Search PubMed⌕ Search

Biomedical subjects

W A Bonner

Publications and source records attributed to W A Bonner.

At least 37 records · Page 2Linked to original sources
PubMed1981

The radiolysis and radioracemization of amino acids on silica surfaces.

L-Leucine, deposited on both 1-quartz powder and on a commercial amorphous silica preparation (Syloid 63), has been subjected to irradiation in a 60Co gamma-ray source, and the ensuing radiolysis and radioracemization have been determined gas chromatographically. The radiolysis and radioracemization observed for leucine on 1-quartz were rather similar to those noted for a crystalline L-leucine control. L-Leucine on Syloid 63, however, was vastly more susceptible to radiolysis as compared to the L-leucine control, and radioracemization was also markedly enhanced - each increasing with larger radiation dosage. L-Isovaline showed a similar, but diminished, enhancement of radiolysis sensitivity when adsorbed on the Syloid surface, but underwent no radioracemization whatsoever. The divergent results of the control and quartz-leucine irradiations versus the Syloid-leucine and Syloid-isovaline irradiations are interpreted qualitatively in terms of the surface area parameters of the two silica adsorbents and the amino acid adsorbates.

Amino Acids↗
PubMed1981

A model for the enantiomeric enrichment of polypeptides on the primitive earth.

A mixture of D- and L-leucine N-Carboxyanhydride (NCA) having an enantiomeric composition of 65.6% L- and 34.4% D-isomer (i.e. 31.2% enantiomeric excess (e.e)) was polymerized to the extent of 52% with sodium methoxide initiator to yield a polyleucine product the enantiomeric composition of which was 72.7% L- and 27.3% D-leucine (45.4% e.e.). This polymer was in turn partially hydrolyzed by acid, whereupon the unhydrolyzed polyleucine residue was found to have an enantiomeric composition of 77.5% L- and 22.5% D-leucine (55.0% e.e.). Thus the e.e. increase in the partial polymerization step (14.2%) and the partial hydrolysis step (9.6%) combined to total 23.8% for the overall polymerization-hydrolysis sequence. On the basis of these model experiments it is proposed that repetitive partial polymerization hydrolysis reactions, driven by environmental dry-wet cycles, might have been operative on the primitive Earth to engender the abiotic evolution of optically enriched polypeptides.

Anhydrides↗
PubMed1981

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.

Chromatography, Gas↗
PubMed1980

Experiments on the amplification of optical activity.

By way of investigating possible mechanisms for the abiotic amplification of small enantiomeric excesses (e.e.'s) in almost racemic mixtures of amino acid enantiomers, we have undertaken a quantitative study of the base-initiated partial polymerization of leucine and valine N-carboxy-anhydride (NCA) enantiomer mixtures containing known excesses of both the R- and S-forms. Polymerization to the extent of ca. 50% of leucine NCA having an 8-70% e.e. of either the R- or S-enantiomer led to an e.e. enhancement in the polymer, which contained a 12-84% e.e. of that enantiomer which predominated in the original monomer NCA. A corresponding decrease in the e.e. of the initially predominant enantiomer was noted in the unpolymerized residue from each reaction. Polymerization to the extent of 25-50% of mixtures of valine NCA enantiomers containing a 12-13% e.e. of either R- or S-isomer led to polymers showing a 7-8% decrease in the e.e. of the initially predominant enantiomer, and to an increase of its e.e. in the unpolymerized residue. these divergent results, the latter of which is quite novel, are compared with earlier qualitative results in the literature and are discussed briefly from the viewpoint of both mechanism and the amplification of optical activity.

Amino Acids↗
PubMed1980

The radiolysis of tryptophan and leucine with 32P beta-radiation.

We have extended earlier experiments on the radiolysis of DL-tryptophan using 32 p beta-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 with 32 P beta's at -196 degrees, 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.

Dose-Response Relationship, Radiation↗
PubMed1979

The radioracemization of amino acids by ionizing radiation: geochemical and cosmochemical implications.

