β-decay,Bremsstrahlen, and the origin of molecular chirality

Summary A brief review is presented of the Vester-Ulbricht -decay Bremsstrahlen hypothesis for the origin of optical activity, and of subsequent experiments designed to test it. Certain of our experiments along these lines, begun in 1974 and involving the irradiation of racemic and optically active amino acids in a 61.7 KCi⁹⁰Sr–⁹⁰Y Bremsstrahlen source, have now been completed and are described. After 10.89 years of irradiation with a total Bremsstrahlen dose of 2.5×10⁹ rads, crystallinedl-leucine, norleucine, and norvaline suffered 47.2, 33.6, and 27.4% radiolysis, respectively, but showed no evidence whatsoever of asymmetric degradation.d- andl-Leucine underwent about 48% radiolysis and showed 2.4–2.9% radioracemization. Other samples in solution were too severely degraded to analyze. Probable intrinsic reasons for the failure of the Vester-Ulbricht mechanism to afford asymmetric radiolysis in the present and related experiments involving -decay Bremsstrahlen are enumerated.A portion of this material was presented at the 7th International Conference on the Origins of Life, Mainz, FRG, July 10–15, 1983     

Search for effect of longitudinally polarized protons on optically active amino acid

As a test of the Vester-Ulbricht hypothesis that the specific optical activity of contemporary biology arose from a differential action of polarized nuclear particles, DL-leucine has been irradiated with protons of both positive and negative longitudinal polarization. Neither kind of protons caused by measurable decomposition difference in the two optical isomers of the amino acid.     

Search for effect of longitudinally polarized protons on optically active amino acids

As a test of the Vester-Ulbricht hypothesis that the specific optical activity of contemporary biology arose from a differential action of polarized nuclear particles, DL-leucine has been irradiated with protons of both positive and negative longitudinal polarization. Neither kind of protons caused any measurable decomposition difference in the two optical isomers of the amino acid.     

The radiolysis of tryptophan and leucine with32Pβ-radiation

Summary We have extended earlier experiments on the radiolysis of DL-tryptophan using³²P-radiation to longer reaction times, observing complete destruction of the tryptophan by secondary, non-radiolytic processes. We have also undertaken the irradiation of DL-leucine with³²P's at -196°, achieving radiolyses to the extents of ca. 20–30%, but observing no concomittant asymmetric bias. The implications of these observations are discussed with regard to the Vester-Ulbricht mechanism for the origin of optical activity.     

The radiolysis and radioracemization of poly-L-leucines.

The brief history of the discovery of radioracemization, the racemization of an optically active substance induced by ionizing radiation, is reviewed. Our early studies involving the radiolysis and radioracemization of D- and L-leucine using gamma radiation from a 111-TBq 60Co gamma-ray source are described briefly, as are later experiments involving other protein amino acids and their salts, as well as the nonprotein amino acid, isovaline. The implications of the results of such studies for the Vester-Ulbricht mechanism which proposes longitudinally polarized beta radiation as the origin of molecular chirality, for the cosmological question of the enantiomeric compositions of amino acids in the Murchison meteorite, and for the use of D/L ratios of amino acids for geochronological and geothermal estimates are reviewed briefly. These past radiolysis-radio- racemization studies have involved only monomeric amino acids. The present research, extending such investigations to two homochiral L-leucine polypeptides, (L-Leu)10 and (L-Leu)78, was undertaken to see if a polymer of an amino acid might be more stable to radiolysis and radioracemization than the corresponding monomer. It was found that these polypeptides were more stable to radiolysis than was the leucine monomer, but that the extents of radioracemization in all samples were comparable.     

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)×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 10⁶ times lower than direct measurements of radiolysis, they are still not small enough to allow us to rule out the V-U hypothesis.     

The radiolysis and radioracemization of amino acids on clays

L-Leucine and its hydrochloride salt have been deposited on the clay minerals kaolin and bentonite, and the amino acid/clay preparations have been irradiated in a 3000 Ci60Co gamma-ray source for radiation dosages that achieved 2-89% radiolysis of the leucine. The undecomposed leucine was thereupon recovered and both percent radiolysis and percent radioracemization were determined. Similar studies were made using solid L-leucine and its hydrochloride, and L-leucine in 0.1 M aqueous solution. It has been found that radiolysis and radio-racemization in these and the previously studied leucine systems follow pseudo-first-order rate laws, and the corresponding specific rate constants are evaluated and compared. Leucine and its hydrochloride salt proved to be the most stable to both radiolysis and radioracemization, followed by leucine and its HCl salt on kaolin, followed by leucine and its HCl salt on bentonite, with leucine (and its HCl and Na salts) in aqueous solution being least stable to both radiolysis and (except for the HCl salt) radioracemization. Implications of these observations as regards the Vester-Ulbricht mechanism for the origin of optical activity are discussed.     

