It is known that if electrons pass matter they are decelerated, due to which they emit electromagnetic radiation that is called Bremsstrahlung (indeed, this German term describes the particular kind of electromagnetic radiation also in English language). In the context of the VU process it is important to note that Bremsstrahlung produced by spin-polarized electrons is equally polarized (Goldhaber et al. 1957)!3 The helicity of the spin-polarized electron is transferred to the helicity of the Bremsstrahlung, which is nothing else than circularly polarized electromagnetic radiation. The interaction of circularly polarized electromagnetic radiation with racemic or prochiral organic molecules was - even to that time - known to induce enantiomeric excesses. 'Chiral photons' can be used for asymmetric photochemical reactions such as asymmetric photolysis and asymmetric synthesis (we will develop a deeper understanding of asymmetric photochemistry in the next chapter). Ulbricht and Vester proposed the chain of the above-mentioned consecutive processes to explain the chiral asymmetry in biomolecules (Fig. 5.3). Today, this sequence is known as the Vester-Ulbricht (VU) process.
Worldwide, an ultimate experimental verification of the Vester-Ulbricht process was searched intensively. Each of the individual steps of the consecutive Vester-Ulbricht process found independent experimental confirmation. Electrons emitted during P-decay are due to the weak nuclear interaction longitudinally polarized (Wu et al. 1957) and Bremsstrahlung emitted from P-rays is circularly polarized (Goldhaber 1957). Long before the VU hypothesis it had become apparent that asymmetric photochemistry of circularly polarized light in ultraviolet wavelengths with organic matter yields optically active products (Kuhn and Braun 1929).
2 To that time, Lee was not aware of the fact that not only parity in physics is violated but that biology's molecular symmetry is broken as well.
3 At the high-energy end of the Bremsstrahlung spectrum emitted photons are almost completely circularly polarized.
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