Is amino-acid homochirality due to asymmetric photolysis in space?

It is well known that the amino acids occurring in proteins (natural amino acids) are, with rare exceptions, exclusively of the L-configuration. Among the many scenarios put forward to explain the origin of this chiral homoge neity (i.e., homochirality), one involves the asymmetric photolysis of amin o acids present in space, triggered by circularly polarized UV radiation. T he recent observation of circularly polarized light (CPL) in the Orion OMC- 1 star-forming region has been presented as providing a strong, or even def initive, validation of this scenario. The present paper reviews the situati on and shows that it is far more complicated than usually apprehended in th e astronomical literature. It is stressed for example that one important co ndition for the asymmetric photolysis by CPL to be at the origin of the ter restrial homochirality of natural amino acids is generally overlooked, name ly, the asymmetric photolysis should favour the L-enantiomer for all the pr imordial amino acids involved in the genesis of life (i.e., biogenic amino acids). Although this condition is probably satisfied for aliphatic amino a cids, some non-aliphatic amino acids like tryptophan and proline may violat e the condition and thus invalidate the asymmetric photolysis scenario, ass uming they were among the primordial amino acids. Alternatively, if CPL pho tolysis in space is indeed the source of homochirality of amino acids, then tryptophan and proline may be crossed out from the list of biogenic amino acids. Laboratory experiments suggested in this paper could shed further li ght on the composition of the set of amino acids that were required for dev elopment of the homochirality of first life.