When Parity Falls The Weak Nuclear Interaction

As we have seen, proteins (enzymes) are exclusively composed of amino acids in L-configuration. If we produce - in contrast to life's biochemical synthesis -, amino acids artificially in the laboratory, e.g., via the well-known Strecker mechanism, we will inevitably produce 50% of L-amino acids and 50% of D-amino acids. The enan-tiomers will be obtained in a racemic ratio and can be separated and quantified by enantioselective chromatography; the racemic mixture will give no signal in chirop-tical measurements such as optical rotation dispersion ORD and circular dichroism CD. This is not surprising. It is the consequence of the symmetry of natural laws, the parity of fundamental laws of physics. Universal forces (or "interactions", as physicists prefer to say) such as the electromagnetic interaction (here photons are gauge bosons), the gravitational interaction (tentatively the boson is called graviton), but also the strong nuclear interaction (gluons as bosons) are symmetric in space. Here parity is not violated. This means that any event based on these forces (interactions) performed in a mirror would show identical results. Energy, colour, and weight, and also rate constants of the formation of the products are for example identical to these parameters in their mirror-image products. In the above reaction, during the Strecker synthesis no preference is given towards the production of L-amino acids or the production of D-amino acids. The two enantiomers will originate with the same probability and therefore be obtained highly symmetrically as a racemic mixture.

But attention: it is important that the above described symmetry of universal interactions is not a general principle even if it was assumed by physicists for quite a while in the past. The so-called weak force, the 4th fundamental interaction of physics that is carried by charged massive W± and neutral Z0 bosons does violate parity! The weak nuclear force or weak interaction distinguishes undeniably between left and right. It can "feel the difference" between left and right (MacDermott 1993). The weak nuclear force is known to influence electrons inside atoms, causing them to travel helical paths that give all atoms a handedness. Due to its weakness -the weak interaction is hardly outreaching towards the exterior of atomic nuclei -it is usually marginal and does not manifest itself in everyday macroscopic events. Its most familiar effects are the P-decay of unstable atomic nuclei such as 40K, 32P, and 14C and the associated radioactivity. The word "weak" derives from the fact that the field strength is some 1013 times smaller than that of the strong nuclear force, which holds together the protons and neutrons in the nucleus of an atom. The strong

U. Meierhenrich, Amino Acids and the Asymmetry of Life. Advances in Astrobiology and Biogeophysics, © Springer-Verlag Berlin Heidelberg 2008

nuclear force is often called the binding force of the nucleus and well known to all of us from nuclear power plants applying nuclear fission and also nuclear fusion as it is prospected for the international ITER project located in Cadarache, France. In radioactive P~ -decay two particles are emitted: an electron e~ and an antineutrino ve as illustrated in Fig. 5.1 for the general P-decay and the 14C-case.

Fig. 5.1 The weak nuclear force is manifested in the radioactive P-decay: unstable nuclei having an excess of neutrons n emit electron and antineutrino via the P--decay (top). The weak force is asymmetric causing that the emitted electron is longitudinally polarized. The unstable 14C isotope serves as example of an isotope undergoing P-decay (bottom). 14C contains 6 protons and 8 neutrons. Due to p--decay, one of the neutrons becomes a proton changing the atomic number from carbon (Z = 6) to nitrogen (Z = 7) emitting - through mediation of a vectorboson W- - an electron and an antineutrino

The big question that we will ask in the frame of this chapter will be: "Is there any link between the parity non-conservation of the weak nuclear force which manifests itself in the interior of atomic nuclei and the asymmetry of molecules in living organisms? Were universal physical processes such as weak nuclear forces involved in prebiotic pathways on the origin of life? And is there thus any connection between the asymmetry of a fundamental law of physics and asymmetric properties of biological life?"

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