical activity (ROA), respectively The principal aim of the article is to provide detailed descriptions of the instrumentation and measurement methods associated with VCD and ROA in general, and Fourier transform VCD and multichannel CCD ROA, in particular. Although VCD and ROA are closely related spectroscopic techniques, the instrumentation and measurement techniques differ markedly. These two forms of VOA will be compared and the reasons behinds their differences, now and in the future, will be explored. 

Conformational changes are easily followed by optical rotation (Hui and Neukom, 1964). Circular dichroism spectroscopy (CD) of polysaccharides (Morris, 1994) exploits optical anisotropy. In a CD instrumental design, the clockwise and counterclockwise rotation of two polarized beams of equal intensity, traversing a 180° path through a chiroptical medium, display a molar ellipticity maximum and minimum. CD is the differential measurement as a function of X. By CD spectroscopy, mixed interchain association rather than nonspecific incompatibility or exclusion was identified as the molecular basis of alginate-polyguluronate interaction (Thom et al., 1982). 

The circular dichroism (CD) spectrum of a number of polysaccharides is close to that of the corresponding monosaccharides (Morris, 1994). CD information gleaned from monosaccharides and extrapolated to polysaccharides is valid, insofar as mono- and oligosaccharides are representative of the complete polymer structure. CD applications to polysaccharides have been reviewed (Johnson, 1987). With the use of CD and complementary instrumentation, Morris (1976) studied xanthan insensitivity to salt and temperature, and observed that stability was introduced by a folding back of side chains around the main chain. Dentini et al. (1991) outlined the use of CD to measure the average charge density of pectin chains. 

More recently the relationship between absolute stereochemistry and spectroscopy has given rise to a good deal of work (11), and the increased availability of instruments for measuring either circular dichroism or optical rotatory dispersion (or both) has led to many applications in studies of molecular shape, including a good deal of work on metallo-enzymes. 

LFnder the influence of a static electric field liquid crystalline solutions of PBLG become uniaxially oriented and show both linear and circular birefringence and linear and circular dichroism. Accordingly, the measured CD shows a dependence of the microscopic angle, a, that the fast optical axis of the oriented solution makes with respect to the plane of polarization of the polarizer in the CD instrument. When the film is reasonably thin (0.01-0.1 mm thick) or/and only partially oriented (less than 5% orientation), the apparent CD of oriented film of deoxyribonucleic acid is expressed by the equation (50) ... 

The measurement of vibrational optical activity (VOA) lacks some of the severe disadvantages mentioned. Vibrational spectral bands are less likely to overlap and can be measured using two complementary techniques namely infrared and Raman spectroscopy. They can be measured as well in the crystalline as in the liquid or gaseous state, and the techniques are applicable to solutions while nearly reaching (complemented with the appropriate theoretical models) the accurateness of the X-ray method. VOA has drawbacks too the effects are quite small and tend to be obscured by artifacts. They are about 10 times weaker than the optical rotatory dispersion (ORD) and the circular dichroism (CD) in the UV-VIS range. However, this apparent disadvantage is more and more relieved by instrumental advances. 

The value of a chiral detector in the analysis of physiologically active materials is clear, but the methods so far used have been found somewhat insensitive. A more encouraging procedure would be the measurement of circular dichroism and such instrumentation employing diode array detection is presently under development. Details of the device are difficult to obtain due to patent applications pending and particulars are not available. The basic arrangement, however, is thought to be similar to that depicted in figure 11. 

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