Dichroism, circular curves

The circular dichroism curves for a variety of penicillanic acid derivatives have been published and discussed (B-77MI51100) and have been used to support extended Hiickel MO calculations (77T711). 

The -position of the cyclopropane ring was established by the observed negative circular dichroism curve (Ae — 2.03 in dioxane at 298 m/i) and consideration of the inverse octant rule for cyclopropyl ketones. ... 

Addition of lithiated heterocycles to aldonolactones yields carbon-linked nucleosides (56). Thus, the reaction of 2,3 5,6-di-O-isopropylidene-L-gu-lono-1,4-lactone (9b) or 2,3-O-isopropylidene-D-ribono-l,4-lactone (16a) with various lithiated heterocycles gave gulofuranosyl derivatives 53a-g or ribofuranosyl derivatives 54b,c. Gulonolactols 53a-g and ribonolactols 54b,c were acetylated with acetic anhydride in pyridine to yield their acetyl derivatives. The stereochemistry of compounds 53a-g and 54b,c was discussed in terms of the Cotton effect of circular-dichroism curves of the ring-opened alcohols formed upon reduction by sodium borohydride. The configuration at C-l of 53g was proved by means of X-ray analysis (57,58). 

Table 1. The induced circular dichroism curves with achiral nitrosamines and chiral alcohols in isooctane.

Their characteristic optical rotatory dispersion or circular-dichroism curves, and their infrared spectra, rich in characteristic frequencies, may be useful. Paper chromatography permits preliminary identification of the glycosyl phosphate or monosaccharide resulting after degradation, and the specific enzymic reactions of these products are widely used to provide additional evidence. 

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. 

The circular dichroism curves of nineteen aporphines have been examined. A generalization worth bearing in mind is that the terminal absorption near 216 nm is positive in the case of the R configuration, and negative for the S.53 A study of natural (—)-crebanine confirmed the R absolute configuration, instead of the S configuration which is usual for aporphines that are disubstituted in ring d. 

Figure 5. Circular dichroism curves for soy 7S protein and modified 7S protein. (----------------------------) Native, (---) modified.

Figure 15. Circular dichroism curve (-—) plotted on the left-hand ordinate and optical rotatory dispersion (ORD) curve (-—) plotted on the right-hand ordinate of d-camphor-10-sulfonate. It is seen that the band width at [df]/e and the distance between the ORD extrema coincide better than does the band width at [0 ]/2, i.e., r. The amplitude of the ORD anomalous dispersion curve (Cotton effect) is indicated as [Amp]. Reproduced, with permission, from [3].

Figure 21. Simultaneous resolution of circular dichroism and absorption curves of a-helical poly-L-alanine. The constraint of correspondence of band maxima and widths was relaxed for the long wavelength band. A, Circular dichroism curve resolved into three bands the positive perpendicularly polarized band near 190 nm, a negative parallel polarized band resolved near 204 nm, and a negative n-K band near 220 nm. B, Absorption curve also resolved into the same three bands. The critical values for these bands are given in Table 2A. Reproduced, with permission, from [70].

Figure 38. A, Circular dichroism spectra of -nicotinamide-adenine dinucleotide ( -NAD) as a function of temperature showing the reciprocity between the adenine band just below 260 nm and the nicotinamide band near 270 nm. These reciprocal relations in optical rotation qualitatively demonstrate a close interaction of the two aromatic rings. Reproduced, with permission, from [98]. B, Circular dichroism curves of flavin-adenine dinucleotide (FAD) as a function of water/dioxane mixtures. As the solvent becomes more polar, there is increased proximity of the two aromatic rings, as shown by the reciprocal changes in the intensity of the adenine band below 260 nm and the isoalloxazine band of flavin near 270 nm. Reproduced, with permission, from [99].

Extensive examinations of the circular dichroism curves of pyrrolizidine alkaloids have been carried out by Culvenor et a/. and Hrbek et The free bases tested were divided into four groups, all possessing 8a-hydrogen atoms. Saturated 1-substituted pyrrolizidines [e.g., 1-methylpyrrolizidine (7) and isoretronecanol (22)] showed negative Cotton effects while with 1,7-disubstituted pyrrolizidines [e.g., platynecine (43)], the Cotton effects were positive. Dehydropyrrolizidines with 1-substituents [e.g., supinidine (96)] displayed positive Cotton effects, and those with 1,7-disubstitution patterns [e.g., retronecine (127) and heliotridine (109)] exhibited large positive... 

