Experiments on the Origins of Optical Activity

Abstract

Two recent reports claim that (1) aqueous L-aspartic acid polymerizes faster than D-Asp in the presence of kaolin at 90°, and (2) L-phenylalanine is adsorbed by kaolin more extensively than D-Phe at pH 6 (the reverse being true at pH 2). The novelty of these observations and their potential significance for the origin of optical activity has prompted us to duplicate these experiments using more sensitive methods. L- and D, L-Asp in 0.01 M solution were incubated with kaolin at 90° for 8 days. Careful examination of the aqueous residues from such experiments failed to demonstrate any preferential polymerization of L-Asp over D-Asp, or indeed any significant gross polymerization of Asp at all.

In other experiments 0.001 M solutions of D, L-Phe at pH 6 and pH 2 were stirred with large excesses of kaolin for 24 hr, and the aqueous extracts from these mixtures were examined for gross adsorption using the amino acid analyzer. No significant gross adsorption was noted. We then looked for asymmetric adsorption in the aqueous residues using optical rotatory dispersion, gas chromatography and thin layer chromatography. By none of these analytical criteria could we find any evidence whatsoever for the preferential adsorption of D-versus L-Phe from either pH 6 or pH 2 solutions.

Finally, in experiments bearing on the origin of optical activity by parity violation during β-decay, we have irradiated solid samples of D-, L- and D, L-leucine in a 61700 Ci Sr-90 source at Oak Ridge National Lab. for 1.34 yr (total dose: 4.2 x 10′ rad). Gas chromatographic examination of the (appropriately derivitized) recovered samples showed that the L-Leu was 16.7% decomposed, the D-Leu 11.4% and the D,L-Leu 13.8% decomposed. The recovered D,L-Leu sample had a gas-chromatographically determined enantiomeric composition of 50.8% D-leu and 49.2% L-Leu. These data, though very close to experimental error, may indicate a slight preferential radiolysis of L-Leu compared to D-Leu by the Bremsstrahlung from Sr-90 β-decay. These high intensity irradiation experiments are being continued on a prolonged basis in order to reach more definitive conclusions.