The radiolysis and racemization of leucine on proton irradiation

D- and L-Leucine have been subjected to 39-55 percent radiolysis using 0-11 MeV protons, both with the proton beam passing through the sample or being absorbed by it, and with quenching the sample immediately on completion of irradiation or after a 21-day interval. Racemization was small (1.1-1.7 percent) and comparable in all cases, suggesting that radioracemization and secondary degradative effects were not important factors in our recent unsuccessful attempts to induce optical activity in DL-leucine by partial radiolysis using 0-11 MeV longitudinally polarized protons.     

The radiolysis and racemization of leucine on proton irradiation

D- and L-Leucine have been subjected to 39–55 percent radiolysis using 0–11 MeV protons, both with the proton beam passing through the sample or being absorbed by it, and with quenching the sample immediately on completion of irradiation or after a 21-day interval. Racemization was small (1.1–1.7 percent) and comparable in all cases, suggesting that radioracemization and secondary degradative effects were not important factors in our recent unsuccessful attempts to induce optical activity in DL-leucine by partial radiolysis using 0–11 MeV longitudinally polarized protons.     

Radiolysis,racemization, and the origin of optical activity

An investigation has been undertaken to determine whether ionizing radiation might engender racemization (radioracemization) of optically active amino acids, along with their well-known radiolysis. We have exposed a number of solid and dissolved optically active amino acids to the ionizing radiation from a 3000-Ci ⁶⁰Co γ-ray source for periods of time which would engender substantial, but not total radiolysis. γ-Ray doses which caused 55–68% radiolysis of solid amino acids typically engendered 2–5% racemization. Aqueous solutions of the sodium salts of amino acids which underwent 53–66% radiolysis typically showed 5–11% racemization. The corresponding hydrochloride salts in aqueous solution, however, underwent little or no racemization. In aqueous solution both percentage degradation and percentage racemization were approximately proportional to γ-ray dosage within the range employed (1–36 × 10⁶ rads). Mechanisms for the radioracemization of amino acids in the solid state and as dissolved sodium salts are proposed, and the absence of racemization for dissolved hydrochloride salts is rationalized. Implications of these observations with regard to the origin of optical activity by the Vester-Ulbricht β-decay mechanism are discussed, as are their implications regarding the use of diagenetic racemization rates of ancient amino acid samples as criteria for geochronological and geothermometric calculations.     

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 (A _Ps) is measured in several highZ optically active molecules using low energy positrons with a net helicity. We findA _Ps<3×10^–4 in selenocystine (Z=34) and thyroxine (Z=53), excluding part of the theoretically predicted range of 4×10^–3> A _Ps>2×10^–6 in these molecules. The connection between these limits and limits on asymmetric radiolysis (A _R) is made, with a new limit ofA _R>10^–9 being placed. This limit onA _R, 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.     

On the origin of biological chirality via natural beta-decay

Experimental evidence that longitudinally polarized electrons having the handedness characteristic of terrestrial beta-decay electrons preferentially remove D-leucine from a racemic mixture, coupled with the probable presence of¹⁴C in pre-biotic molecules, offers a plausible hypothesis for the origin of biomolecular handedness.     

Experiments on the origins of optical activity

Two recent reports claim that (1) aqueous L-aspartic acid polymerizes faster than D-Asplin 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 0.01M 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 0.001M 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 ratatory 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: 4.2×10⁸ 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 expermental 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.     

