THE VISUAL PIGMENTS 



6. There are often large changes in optical rotatory power (if the 

 molecule contains one or more asymmetric carbon atoms). 



Some of these effects are illustrated for the case of ^-carotene in 

 Fig. 5.1. The reduction in intensity of the absorption band in the 



150.000 



100.000 



Z 50000 



300 



400 

 Wavelength in m|ji 



500 



Fig. 5.1. Light-absorbing characteristics of /S-carotene, a typical Qo 



carotenoid, and the effect of molecular shape (isomeric form). 



trans-P-carotQUG, cw-/3-carotene. 



visible spectrum and the appearance of the cis peak in the near 

 ultraviolet (340 mju) on isomerization are well shown. In most C40 

 carotenoids (in hexane solution) the centre of the cis peak lies at 

 142 ± 2 mju from the longwave maximum. 



Interpretation of absorption spectra. As exemplified in Fig. 5.1 for 

 /S-carotene, the carotenoids absorb Ught in three spectral regions, 



132 



i 