A number of optically active amino acids, both in the solid state and as sodium or hydrochloride salts in aqueous solution, have been exposed to ionizing radiation from a 3000 Ci60Co gamma-ray source to see if radioracemization might accompany their well-known radiolysis. gamma-Ray doses causing 55--68% radiolysis of solid amino acids typically engendered 2--5% racemization, while aqueous solutions of the sodium salts of amino acids which underwent 53--66% radiolysis showed 5--11% racemization. Amino acid hydrochloride salts in aqueous solution, on the other hand, showed little or no radioracemization accompanying their radiolysis. Both radiolysis and radioracemization were roughly proportional to gamma-ray dose in the range studied (1--36 x 10(6) rads). Mechanisms for the radioracemization of amino acids in the solid state and as aqueous sodium salts are discussed, and the absence of radioracemization for aqueous hydrochloride salts is rationalized. Isovaline, a non-protein amino acid which has been isolated from the Murchison meteorite, contains no alpha-hydrogen atom and is therefore incapable of racemization via the chemical mechanisms by which ordinary amino acids racemize. Nevertheless, isovaline suffers radioracemization in the solid state to an extent comparable to that shown by ordinary amino acids, as do its sodium and hydrochloride salts in the solid state. The sodium salt of isovaline in aqueous solution, however, fails to racemize during its radiolysis. Several implications of the newly described phenomenon of radiomization are pointed out for the fields of geochemistry and cosmochemistry.

Amino Acids↗
PubMed1978

Radiolysis, racemization and the origin of molecular asymmetry in the biosphere.

An investigation has been undertaken to determine whether ionizing radiation might engender racemization of optically active amino acids, along with their usual radiolysis. As prototypes, crystalline D- and L-leucine, as well as aqueous solutions of their sodium salts were exposed to the radiation from a 3000 Ci 60Co gamma-ray source. Gamma-ray doses which caused about 68% radiolysis of solid leucine left a residue which was about 5% racemized, while racemization proved even greater at lower doses for the dissolved sodium salts. In aqueous solution both percent degradation and percent racemization of the sodium salts were proportional to gamma-ray dosage within the range employed (1--27 x 10(6) rads). Implications of these observations for the origin of molecular asymmetry by the beta-decay parity violation mechanism are discussed.

Gamma Rays↗
PubMed1977

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 of 14C in pre-biotic molecules, offers a plausible hypothesis for the origin of biomolecular handedness.

Carbon Radioisotopes↗
PubMed1975

On the origin of molecular "handedness" in living systems.

Elementary particle effects (beta-decay) provide at best only a weakly handed radiation in the biologically effective energy ranges. Global magnetic effects coupled to sunlight are randomized by paleomagnetic reversals. Hence a persistent terrestrial handed bias at possible local biopoetic sites offers a more promising explanation for the origin of the "handedness" of the molecules found among living systems on earth. Magnetite in lava flows maintains a handed bias for surface catalysis through many magnetic reversals. Magnetite contaminated with sulfur has already been proposed by Granick as a biopoetic site because it provides a weak source of chemical energy derived by photochemical conversion. Indirect evidence for this hypothesis has been provided by the molecular structure of ferredoxin - a single strand of the 14 primordial amino acids wrapped around an FeS core. Lava flows have been suggested as biopoetic sites by Fox, since their temperature and chemical composition might allow for the rapid synthesis of prebiotic compounds at the surface of the primitive earth. The additional fact that magnetite in lave flows also provides a persistent handed site for surface catalysis offers a further argument for the experimental investigation of this specific biopoetic environment.

Biological Evolution↗
PubMed1975

Asymmetric adsorption by quartz: a model for the prebiotic origin of optical activity.

One mechanism previously proposed for the abiotic accumulation of molecules of one chirality in nature is asymmetric adsorption on the chiral surfaces of optically active quartz crystals. Earlier literature in this field is reviewed, with the conclusion that previous investigations of this phenomenon, using optical rotation criteria, have afforded ambiguous results. We now have studied the adsorption of radioactive D- and L-alanine on powdered d- and l-quartz, using change in radioactivity level as a criterion for both gross and differential adsorption. d-Quartz preferentially adsorbed D-alanine from anhydrous dimethyl-formamide solution, and l-quartz L-alanine. The differential adsorption varied between 1.0 and 1.8%. The implications of these observations are discussed from the viewpoint of early chemical evolution and the origin of optically active organic compounds in nature.

Alanine↗