Induced cotton effects

Figure 11. Correlation of the induced Cotton Effect with nitrosamines and chiral alcohols and the configuration of the alcohol...

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... 

Binding of related symmetrical dyes (naphthalene derivatives) to apohemoglobin resulted in different CD-Cotton effects in each case. As in the case of heme (see above), a coupled oscillator interaction with Ti-Ti transition moments of aromatic side chains of the protein was suggested, but no calculations were reported (194). The binding sites of the dyes used have not been identified and may include the heme pocket in some cases (195). The induced Cotton effects appear to be very sensitive to the local protein environment of the dye chromophores (194). 

Mercurated fluorescein derivatives, which were covalently linked to the sulfhydryl group of bovine serum albumin (BSA), showed induced Cotton effects in the region of the dye absorption, which disappeared in 8 M urea (304). The dependence on />H of the Cotton effects indicated... 

Interestingly when a racemic mixture of the helical complexes M(phen)3 + [M = Fe(II), Co(II) and Zn(II)] was added to the per-C02 - S-CD host, the circular dichroism spectra showed induced Cotton effects which in the case of the iron complex indicates that the per-C02 - S-CD enriches the A-enantiomer of Fe(phen)3 +. This behaviour, known as the Pfeiffer effect, occurs when a chiral complex with labile configuration interacts with optically active species to form a pair of disatereomers with one being more stable and hence interconversion between the A- and the A-enantiomers easily occurs. 

Value [s] is sensitive to changes of the bond strengths of both ligand S (MAA, EAA, acac) with complex [C-AA] and AA with [C-S]. With the diminishing of bond strengths CM-S and CS-M, there is a trend of increasing of [s] as the differential value of the induced Cotton effect in the absorbance band of the chromophore of the substrate molecule. 

The spectral properties of the guest are modified. For example, the chemical shifts of the anisotropically shielded atoms are modified in the NMR spectra. Also, when achiral guests are inserted into the chiral CD cavity, they become optically active and show strong induced Cotton effects on the circular di-chroism spectra. Sometimes the maxima of the UV spectra are shifted by several nm, and fluorescence is strongly improved, because the fluorescing molecule is transferred from the aqueous milieu into an apolar surrounding. 

Liquid crystal induced circular dichroism (LCICD) examines the differential absorption of circularly polarized radiation by an achiral solute oriented in a cholesteric liquid crystal. The circular dichroism results from an induced Cotton effect in the achiral solute due to the macroscopic chirality of its ordering in the helical solvent matrix. The effect has been shown theoretically to arise... 

Quite recently we synthesized polyvinyl alcohol with a small number of camphorsulfonic acid ester groups [19]. The optical activity corresponds to a conversion of about 6%. Although the aqueous solution of this optically active PVA forms a blue complex with iodine-potassium iodide, there is no induced Cotton effect in the visible region (see Section 6). 

The dye-salts with the D-polymer and the L-polymer give Cotton effects at the same wavelength, but with opposite signs. This indicates that this induced Cotton effect must be connected with the secondary structure of the polymer in solution. 

Numerous other polyelectrolytes and biopolymers as well as other dyes were therefore also examined, and it was discovered that induced Cotton effects also occur in interactions between dyes as acridine orange, proflavine or acridine yellow and the j5-form of for example poly-5-carboxymethyl-L-cysteine [31, 32]. Ikeda and Imae assumed an ordered addition of dimeric dye molecules to an antiparallel pleated sheet structure of the polymer [33]. 

Numerous publications, particularly by Stone [38, 39], describe the induced Cotton effects in dye complexes with acidic polysaccharides such as heparin, chondroitin sulfate, carboxy me thy 1-cellulose, pectic acid and hyaluronic acid [40—42]. Methylene blue is usually used for these investigations, as its metachromasy depends to a high degree on the polysaccharide structure, As an example of a 4-1 linked amino sugar, heparin with methylene blue has a metachromatic absorption maximum of X = 565 nm. It its ORD a trough appears at 587 nm ( 3), and the CD also shows an exciton split with a zero crossover at 578 nm. These observations indicate dipole coupling and a helical arrangement of the bound dye molecules. 

Nakajimo and Matsamura [42] studied the effect of temperature on the induced Cotton effects in toluidine blue complexes with various acidic polysaccharides. They concluded that the signs of the induced Cotton effects do not reflect the entire conformation of the macromolecules local conditions may influence the interaction of the dye and the polymer. Thus affecting the optical activity. 

If however the interaction of the chiral tube and the achiral iodine chain is dissymmetrical, then an induced Cotton effect should be present here too. We have in fact been able to show that an aqueous iodine-starch solution has a strong Cotton effect around the absorption maximum at 600 nm [49]. 

A plot of the ellipticity and optical rotation of the iodine adduct as a function of the DP, shows a maximum for DP of between 70 and 100 [51]. The amplitude of the Cotton effect for a DP of about 20 is very small, and oligomers with DP <12 display no colour reaction and no induced Cotton effect. Thus the chiroptical phenomena in the long wave region are not based on the asymmetric carbon atoms of the base units they are induced by the secondary structure and come from the chiral exterior of the iodine chain, although this is not bound by covalent bonds or salt formation. The induced Cotton effects therefore show that iodine-amylose must be present in the form of a helical channel inclusion compound even in diluted aqueous solution. 

In the case when the compound reacting with the optically active molecule also possesses one or more electronic transitions the evidence of the chemical interaction can become indiscutable. The very existence of an association submits the electronic transitions of the compound to the dissymmetry of the chiral molecule. They become optically active and, consequently, detectable by the intermediary of the corresponding induced COTTON effects. Figure 9 shows the induction of optical activity in the d-d transition of the Cuii ion complexed with the optically active ligand N-tosyl L-alanine(YI) in aqueous solution at pH 10 [18]. 

As for interactions with ions or small molecules in solution, ORD and CD are effective only in the cases where these ions or small molecules show absorption bands in the visible region since one observes induction of optical activity in these bands. For example, the CD curve of poly L-lysine in helix at pH = 10.5 with Cu " shows a induced COTTON effects at the d-d transition of the Cu ion near 500 nm. The decrease of pH to 8.5 suppresses interactions with the main chain and consequently the induced optical activity [27]. Similarly, for degrees of neutralization below 0.7, the interaction of aery dine orange with poly-S-carboxy methyl L-cysteine VII is demonstrated by the induction of a COTTON effect at the level of the absorption band of the dye. Above 0.8, induced COTTON effects do not appear any more, the variation of the ORD of the polymer alone during the neutralization is then considered as resulting from a jS-structure-random coil transition [28]. It may be noted that induced optical activity in the side chain chromophores could also explain such behaviour. 

---