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130 150 170 190 210 230 250 270 290 310 330 Wavelength (nm)

Fig. 6.9 Vacuum ultraviolet circular dichroism spectra of solid-state amino acids. a, CD spectra between 130 and 330 nm of solid amorphous l-valine sublimate (red line), l-alanine sublimate (blue line), and l-leucine sublimate (yellow line) immobilized on MgF2. b, l-proline sublimate. A CSA standard was used for instrument calibration purposes; d-enantiomers gave CD spectra of opposite sign. The band of l-proline matched the band of l-alanine, l-valine, and l-leucine producing the achiral Z-cyclooctene as given in Fig. 6.10. Interestingly, during this photoreaction, the optical activity of the reaction mixture rises, passes a maximum, and disappears again. However, over long periods, a photostationary state will be reached due to the inverse Z ^ E isomerization, which is also active under irradiation (Rau 2004). Very recently, functional groups were incorporated along the cyclooctene skeleton leading to changes in its photochemistry.

It was proposed that these results give additional hints, that indeed photoreactions involving circularly polarized light are capable of inducing enantiomeric excesses.

(-)-ff-(E)-cyclo- (+)-S-(E)-cyclo- (Z)-cyclooctene octene octene

Fig. 6.10 Photoisomerization of racemic E-cyclooctene towards achiral Z-cyclooctene. Triggered by a photoreaction with circularly polarized Light, the initially racemic reaction mixture develops optical activity

(-)-ff-(E)-cyclo- (+)-S-(E)-cyclo- (Z)-cyclooctene octene octene

Fig. 6.10 Photoisomerization of racemic E-cyclooctene towards achiral Z-cyclooctene. Triggered by a photoreaction with circularly polarized Light, the initially racemic reaction mixture develops optical activity

Similar reactions with amino acids and other biomolecules were assumed to provide further insights into the crucial role of 'chiral light' in chemical evolution.

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