Darwin's left hand. Analysis of chiroptical properties of unirradiated and irradiated L- and D-amino acids

Complete text of publication follows. Although normal chemical reactions roughly produce equal mixtures of left-handed and right-handed types, life used some specialized machinery to produce only left-handed forms of amino acids. Origin of life theories must explain how nature could produce the proper mirrored building blocks. There appear, however, to be no established reasons why left-handed amino acids should be favoured in biological systems, and the possible emergence of chiral uniqueness in living processes is still an unresolved riddle. In a multitude of experiments, we have tested the sensitivity of all L- and D- amino acids against X-irradiation and UV light exposure, with special emphasis on any differences in degradation, (thermal) stability and ability to act as substrates. Among the techniques tested, the spectroscopic techniques (UV absorption, fluorescence, and in particular far-UV circular dichroism) turned out to be most effective, in addition to crystallisation experiments. In this context, a variety of experimental conditions (pH, gassing conditions etc.) were chosen. For analysing the data of aromatic and non-aromatic amino acids, respectively, appropriate precautions have to be taken. As a result of our investigations, indeed, several significant dissimilarities between different types of amino acids and different enantiomers were established. For example, among the aromatic representatives, L and D forms of tyrosines turned out to show a quite different behaviour, while among the non-aromatics, L and D enantiomers of asparagines and glutamines established distinctly different characteristics. Overall, under definite conditions, D-isomers tend to be more sensitive to radiation than their L-counterparts. If this experimentally observed radiosensitivity of L and D enantiomers is indeed the reason for amino acid homochirality and the 'handedness of life' has to be elucidated in future experiments on earth and in space.