Evidence for the relative and absolute stereochemistry, which was reported in 1965, was based on a more detailed analysis of the NMR spectra and on the signs of the circular dichroism curves of various ketones when compared to steroid models. The stereochemistry was finally confirmed by X-ray crystallography. 

Fig. 10. Circular dichroism curves of a-NADH at various temperatures in 1 M NaF at pH 7. Reciprocal relations are observed between a reduced nicotinamide band near 215 nm and an adenine band near 210 nm [Reproduced from Ref. (502)]...

The two 3jS-dimethylaminoandrost-5-en-17-ones, epimeric at C-14, namely (109), 14a, and (110), 14) , give the same 3/ -dimethylamino-5) -methyl-14j -oestr-8-en-17-one (111) on treatment with H2SO4 at 0 °C. The 10/S configuration of (111) is deduced from the negative circular dichroism curve of the ketol (112). 

The evolution of the circular dichroism curves during the neutralization of the pectinic acids by sodium and calcium hydroxides were therefore followed in order to obtain more informations on the conformational changes. The neutralization of sample 5 (a pectin with a DE of 40% with a statistical distribution of the free carboxyl groups), either with sodium or calcium hydroxides leads to a regular decrease both in X and in differential extinction coefficient (A e) which progressively reach values for pure sodium and calcium forms, respectively. This is entirely consistent with a simple ionization process and all the curves pass through the isodichroic point at about 200 nm. 

Fig. 9. Circular dichroism curves for poly-L-glutamic acid in the reference state, pH 3.85, and as suspensions. The concentration of PGA is constant at 2mg/ml only the particle size is varied. Curves , h, and c were obtained simultaneously on the same sample and phototube and with the same light beam as the absorption curves in Fig. 8. Curve d is the calculated cui ve, using (2), and curve e is the difference between the calculated and experimental curves. Note that curve e resembles the ORD curve for an a-helix. From Urry et al. ...

Chart 2 Circular Dichroism Curves of Dibenzylbutyrolactone in EtOH... 

H. Liu and K. Nakanishi, Additivity relation found in the amplitudes of exciton-split circular dichroism curves of pyranose benzoates, J. Amer. Chem. Soc., 103 (1981) 5591-5593. 

Each diastereoisomer can be isolated in a pure optically active form. They display quasi mirror image circular dichroism curves which illustrates the enantiomeric relationship between the tantalum asymmetric centers. 

Comparison of the NMR data and the circular dichroism of toxlsterol C2 with those of the now known Cl suggests that C2 should have formula II. The circular dichroism curve of C2 is opposite in sign (Ae227 -7.09) to that of Cl (Ae233 +4.55), The sign of the Cotton effect, associated with the absorption band at 227 to 229 nm, reflects the configuration in the chromophore concerned—that Is, the relative orientation of the cyclopropane ring and the double bond. This rules out formula... 

In order to ensure the stereochemistry of these molecules at C-6 and C-10, they were subjected to X-ray analysis (Fig. 1) which showed that ring A deviates from the preferred half-chair conformation of cyclohexane, is slightly distorted from perfect chair. This is possibly due to constraint imposed by the y-lactone ring. The sign of circular dichroism curve associated with the a, P unsaturated lactone has been taken as evidence for the absolute stereochemistry of this series. Teuflidin (15) showed the same sign and the magnitude of CD curve as teucvidin (14) and, hence, it was assigned to the same absolute stereochemistry. 

The cyclization reactions of manool derivatives have continued to attract attention. A synthesis of the strobane skeleton based on oxymercuration reactions of 13-epimanool has been described. The oxidation of sclareol by chromium salts has been re-examined in a series of papers. The circular dichroism curves of manoyl oxide and its 13-epimers have been examined. Some further 11-oxomanoyl oxide derivatives, coleonol E and F, (13) and (14),... 

The phenomenon is shown up even more convincingly in the circular dichroism curves (fig. 4.1.11). For a left-handed structure (i.e., negative p), 3 exhibits an anomalous increase and for 5k < 0, the peak transmission occurs on the long wavelength side of the reflexion. 

Fig. 4.1.12. Experimental circular dichroism curves versus wavelength, (a) Pure cholesteryl nonanoate (CN), (6) CN+0.98 per cent by weight of PAA. Sample thickness in both cases 6.S foa. (After reference 28.)...

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