Bisignate

This knowledge and understanding may be helpful to characterize local conformations of other optically active polysilanes in solution. For example, poly(methyl-(-)-(3-pinanylsilane) [(+)-7 Mw = 10,200] prepared by Shinohara and co-workers.281 showed a bisignate CD band at 280 and 303 nm, associated with a broad UV absorption at 300 nm in chloroform at 15°C. Since the spectroscopic features are quite similar to those of i,28d-28e it is possible that the main chain in 7 may contain diastereomeric helical motifs with opposite screw senses and different screw pitches. 

Chiral aggregates of 40 were subjected to multiple cycles of heating (50°C) and cooling (-10°C) to investigate the thermal stability of the aggregate chirality.29 In the CD spectra of the aggregates at 60%/40% THF/methanol, the bisignate Cotton effect observed at 20°C completely disappears at 50°C... 

Figure 4.23 Chiral orientation of the preferred helicity of single molecules of the polysilane for the origin of bisignate Cotton effect.

The (—) and racemo forms were also prepared. The CD spectrum of the (+) and (—) polymers in ethanol showed negative and positive bisigned profiles respectively, indicating main-chain helical screw sense chirality. The origin of... 

Upon threading the oligosilane molecules into the asymmetric y-cyclodextrin cavity to form a pseudorotaxane-type aggregate,335 a bisignate Cotton effect became apparent at about 290 nm, indicating induced PSS helicity. The reason for the bisignate nature of the signal was not clear. 

Often the UV absorption tails into the visible due to light scattering by the microparticles, although the Amax is usually very close to that in solution. The aggregate CD spectrum, however, is markedly different from the solution-state spectrum, showing an intense bisigned Cotton effect coincident with the UV due the Davidov coupling. 

With the same (.y)-2-methylbutyl side chain in each case, the solution CD was, as expected, almost identical, with positive Cotton effects. However, on addition of MeOH to form aggregates, while 127 showed a negative bisigned Cotton effect, 128 showed a positive bisigned Cotton effect, as shown in Figure 53. 

A special mention in the field of enantioselective HPLC separations must be made of chiro-optical detection systems, such as circular dichroism (CD) and optical rotation (OR), which can be also used to circumvent the low UV detectability of chromophore-lacking samples [40, 61]. While sensitivity of chiro-optical detection is not always sufficient to perform enantiomeric trace analysis, the stereochemical information contained in the bisignate spectropolarimetric response is useful in establishing elution order for those compounds not available as single enantiomers of known configuration. An example of application of different online detection systems (UV and CD at 254 nm) in the enantioselective separation of a racemic sulfoxide on a commercially available TAG CSP is reported in Figure 2.12, under NP conditions. 

The biphasic reaction with CO points to the existence of multiple heme-hemopexin conformers, and this is borne out by spectral analyses. The absorbance spectra of rabbit ferri-, ferro-, and CO-ferro-mesoheme-hemopexin are entirely analogous to those of other bis-histidyl heme proteins such as cytochrome 65 142), but the CD spectra exhibit unusual features (Fig. 11). Of particular interest are the weak signal of the ferro complex and the bisignate signal of the CO-ferro complex (also seen in the NO-ferro-mesoheme-hemopexin complex (140) and in human ferri-protoheme—hemopexin (139)). 

However, this explanation is not sufficient to accoimt for the bipha-sic CD spectrum of human ferri-protoheme—hemopexin (with 2,4-vinyl substituents), as well as the much weaker human CO-ferro-heme-hemopexin bisignate signal compared to the rabbit congener (139), and hence other factors must be involved. Several potential effectors exist (a) exciton coupling (b) the conformers produced by a 180° rotation about the a- and y-meso-carbon axis and consequent nonisometric interactions of the as5unmetric 2,4- and 9,10-substituents (c) the aromatic tryptophan residues near the heme binding site (s) and (d) two independent binding modes or sites. 

A bisignate spectrum is characteristic of exciton coupling between identical chromophores on one molecule (144), an impossibility with the 1 1 heme-hemopexin complex. Bisignate CD spectra have been observed with bilirnbin-albnmin however, bilirubin, nnlike the planar molecnle heme, can adopt an extended, helical stacked conformation... 

The first term in eq. [21] is the contribution of the intrinsic rotational strengths if oscillators a and/or b are themselves chiral. The second term is the coupled oscillator contribution due to the intrinsic moments and the third term is the coupled oscillator contribution due to the geometric arrangement of the two electric dipole oscillators. The latter two contributions give rise to a conservative bisignate couplet in the observed spectrum, if the coupled modes are sufficiently separated in frequency such that the positive and negative contributions do not cancel. 

In this case the coupling still generates a bisignate VCD couplet, but the magnitude of the intensity and the degree of mixing depends on the energy separation of the uncoupled modes, — Ey,. 

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