Attempted asymmetric radiolysis of D,L-tryptophan with ³²P β radiation

Abstract

In 1957 Vester¹ suggested that the then recently discovered^2,3 violation of parity in weak interactions might be ultimately responsible for the present unique chirality of asymmetric molecules in the biosphere. Since then there has been a growing number of investigations involving the longitudinally polarised electrons from natural β decay. Initial attempts^4,5 to detect asymmetric effects in several synthetic and degradative organic reactions using a number of β emitters failed and it was not until 1968 that Garay reported⁶ that β radiation from ⁹⁰Sr/⁹⁰Y caused D-tyrosine to degrade more than L-tyrosine in alkaline aqueous solution. Studies have subsequently looked at the possible asymmetric interaction of β rays (or their bremsstrah-lung) from a variety of sources (for example ¹⁰⁴Rh (ref. 7), ⁹⁰Sr/⁹⁰Y (refs 8,9), ¹⁴C (refs 10–12) ³²P (ref. 13) and polarised electrons from an accelerator^14,15) with a number of racemic or optically active substrates (usually amino acids). Conflicting results emerging from these and certain related experiments have been tabulated¹³. Perhaps the most striking positive report among these studies is the claim by Darge and coworkers¹³ that D,L-tryptophan (4.9 × 10⁻⁷ mol in 2 ml frozen (−25°C) aqueous solution) was 33% decomposed with a 19% relative enrichment of the D-enantiomer in the undestroyed residue by the action during 12 weeks of the β rays from 5 mCi of co-dissolved ³²P phosphate. The 33% gross decomposition was estimated from the UV absorption spectrum, and the 19% optical enrichment from the optical activity of the crude diluted reaction mixture, as compared with L-tryptophan at the same concentration (∼10⁻⁵M). The observed optical rotations used in the latter calculation were minute, −0.0036° for L-tryptophan and +0.0007° ±0.0004° (average of 16 observations) for the crude diluted reaction mixture. Because the remarkably large asymmetric effect claimed was based solely on these small observed rotations, and as the results have already been used by others in additional calculations¹⁶, we thought it desirable to attempt to duplicate the observations of Darge et al.¹³. We report here the use of a different analytical technique to estimate the enantiomeric composition of the undecomposed residual tryptophan, namely, gas chromatography (GC). The significant advantages of GC over optical rotation for determining the enantiomeric composition of small amounts of an optically active component admixed with impurities have already been described¹⁷.