First image from a positron reemission microscope.
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Copper↗
Biomedical subjects
J Van House
Publications and source records attributed to J Van House.
New estimates of asymmetric decomposition of racemic mixtures by natural beta-radiation sources.
The Vester-Ulbricht hypothesis suggests that the chirality of biological molecules originates from the beta-radiolysis of prebiotic racemic mixtures. Despite the inconclusiveness of past investigations, recent calculations have shown that beta particles, because of their helicity, radiolyse L- and D-enantiomers at slightly different rates, the asymmetry, AR, being predicted to be 10(-11) (new experimental tests, give /AR/ < 2 x 10(-9)). Before this, the size of the radiolysis-induced chiral polarization, eta R (eta triple bond (nL - nD)/(nL + nD) where nL and nD are the numbers of L and D molecules present), was estimated for different values of AR; according to Keszthelyi et al., if /AR/ approximately 10(-11), /eta R/ can never exceed the chiral polarization, /eta F/, produced by statistical fluctuations, thus invalidating the V-U hypothesis. Here we re-examine the major assumptions on which these calculations were based and find that several overly restrictive conditions were imposed, which, when relaxed, allow the condition /eta R/ > /eta F/, in accordance with the V-U hypothesis.
Amino Acids↗
Limits on asymmetric orthopositronium formation in high Z optically active molecules.
The proposed connection between the parity violating handedness of beta particles in radioactive decay and the sign (L) of biological chirality (the Vester-Ulbricht [V-U] hypothesis) is experimentally tested. The theoretically predicted asymmetry in triplet positronium formation (Aps) is measured in several high Z optically active molecules using low energy positrons with a net helicity. We find Aps less than 3 X 10(-4) in selenocystine (Z = 34) and thyroxine (Z = 53), excluding part of the theoretically predicted range of 4 X 10(-3) greater than Aps greater than 2 X 10(-6) in these molecules. The connection between these limits and limits on asymmetric radiolysis (AR) is made, with a new limit of AR greater than 10(-9) being placed. This limit on AR, which is thirty times lower than a previous measurement in the amino acid leucine (Z = 6), is still not small enough to rule out the V-U hypothesis. rule out the V-U hypothesis.
Cystine↗
Beta decay and the origin of biological chirality: new experimental results.
The proposed connection between the parity-violating handedness of beta particles in radioactive decay and the sign (L) of biological chirality (the Vester-Ulbricht [V-U] hypothesis) is being investigated by measuring the theoretically predicted asymmetry in the formation of triplet positronium in amino acid enantiomers by low energy positrons under reversal of the helicity of the positrons. We find the asymmetry in leucine to be (0.8 +/- 1.0) X 10(-4), i.e. consistent with the theoretical prediction of 10(-6) to 10(-7). The apparatus is now sensitive enough to test the predicted asymmetry in optically active molecules which have heavy atoms at their chiral centers. The connection between these results and asymmetry in radiolysis by beta-decay electrons is made, and the implications of our limits for the V-U hypothesis discussed. Although the above limits are 10(6) times lower than direct measurements of radiolysis, they are still not small enough to allow us to rule out the V-U hypothesis.
Elementary Particles↗
beta Decay and the origins of biological chirality: experimental results.
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Amino Acids↗