In 1977, molecules of assumed relevance for prebiological pathways, such as the amino acids alanine (Norden 1977) and leucine (Flores et al. 1977), were decomposed enantioselectively. Irradiation was performed at 212 nm in acidic solution provoking a particular electronic transition that is called the (n*, n)-transition. The corresponding photolysis mechanisms were recently developed (Nishino et al. 2001, 2002). It was demonstrated for each amino acid that the enantiomeric purity achievable during photolysis of a racemate is dependent on (a) the anisotropy factor (g) that is the ratio between the circular dichroism (Ae) value and the extinction coefficient (e), and (b) the extent of reaction (Balavoine et al. 1974). The anisotropy factor g was formerly called the dissymmetry factor and is given by Eq. (6.2).
Due to its dependence on e, the anisotropy factor is a function of the wavelength. In order to determine g at a given wavelength, one can choose between two possibilities.
1. One measures the differential absorption Ae of one individual enantiomer (R- or S-enantiomer) against right- and left-circularly polarized light, designated in Eq. (6.2) as r and l.
2. One determines g according to Eq. (6.3) by recording the differential absorption Ae of a pair of enantiomers (R- and S-enantiomer) against one kind of circularly polarized light, e.g. individual R- and S-enantiomers against left-circularly polarized light or individual R- and S-enantiomers against right-circularly polarized light (Rau 2004).
Table 6.1 Anisotropy factors (g) of selected amino acids
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