Cotton effect, optical rotatory dispersion

Enantiomers can be distinguished by their rotation of plane-polarized light at a specific wavelength, or over a range of wavelengths (optical rotatory dispersion, ORD), as well as by the difference in absorption of right and left circularly polarized light (circular dichroism. Cotton effect, CD). 

Neville and Bradley 79) were first to report that DNA induced Cotton effects in the optical rotatory dispersion spectrum of acridine orange. The investigation of DNA-induced Cotton effects has been extended to other aminoacridines. According to Peacock 80), aminoacridines may be divided into two groups. The DNA-induced ICD of the first class (aminoacridines having a 3-amino group) increases steeply and cooperatively with increasing amounts of bound aminoacridine /% where r is... 

At alkaline pH values, even though no observable association or aggregation occurs, some changes in configuration are observed. Changes in the Cotton effect between 250 and 300 nm and in optical rotatory dispersion occur at pH 11.5. The fourth tyrosine, which was somewhat buried at neutral pH values, reacts with cyanogen fluoride at pH 10.0 and above (Gobrinoff 1967). 

This means that the molar extinction coefficients of the two enantiomers (e, and er) are unequal in circularly polarized light. These differences in absorption (e, and er) can be measured as a function of wavelength, and the curves obtained are called circular dichroism curves. They have positive or negative signs (Cotton effect) just as for optical rotatory dispersion curves. 

After decarboxylation, enolization in the alternative mode would cause racemization. However, this reasoning does not explain why 199 was racemized. The racemic a-aminoketones were eventually resolved via their bromocamphorsulfonates. Optically pure (—)-indolizidin-l-one (196) was reduced with lithium aluminum hydride to the alcohol, tosylated, and again reduced to (+)-indolizidine [Eq. (28)]. Since indolizidine obtained from R-pipecolic acid [Eq. (29)] was levorotatory, it followed that the absolute configuration of the original ketone was S.254 The optical rotatory dispersion (ORD) curve of the S(-)-ketone showed a strong negative Cotton effect as predicted by the octant rule. 

Optical rotatory dispersion studies on lactones of aldonic acids and other carbohydrate acids were made by Hirst et al.11 and later by Okuda et al.7S The latter workers related the absolute configuration at C-2 of aldono-1,4-lactones to the sign of the Cotton effect. 

A recent detailed investigation530 was carried out on the UV optical rotatory dispersion and the circular dichroism of the compounds i/>3SiSi (me)X (with X = H, F, OH, OCH3). A Cotton effect was seen and discussed in relation to the structure. 

Fig. 15.30 Anomalous optical rotatory dispersion (ORD) and circular dichroism (CD) spectra for the positive Cotton effect of a single, isolated electronic transition.

As discussed, optical rotatory dispersion (ORD) is determined by the unequal indices of refraction for left- and right-circularly polarized light in a chiral medium, within an absorption band, the ORD spectrum exhibits anomalous dispersion, which is referred to as a Cotton effect. Full understanding of ORD and anomalous dispersion requires a more detailed examination of the properties associated with refractive indices. 

Table III. (Also continued on next page) Predicted and observed n - t Cotton effects for steroid ketones from circular dichroism and optical rotatory dispersion spectroscopy"... 

ABA absorbs ultraviolet with maxima at 240 nm (e 2.1 x 104, a shoulder peak), 260 nm (e 2.6 x 104), and 320 nm (e 50) in an acidic methanol solution.590 Irradiation with UV with a wavelength shorter than 305 nm isomerizes the 2-(Z)-double bond to (E) to give an equilibrium mixture of ABA and its 2-( )-isomer with a ratio of 1 1, and also causes decomposition of ABA to unidentified compounds by the excitation of the 7t-7t transition of the side chain and the enone groups. ABA has a strong optical activity, and its specific optical rotation is +430° in an acidic methanol solution.591 In the optical rotatory dispersion (ORD)591 and the circular dichroism (CD) spectra,558 ABA shows a positive Cotton effect from 300 to 200 nm. Phaseic acid and dihydrophaseic acid and its epimer, which did not have the enone group, show a small specific optical rotation with a minus value and also a negative plain curve in the ORD. 

