γ-Radiation Induced Polymerization of a Chiral Monomer: A New Way to Produce Chiral Amplification

Abstract

The treatment of the terpene beta(-)pinene with gamma radiation (at dose level: 150, 300 and 600 kGy) causes its polymerization into a resin and into a dimer. The yield of the resin and of the dimer appears to be linearly dependent to the radiation dose. The structure of the products was studied by FT-IR spectroscopy also in comparison to a reference beta(-)pinene resin prepared by cationic polymerization. A highly ordered structure was found in the case of the radiopolymer in comparison to the resin from cationic polymerization. Polarimetric measurements have shown astonishing enhancement in the optical activity of the radiopolymer and radiodimer in comparison to the starting optical activity of the beta(-)pinene monomer. Also DSC (differential scanning calorimetry) data supports the unexpected highly ordered structure for the beta(-)pinene radiopolymer in comparison to the resin prepared by cationic polymerization. The results have been discussed in terms of amplification of chirality caused by gamma radiation and the implications of this fact on the mechanism of chiral amplification on prebiotic molecules.

A Study on the Optically Active Polymer Poly‐β‐pinene

“…A Synthesis of Poly-b-pinene 1227 linear response has been found by other authors working under similar conditions (11). Considering all the yield data available from this and previous studies (4,6,9,10), including also the data obtained at very high radiation dose of the order of MGy, the average radiation chemical yield for the b(-)pinene polymerization in the presence of air is G(PBP) monom ¼ 6 molecules/100 eV. The value reported considers only the amount of monomer lost in the formation of the PBP polymer and does not consider the amount of monomer lost in the formation of PBP oligomer or other possible products.…”

“…In previous works (4,6,9,10), the irradiation of b(-)pinene was conducted in screw-cup closed vials where the air was not completely excluded. The irradiations were made at 50, 100, 150, 300, 600 kGy (4,6,9,10).…”

“…The irradiations were made at 50, 100, 150, 300, 600 kGy (4,6,9,10). In the present work, the b(-)pinene irradiations have been made at 1000, 2000 and 3000 kGy, again using the screw-cap vials (25 ml each) without the complete exclusion of air at a dose rate of 2.2 kGy/h.…”

“…The amount of insoluble fraction in acetonitrile was dependent on the radiation dose administered and separated out as a viscous oil from the solution and determined gravimetrically. The acetonitrile in the soluble fraction of each sample was then dissolved in 25 ml of toluene and the optical activity of this fraction was recorded previously (4,6,9,10). More details on the fractionation of the crude irradiated b(-)pinene have been reported previously (4, 6, 9, 10).…”

“…Reference b(-)pinene resin was prepared by treating the monomer with ZrCl 4 in toluene as detailed previously (4,6,9,10). The specific optical rotation of the resin in toluene (c ¼ 2) was found [a] D ¼ 24.2 + 0.6.…”