Chiral molecules molecular origins

In most common chiral molecules, chirality arises from chiral tetravalent atoms. A conformation-independent chirality code (CICC) was developed that encodes the molecular chirality originating from a chiral tetravalent atom [42], For more generality, a conformation-dependent chirality code (CDCC) is used [43]. CDCC ti cats a molecule as a rigid set of points (atoms) linked by bonds, and it accounts for chirality generated by chirality centers, chirality axes, or chirality planes. 

Chirality at surfaces can be manifested in a number of forms including the intrinsic chirality of the surface structure and even the induction of chirality via the adsorption of achiral molecules onto achiral surfaces. The ability of STM to probe surfaces on a local scale with atomic/molecular resolution has revolutionized the understanding of these phenomena. Surfaces that are globally chiral either due to their intrinsic structure or due to the adsorption of chiral molecules have been shown by STM to establish control over the adsorption behavior of prochiral species. This could have profound consequences for the understanding of the origin of homochirality in life on Earth and in the development of new generations of heterogeneous chiral catalysts that may, finally, make a substantial impact on the pharmaceutical industry. 

The question also arises as to where the chiral molecules came from. Were the L-amino acids or the D-sugars selected on the primeval Earth, or are exuaterresuial sources responsible for the homochirality This second possibility is dealt with by hypotheses on the effect of circularly polarised light, of extraterrestrial origin, on chiral molecules in the molecular clouds from which the solar system was formed. One such hypothesis was proposed by Rubenstein et al. (1983) and developed further by others, particularly A. W. Bonner (Bonner and Rubenstein, 1987) both scientists worked at Stanford University. The authors believe that the actual radiation source was synchrotron radiation from supernovae. The excess of one enantiomeric form generated by this irradiation process would have needed to be transported to Earth by comets and meteorites, probably during the bombardment phase around 4.2-3.8 billion years ago. 

There are two different kinds of sources of molecular chirality central chirality and axial chirality (Fig. 1). Central chirality is due to the existence of chiral carbon, whereas axial chirality originates from twisted structures of molecules, between which a sufficiently high energy barrier exists, preventing the chiral conformational interconversion in ambient conditions. Surprisingly, however, the introduction of nonchiral molecules to chiral liquid crystalline environments sometimes enhances the chirality of the systems [3-5]. This means that inherently nonchiral molecules act as chiral molecules in chiral environments. This occurs in the following way. Molecules with axial chirality behave as nonchiral molecules when the potential barrier is low enough for chiral conformational interconversion. But when such... 

The measured crystal optical activity, in general, can be either of molecular origin or due to the chiral helical arrangement of chiral or achiral molecules in the crystal, or both. The two factors are difficult to separate. Kobayashi defined a chirality factor r = (pc — ps)/pc = 1 — pslpc, where pc is the rotatory power per molecule of a randomly oriented crystal aggregate derived from the gyration tensors determined by HAUP, and ps that in solution [51]. It is a measure of the 4 crystal lattice structural contribution to the optical activity and represents the severity of the crystal lattice structural contribution to the optical activity, and represents the severity of the restriction of the freedom of molecular orientation by forming a crystal lattice. Quartz is a typical example of r = 1, as it does not contain chiral molecules or ions and its optical activity vanishes in random orientation (ps = 0). 

Chiral molecules are characterized by three-dimensional handedness and can exist in two enantiomeric forms of opposite absolute configuration (AC). Most natural products and biologically active compounds are chiral and their biological and molecular functions are closely related to their chirality, that is, AC and conformation. Furthermore, many drugs derived from natural products or of purely synthetic origin are currently used in enantiopure form. Therefore, the unambiguous determination of the AC of chiral compounds is critical for the studies of natural products and biomolecular systems.1... 

From the chirality standpoint the next fundamental development occurred in 1874, when the tetrahedral carbon atom was proposed as a basis for molecular chirality by the Dutch and French chemists Jacobus Henricus van t Hoff (1852— 1911) [47, 48] and Joseph Achille LeBel (1847-1930) [49], respectively, independently and almost simultaneously. The discovery of the asymmetric carbon atom (van t Hoff s terminology) finally provided the explanation for the existence of optical isomers and for the chiral nature of the molecules of optically active substances, including many drugs. In his original 1874 pamphlet proposing the tetrahedron [47] van t Hoff listed camphor as a chiral molecule, but the structure he gave (19) was incorrect. 

In the course of the development of CSPs, a broad variety of chiral molecules (and materials) has been the subject of scrutiny with respect to chromatographic enantiomer separation capacity. The chiral molecules studied as potential SOs cover virtually the entire chemical and structural diversity space, ranging from low-molecular-weight compounds to polymers of both synthetic and biological origin. So far, the (stiU ongoing) quest for efficient SOs has resulted in the synthesis of more than 1400 CSPs [94], the properties of which are documented in an almost intractable number of dedicated scientific publications. The outcome of these efforts is manifest in an enormously rich toolbox of more than 200 commercially available CSPs offered by various speciahzed suppliers. 

Observations on the chirality of crystals made it possible for Pasteur and others to identify dissymmetry as the true origin of optical activity. It became quickly evident that the molecular chirality associated with a given compound could be directly evident in the bulk crystallography of that compound. This in turn led to observable differences in a variety of physical properties, such as the melting point and the solubility of such species. Many chiral molecules have been observed to resolve spontaneously upon crystallization, forming enantiomorphic crystals that can be physically separated. Others can only be resolved through the formation and separation of diastereomeric species. 

With this, one has obtained a significant semiquantitative statement about question 1 from the introduction, namely, to the question as to the nature of and the quantum dynamical origin of molecular chirality. The parity violation de lege is the dominant effect in the characterization of quantum dynamics of molecular chirality for all long lived (t ) > Is), isolated chiral molecules, and is much more important than the symmetry breaking de facto as described in the work of F. Hund. This importance of parity violation for the normal case of chiral molecules is perhaps surprising and provides, at least for the time being, valid theoretical answers to question 1 about the nature of molecular chirality. The experimental confirmation of the theoretical values for is not yet available, but can be expected in the near future. 

Nanofibers are also formed in water by amphiphilic ion pairs. Hue et al. showed formation of helical nanofibers and hydrogels from Gemini surfactants 7 with L-tartrate and o-tartrate counterions (Fig. 6) [41]. These chiral structures are originating from tartrate molecules, and their molecular orientation and hydrogen bonding at the membrane surface are responsible the morphology. The ion pairs of dialkyldimethylammonium (8, n = 12, 14) and guanosine 5 -monophosphate (GMP), adenosine 5 -monophosphate (AMP) were recently shown to form nano to micrometric left-handed helices in water [42],... 

The connection between the asymmetry of the physics of the universe and the production of chiral molecules may well exist, but the mechanisms for it remain tenuous. Another theory suggests that initial symmetry breaking in molecular terms may have been random, rather than deterministic. By chance, an aggregate of molecules had a small preponderance of one enantiomer over the other, and this was then amplified over time. Up to 60 amino acids have been isolated from carbonaceous meteorites, and in all of these, the S-amino acids are in slight preponderance (begging another question as to how this comes about). Were these amino acids, of extraterrestrial origin, the start of chirality... 

---