Knowledge Bank

Thin films of all trans-$\beta$-carotene crystals at room temperature show three main absorption bands in the visible with $\lambda max$ at $502m\mu ,468m\mu$ and $442m\mu$. The bands are broader in crystal films than in solution and the central one is most intense as in solution at room temperature. Cooling the film to $77\circ K$ does not show any appreciable change except that the bands are slightly sharpened and red shifted with four bands now fairly resolved. The position of $\lambda max$ are at $505m\mu ,471m\mu$, $441m\mu$ and $415m\mu$. The average separation between bands is $1430cm-1$, the same as that observed in EPA glassy solution at $77\circ K$. When the film of all-trans $\beta$-carotene crystals is exposed to vapors of toluene, pyridine, $CS2$, 3-methyl pentane, methanol, ethanol, benzene, acetone, chloroform, hexane, or ether, a drastic change in the spectrum is observed. A new intense absorption band appears in the $535m\mu$ region and the intensity of the main absorption region decreases as the intensity of the new band increases. The position of the new band depends on the adsorbed molecule, being $550m\mu$ for $CS2$ and $527m\mu$ for hexane. This new band is separated from the main absorption band by about $1600cm-1$. With desorption of the foreign molecules, the intensity of the new band decreases and the spectrum tends to return to the original $\beta$-carotene crystal film absorption spectrum. All trans $\beta$-carotene when melted in the dark and in an inert atmosphere, cools into a deep red colored glass. Thin films of this glass, when exposed to different vapors do not show the new band in most cases---and when it does, the new band appears very weak compared to that in $\beta$-carotene crystals films. Similar results have been observed with $15,151$ cis $\beta$-carotene, isozeaxanthin, $\beta$-APO-8'-carotenal and astacene. Possible interpretations of these results will be discussed.