Palytoxin is a white, amorphous, hydroscopic solid that has not yet been crystallized. It is insoluble in nonpolar solvents such as chlorophorm, ether, and acetone sparingly soluble in methanol and ethanol and soluble in pyridine, dimethyl sulfoxide, and water. The partition coefficient for the distribution of palytoxin between 1-butanol and water is 0.21 at 25°C based on comparison of the absorbance at 263 nm for the two layers. In aqueous solutions, palytoxin foams on agitation, like a steroidal saponin, probably because of its amphipathic nature. The toxin shows no definite melting point and is resistant to heat but chars at 300°C. It is an optically active compound, having a specific rotation of -i-26° 2° in water. The optical rotatory dispersion curve of palytoxin exhibits a positive Cotton effect with [a]25o being -i-700° and [a]2,j being +600° (Moore and Scheuer 1971 Tan and Lau 2000). 

As it is easily derived from Eqs. 6.4-18, the relations are valid also for birefringence and dichroism. As far as optical activity in the UV/VIS range is concerned transformations on this basis have already been widely used for decades to correlate circular-dichroism bands and Cotton effects, i.e. anomalies of the optical rotatory dispersion (Moffit and Moscowitz, 1959 Blout et al., 1967). Another relation is mentioned in the following section. 

Figure 8 also shows dichroism in the weak band in the 2200-2300 A range, which may correspond to the weak amide absorption assigned as an n —> 7T transition by Hunt and Simpson (1953). This may quite possibly be the transition involved in an apparent Cotton effect seen in the optical rotatory dispersion of a-helical structures (Simmons et al., 1961). If this is true, and it appears possible, it raises a serious question about the... 

Measurements of light absorption, optical rotatory dispersion, the Cotton effect, and circular dichroism have been made.i i5 The mechanism, kinetics, and stereochemistry of the aquation and basic hydrolysis of [Coen2NH3Br] + complexes have been investigated. Change in optical activity with aquation has also been determined. 

Amino-5-deoxy-D-xylopyranose (34 = 17) is like 2-piperidinol (2-hydroxypiperidine) in existing in a pH-dependent equilibrium with its dehydration product (33 = 16). The ultraviolet peak of the n-7T transition of the C=N chromophore of 33 is not suitable for structural elucidation. However, the asymmetric nature of this chromophore gives rise to a Cotton effect. A solution of free 5-amino-5-deoxy-D-xylose shows a negative Cotton effect at 300 nm that is well demonstrable by measurement of circular dichroism for this purpose, the optical rotatory dispersion is much less sensitive. The Cotton effect is ascribable to 33, as 34 and 22 would exhibit no Cotton effect in this region. Thin-layer chromatograms of 34 always show a second spot which, like 34, gives a red coloration with o-aminobenz-aldehyde this is, presumably, caused by 33. 

FIGURE 9-20 The Cotton Effect in ORD and CD, (a) Idealized optical rotatory dispersion (ORD) and circular dichroism (CD) curves at an absorption peak, with a positive Cotton effect, (b) Structures of tris-(S-alaninato) cobalt(IIl) complexes, (c) Absorption and circular dichroism spectra of the compounds in (b). (Data and structures in (b) adapted with permission from R. G. Denning and T. S. Piper, Inorg. Chem., 1966, 5, 1056. 1966 American Chemical Society. Curves in (c) adapted with permission from J. Pujita and Y. Shimura, Optical Rotatory Dispersion and Circular Dichroism, in K. Nakamoto and P. J. McCarthy, eds.. Spectroscopy and Structure of Metal Chelate Compounds, John Wiley Sons Inc., New York, 1968, p. 193. 1968 John Wiley Sons, Inc. Reprinted by permission of John Wiley Sons, Inc.)... 

In Figure 5 the optical rotatory dispersions and CD spectra of some frans-disubstituted cyclopropane hydrocarbons are displayed Compound 1 has C2 symmetry and with respect to the cyclopropane subunit also 114 and 115 have a local C2 symmetry. Trans-1,2-dimethylcyclopropane (1) exhibits a continuous CD rising to a positive Cotton effect... 

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