D-enantiomers (Cerf and Jorissen 2000). This would, of course, be problematic! However, the referred circular dichroism spectra were exclusively recorded in aqueous solution, a state of aggregation that is - as outlined above - not relevant to astrophysical and interstellar conditions.

In order to further investigate this argument we subjected solid amorphous films of the amino acids L-valine, L-alanine, L-leucine, and L-proline to circular dichroism measurements. Synchrotron light between 130 and 330 nm was used at the Institute for Storage Ring Facilities at Arhus University. Newly recorded circular dichroism spectra showed that not only L-valine, L-alanine, and L-leucine exhibit characteristic bands in the vacuum ultraviolet at 182 nm, but also L-proline showed a signal of the same sign exactly matching this wavelength (energy).

Circular dichroism spectra given in Fig. 6.9 were recorded for amorphous films of the individual amino acids after vacuum sublimation. The corresponding bands are red-shifted compared to the circular dichroism spectrum of leucine depicted in Fig. 6.4, because in Fig. 6.4 microcrystalline D-leucine was used instead of an amorphous film. For more detailed information on the comparison of circular dichroism spectra of microcrystalline and amorphous films, see Kuroda (2004).

The obtained chiroptical data clearly demonstrate that at least the four tested amino acids show very similar circular dichroism spectra in the vacuum ultraviolet region in the solid state. Furthermore, diamino acids were experimentally and theoretically, i.e., based on ab initio calculations, shown to possess nearly identical circular dichroism spectra (Bredehoft et al. 2007). In contrast to the skeptical comments of Cerf and Jorissen (2000), photolysis reactions of amino acids in the solid state thus remain suitable possibilities for the induction of an enantiomeric enhancement into prebiotic and interstellar amino acid structures.

What about the mentioned tryptophan? In the next chapter (Sect. 7.3.3), we will discuss experimental studies on the recruitment order of amino acids at the beginning of biological evolution. These studies come to the unisonous conclusion that complex amino acids such as tryptophan seem to be recruited late in proteins. In contrast, the above tested amino acids L-valine, L-alanine, L-leucine, and L-proline are believed to be recruited very early during the origin of life, which may underline the above hypothesis.

6.2.3 Asymmetric Photoisomerization

In contrast to asymmetric photolysis, asymmetric photoisomerizations include processes in which chiral organic molecules undergo enantioselective isomerization induced by circularly polarized light. The pioneering work on this topic was performed by Inoue et al. (1996) studying the direct photoderacemization of E-cyclooctene. E-Cyclooctene is chiral and composed of the (-)-R-E-cyclooctene and (+)-S-E-cyclooctene enantiomers. It was selected for this kind of experiment because it shows extraordinary high optical rotation with [a]D values of -426 for the R-enantiomer. A photoreaction of racemic E-cyclooctene induced by circularly polarized light at X = 190 nm in pentane solution provokes a E ^ Z isomerization a

Was this article helpful?

0 0

Post a comment