Chromophores intrinsically dissymmetric

The planar conformations s-cis and s-trans cannot be appreciably populated owing to the repulsive steric interaction between the Cg—H and C5-methyl (in the s-cis) or between the same Cg—II and the methyl group on (Q in the s-trans). This repulsion is minimized by introducing a twist around the Q,—( 7 bond. Limiting conformations, with skew angles of about 40° and 140°, can be assumed, as shown in Scheme 8. In this way, an intrinsically dissymmetric chromophore is created. 

In the skewed form, instead, the transition is allowed both electrically and magnetically, with parallel transition moments. The product in equation 1 is hence non-vanishing, implying that this transition has finite rotational strength. This observation leads to the conclusion that skewed 1,3-butadiene is an intrinsically dissymmetric chromophore. 

The power of circular dichroism (CD) spectroscopy is unfortunately limited to the study of chiral molecules having accessible chromophores, and cannot be applied to the study of achiral compounds. However, a great many compounds of interest are not intrinsically dissymmetric, so the induction of chirality into such species is a fundamental requirement for the performance of CD studies. In the present work, investigations into the CD induced in achiral molecules by the action of chiral agents is reviewed. 

A distorted conjugated pair of double bonds is an intrinsically chiral chromophoric system, and its overall chiroptical properties depend on the reduced symmetry of the chromophore itself as well as on the perturbing action of a dissymmetric environment. 

The diene chirality rule (hereafter referred to as DR) constitutes a simple tool for correlating the sign of the lowest energy tt —> n transition (] A —> 1B in C2 symmetry) of the distorted diene to the chirality (left or right-handed) of the chromophore. The validity of this rule is based on the assumption that the CD of the distorted chromophore is determined by its intrinsic helicity alone and that external dissymmetric perturbations have only minor effects on the optical activity. 

The rotational strength calculated for I is as large as that of a butadiene twisted by 20°. In II, with an out-of-plane methyl, R increases by a factor of about 2. This shows that the contributions to R of dissymmetric substituents of chiral cisoid dienes may be comparable to and even outweigh the contributions arising from the intrinsic dissymmetry of the chromophore. 

In protein systems in particular, a distinction between intrinsic and extrinsic Cotton effects has been made 31). The intrinsic effects are the result of internal dissymmetric interactions of protein chromophores such as the amide transitions in helical arrays. Extrinsic effects relate to non-protein substances which are usually optically inactive but exhibit optical activity when conjugated to a protein or to an... 

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