Low dose ionizing radiation produces too few reactive oxygen species to directly affect antioxidant concentrations in cells

It has been hypothesized that radiation-induced oxidative stress is the mechanism for a wide range of negative impacts on biota living in radioactively contaminated areas around Chernobyl. The present study tests this hypothesis mechanistically, for the first time, by modelling the impacts of radiolysis products within the cell resulting from radiations (low linear energy transfer β and γ), and dose rates appropriate to current contamination types and densities in the Chernobyl exclusion zone and at Fukushima. At 417 μGy h(-1) (illustrative of the most contaminated areas at Chernobyl), generation of radiolysis products did not significantly impact cellular concentrations of reactive oxygen species, or cellular redox potential. This study does not support the hypothesis that direct oxidizing stress is a mechanism for damage to organisms exposed to chronic radiation at dose rates typical of contaminated environments.     

Extraterrestrial Handedness: A Reply

Recent investigations of stable isotope ratios of amino acids from the Murchison meteorite have shown them to be of unambiguous extraterrestrial origin, and examinations of their enantiomeric compositions, where terrestrial contamination can be excluded, have found a consistent excess of L-enantiomers. One explanation for this observation has been the asymmetric photolysis of racemic extraterrestrial amino acids by circularly polarized light (CPL) in the synchrotron radiation from orbiting electrons around the pulsar remnants of supernovae. Mason (1997) has attempted to discredit this mechanism on the grounds that circular dichroism (CD) bands for optically active molecules alternate in sign and sum to zero over the entire spectrum, and hence enantioselective photochemical reactions cannot be induced by broad band CPL. We submit arguments disputing this conclusion and present reasons for expecting that broad band CPL synchrotron radiation would be quite capable of inducing asymmetric photolysis, particularly in aliphatic amino acids.     

In Polarized MDCK Cells Basolateral Vesicles Arise from Clathrin-γ-adaptin–coated Domains on Endosomal Tubules

Human transferrin receptors (TR) and receptors for polymeric immunoglobulins (pIgR) expressed in polarized MDCK cells maintain steady-state, asymmetric distributions on the separate basolateral and apical surfaces even though they are trafficking continuously into and across these cells. The intracellular mechanisms required to maintain these asymmetric distributions have not been located. Here we show that TR and pIgR internalize from both surfaces to a common interconnected endosome compartment that includes tubules with buds coated with clathrin lattices. These buds generate vesicles that carry TR to the basolateral border. The lattices contain γ-adaptin and are dispersed by treatment with brefeldin A (BFA). Since BFA treatment abrogates the vectorial trafficking of TR in polarized MDCK cells, we propose that the clathrin-coated domains of the endosome tubules contain the polarized sorting mechanism responsible for their preferential basolateral distribution.     

Microtubule reorganization accompanying preprophase band formation in guard mother cells ofAvena sativa L.

Summary In order to study developmental changes in microtubule organization attending the formation of a longitudinally oriented preprophase band, the guard mother cells ofAvena were examined using a new procedure for anti-tubulin immunocytochemistry on large epidermal segments. We found that the interphase band (IMB) of transverse cortical microtubules present in these cells following asymmetric division is replaced after subsidiary cell formation by mesh-like to radial microtubules that extend throughout the cytoplasm. Many of the Mts are also grouped in bundles. Gradually, this intermediate array is succeeded by longitudinal elements of the PPB. Thus, preprophase band formation is accompanied by a 90° shift in Mt orientation, with a radial arrangement serving as an intermediate stage. The micrographs are most consistent with the rearrangement of intact Mts, although changes in Mt assembly are possible as well. The role of the IMB in guard mother cells is also discussed.Abbreviations GMC guard mother cell - IMB interphase microtubule band - Mt microtubule - PPB preprophase band     

Caveolin-1 polarization in migrating endothelial cells is directed by substrate topology not chemoattractant gradient

Polarization is a hallmark of migrating cells, and an asymmetric distribution of proteins is essential to the migration process. Caveolin-1 is highly polarized in migrating endothelial cells (EC). Several studies have shown caveolin-1 accumulation in the front of migrating EC while others report its accumulation in the EC rear. In this paper we address these conflicting results on polarized localization of caveolin-1. We find evidence for the hypothesis that different modes of locomotion lead to differences in protein polarization. In particular, we show that caveolin-1 is primarily localized in the rear of cells migrating on a planar substrate, but in the front of cells traversing a three-dimensional pore. We also show that a chemoattractant, present either as a gradient or ubiquitously in the medium, does not alter caveolin-1 localization in cells in either mode of locomotion. Thus we conclude that substrate topology, and not the presence of a chemoattractant, directs the polarization of caveolin-1 in motile ECs.