Corinne Cerf

Asymmetric photoreactions as the origin of biomolecular homochirality: A critical review

The role of asymmetric photoreactions (occurringin space or on the primitive Earth) in the origin of biomolecularhomochirality is critically reviewed. A general description of thevarious possible ways for light to interact with chiral moleculesis first presented on the basis of a series expansion of thedielectric constant: natural, magnetic and magnetochiral circulardichroism are identified with the first three terms in thisdevelopment. Natural and magnetochiral circular dichroismmay cause, through asymmetric photolysis, an enantiomeric excessin a racemic mixture of chiral molecules irradiated, respectively,by circularly polarized ultraviolet light, or by unpolarizedultraviolet light in the presence of a magnetic fieldnon-perpendicular to the light beam. Terrestrial and extraterrestrialsites matching these conditions are then critically reviewed.Finally, we stress the possibility to arrive at the homochiralityof amino acids through a path involving D-ribose during RNA worldas an alternative to the usual scenarios operating directly onamino acids.

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 homogeneity (i.e., homochirality), one involves the asymmetric photolysis of amino acids present in space, triggered by circularly polarized UV radiation. The recent observation of circularly polarized light (CPL) in the Orion OMC-1 star-forming region has been presented as providing a strong, or even definitive, validation of this scenario. The present paper reviews the situation and shows that it is far more complicated than usually apprehended in the astronomical literature. It is stressed for example that one important condition for the asymmetric photolysis by CPL to be at the origin of the terrestrial homochirality of natural amino acids is generally overlooked, namely, the asymmetric photolysis should favour the L-enantiomer for all the primordial amino acids involved in the genesis of life (i.e., biogenic amino acids). Although this condition is probably satisfied for aliphatic amino acids, some non-aliphatic amino acids like tryptophan and proline may violate the condition and thus invalidate the asymmetric photolysis scenario, assuming they were among the primordial amino acids. Alternatively, if CPL photolysis 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 light on the composition of the set of amino acids that were required for development of the homochirality of first life.

Nullification Writhe and Chirality of Alternating Links

In this paper, we show how to split the writhe of reduced projections of oriented alternating links into two parts, called the nullification writhe wx, and the remaining writhe wy, such that the sum of these quantities equals the writhe w and each quantity remains an invariant of isotopy. The chirality of oriented alternating links can be detected by a non-zero wx or wy, which constitutes an improvement compared to the detection of chirality by a non-zero w. An interesting corollary is that all oriented alternating non-split links with an even number of components are chiral, a result that also follows from properties of the Conway polynomial.

A family of brunnian links based on Edwards' construction of Venn diagrams

We construct an infinite family of brunnian links whose projections give the family of Venn diagrams for many sets constructed by Edwards.

Linear relations between writhe and minimal crossing number in Conway families of ideal knots and links

We showed earlier how to predict the writhe of any rational knot or link in its ideal geometric configuration, or equivalently the average of the 3D writhe over statistical ensembles of random configurations of a given knot or link (Cerf and Stasiak 2000 Proc. Natl Acad. Sci. USA 97 3795). There is no general relation between the minimal crossing number of a knot and the writhe of its ideal geometric configuration. However, within individual families of knots linear relations between minimal crossing number and writhe were observed (Katritch et al 1996 Nature 384 142). Here we present a method that allows us to express the writhe as a linear function of the minimal crossing number within Conway families of knots and links in their ideal configuration. The slope of the lines and the shift between any two lines with the same slope can be computed.