Homochirality problem

The topical homochirality problem is presently being investigated in several research laboratories across the world. One new object of study is systems with eutectic mixtures. The addition of chiral dicarboxylic acids that co-crystallise with chiral amino acids to aqueous mixtures of d- and L-amino acids allows tuning of the eutectic composition of the amino acids in several cases, these systems yield new eutectic compositions of 98% ee or higher. Thus, solid mixed crystals with a ratio... 

In conclusion, we can say that, in spite of the important results already obtained, the homochirality problem is still awaiting a solution ... 

While a sense of chirality can be defined arbitrarily for the members of each individual heterochiral pair in such a set, so that one member within each pair is right-handed and the other left-handed, it does not follow from this that the right-handed member of one pair bears a well-defined relationship to the right-handed member of another pair. In other words, it is not, in general, possible to partition the members of the set C U C into two homochirality classes, right-handed (R) or left-handed (L). This is the homochirality problem of the title. 

The homochirality problem arises whenever there exists a pathway of continuous deformation that connects two enantiomorphous objects but does not require passage through an achiral point. Under these circumstances, it becomes meaningless to speak of such objects as right-handed or left-handed. The case of the asymmetric tetrahedron is the simplest example. 

The unlabeled triangle is the simplex in E (2-simplex) and the unlabeled tetrahedron is the simplex in (3-simplex) evidently, whether enantiomorphous -simplexes can be partitioned into homochirality classes depends on the dimension of E". Recall that an /j-simplex is a convex hull of + 1 points that do not lie in any (n - l)-dimensional subspace and that are linearly independent that is, whenever one of the points is fked, the n vectors that link it to the other n points form a basis for an n-dimensional Euclidean space An n-simplex may be visualized as an n-dimensional polytope (a geometrical figure in E" bounded by lines, planes, or hyperplanes) that has n + vertices, n n + )/2 edges, and is bounded by n + 1 (u — l)-dimensional subspaces. It has been shown that the homochirality problem for the simplex in E is shared by all -sim-... 

In this chapter the question of homochirality has also been considered according to Meir Lahav breaking of symmetry is not the problem. I do not know how many scientists would agree with him, but it is certainly true that in the laboratory chiral compounds can be obtained starting from racemic mixtures -and this by simple means, without invoking subtle effects of parity violation. Of course we do not know how homochirality really evolved in nature however, it is comforting to know that there is in principle an experimental solution to the problem. 

Such a capability of an oligonucleotide system deserves special attention in the context of the problem of the origin of biomolecular homochirality breaking molecular mirror symmetry by de-racemization is an intrinsic property of such a system whenever the constitutional complexity of the products of co-oligomerization exceeds a critical level. 

Since Pasteur s discovery this problem of the origin of homochirality in life has attracted the attention of many scientists in relation to the origin of life itself [3,5-7]. Japp expected a directive force when fife first arose [3], and various directive forces have been proposed later, such as different intensities of circularly polarized light in a primordial era, adsorption on optically active crystals, or parity breaking in the weak interaction. However, the expected degree of chiral asymmetry or the value of the enantiomeric excess (ee) turns out to be very small [6], and one needs a mechanism to amplify ee enormously to a level of homochirality. 

The author originally reviewed three important aspects of semiochemical research that has been discussed in Volume 9. They are as follows (1) determination of structure including the absolute configuration of bioactive natural products, (2) problems of biological homochirality, and (3) study of structure—activity relationships to invent mimics of bioactive small molecules so as to utilize them as pesticides or medicinals. 

Because of the nature of the priority rules [32], substrates sometimes differ in formal chirality, but nevertheless have essentially the same handedness [51,52]. Such reactions are homofacial rather than homochiral. The problem is of nomenclature interpretation for the enzymologist, not of stereospecific recognition by the enzyme. 

The solution to this problem is straightforward either chemically attach a homochiral auxiliary to the achiral reactant or cocrystallize the achiral reactant with an external homochiral auxiliary. In botlreases, the crystals formed must be non-centrosymmetric, i.e., chiral, because it is not possible for homochiral objects to pack with a center of symmetry between them. In our research, we have pioneered the use of internal, built-in chiral auxiliaries to generate chiral... 

Whereas the direct exploitation of homochiral tectons would seem to be the most obvious approach because the absence of a center of inversion is guaranteed, it in no way implies or affords any control of molecular orientation and therefore optimization of bulk polarity. Furthermore, reliance upon the use of pure enantiomers raises the substantial problem of needing to control the stereochemistry at the molecular level without yet solving the problem of controlling the supramolecular... 

Second, problems related to biological homochirality have been discussed to contemplate its origin and also to see exceptions to the homochirality principle. The presence and roles of D-amino acids in organisms as well as the presence of stereochemically heterogeneous compounds have been illustrated with examples. 

Furthermore, does an intrinsic reason exist in our world to exclusively use the L-amino acids instead of the D-series It is remarkable that the amino acids isolated from extraterrestrial organic matter from meteorites that are at least somewhat carbonaceous are slightly enriched in the enantiomers of the same L-series as those present in the amino acids of living organisms (Pizzarello, 2004). This would become a serious question only once we have discovered a few other life-forms in the universe and found them all to be based on L-amino acids. I know that others have a different point of view and consider the origin of chirality and of homochirality to be an important problem. I think it is not, in contrast with some related but simpler questions, such as that of the propagation of homochirality. 

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