Optical activity helical conformation, circular dichroism

Cobalt(lll).—Complexes. Ammine complexes. Optical activity can be induced in the complexes [Co(NH3) ] and [Cofenlj] by means of outer-sphere association with chiral anions, e.g. (- - )-tartrate. Circular dichroism is observed in the d-d bands of the cations and it is suggested that this is due to (a) direct interaction between the chiral anion and the metal f/-orbitals and (b) the preferred conformation adopted by the inner-sphere ligands in the presence of a helical outer-sphere ligand. 

It has also been reported from circular dichroism (CD) studies [36] that polysaccharide-based CSPs can induce chirality in enantiomeric guests such as (4Z,15Z)-bilirubin-Ixoc (BR) (Fig. 5). Although not optically active, BR has two enantiomeric helical conformations maintained by six intramolecular hydrogen bonds between two carboxylic acid moieties and two pyrromethenone — NH— protons. These (R)- and (5)-helical conformers are in dynamic equilibrium in an achiral solution [37], but some optically active compounds can enantioselectively bind to BR to induce CD spectra in solution [38-40]. A significant induced CD... 

In a suitably chosen mixed solvent system, such as acetic acid and 1-propanol (21), a sharp reversible transition between Form I and Form II can be achieved with a small change in solvent composition. Measurements of optical activity provide a convenient way to follow the transition in dilute solution. There are large changes in optical activity because Forms I and II are helices of opposite handedness. The left panel in Figure 3 depicts the reversible transition that is detected by circular dichroism measurements in mixtures of trifluoroethanol and 1-propanol. Form II is the only conformation present in trifluorethanol. A solution of poly(L-proline) in 35 65 trifluoroethanol 1-propanol exhibits the same circular dichroism pattern as does a solution where the solvent is pure trifluoroethanol. However, further addition of 1-propanol produces a dramatic change in the circular dichroism. At 20 80 trifluoroethanol 1-propanol the circular dichroism pattern is that characteristic of Form I. Data in Figure 3 do not extend beyond 10 90 trifluoroethanol 1-propanol because of the low solubility of poly(L-proline) in 1-propanol. 

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