Chirality of biomolecules

The origin of chirality of biomolecules may have involved the potentially achiral nucleobase cytosine. Spontaneously formed chiral crystals of achiral cytosine by mechanical stirring might have been responsible for the origin of chiral biomolecules with subsequent amplification of autocatalysis. 

The second report from La Jolla attempts to cast some light on the question of the homochirality of biomolecules (see Sect. 9.4). Put simply, the question is why only one of the two possible chiral forms is always found in some important classes of biomolecules. 

Another attempt to explain the homochirality of biomolecules is based on autocatalysis. The great advantage of asymmetric catalysis is that the catalyst and the chiral product are identical and thus do not need to be separated (Buschmann et al., 2000). The racemic mixture must have been affected by a weak perturbation in order that autocatalysis, which acts as an amplifier of enantioselectivity, could have led to only one of the two enantiomeric forms. This perturbation could have been due to the slight energy difference of the enantiomers referred to above, or to statistical fluctuations. 

Another hypothesis on homochirality involves interaction of biomolecules with minerals, either at rock surfaces or at the sea bottom thus, adsorption processes of biomolecules at chiral mineral surfaces have been studied. Klabunovskii and Thiemann (2000) used a large selection of analytical data, provided by other authors, to study whether natural, optically active quartz could have played a role in the emergence of optical activity on the primeval Earth. Some researchers consider it possible that enantioselective adsorption by one of the quartz species (L or D) could have led to the homochirality of biomolecules. Asymmetric adsorption at enantiomor-phic quartz crystals has been detected L-quartz preferentially adsorbs L-alanine. Asymmetrical hydrogenation using d- or L-quartz as active catalysts is also possible. However, if the information in a large number of publications is averaged out, as Klabunovskii and Thiemann could show, there is no clear preference in nature for one of the two enantiomorphic quartz structures. It is possible that rhomobohedral... 

It should be possible to use the special properties of chiral structures for particular separation problems. According to Belinski and Tencer, one possible way in which nature solved the ribose problem could have involved an enantioselective and diastereoselective purification process acting on a mixture of biomolecules, which left ribose as the only molecule available for further reactions. The authors propose a theoretical mechanism in which a type of chromatographic process occurs at chiral mineral surfaces. This paper is likely to stimulate new experiments as well as the quest for as yet unknown surfaces which can separate racemic carbohydrate mixtures. The question arises, however, as to whether there were minerals present on the young Earth which are now unknown, as they no longer exist on the Earth of today (Belinski and Tencer, 2007). 

Laboratory data from two groups (see Sect. 3.2.4) indicate that chiral amino acid structures can be formed in simulations of the conditions present in interstellar space. The experimental results support the assumption that important asymmetrical reactions could have taken place on interstellar ice particles irradiated with circularly polarised UV light. The question as to whether such material was ever transported to the young Earth remains open. But the Rosetta mission may provide important answers on the problem of asymmetric syntheses of biomolecules under cosmic conditions (Meierhenrich and Thiemann, 2004). 

ASYMMETRIC AUTOCATALYSIS WITH AMPLIFICATION OF CHIRALITY AND ORIGIN OF CHIRAL HOMOGENEITY OF BIOMOLECULES... 

Of all asymmetrical forms of matter, chirality has always exerted a particular fascination. Pasteur s bewilderment over the optical asymmetry of biomolecules, which he considered to be one of the basic aspects of life, is well known. The observation of the morphological asymmetry of adult organisms has introduced into human thinking the notions of right and left, which subsequently turned out to be deeply rooted in a variety of other natural phenomena ranging from elementary particles to astrophysics (see, e.g., Gardner43). 

The most common form of TOT clathrate crystallises as discrete C2 symmetric cavities in the chiral space group 1, implying that the (—)-(M) and (+)-(/") forms separate spontaneously as crystallisation occurs. This property of TOT has been used in an ambitious model experiment designed to test the theory that the parity-violating energy difference (the violation of parity or symmetry in elementary particles), with autocatalytic amplification (in the case of TOT during crystallisation) is responsible for the observed chirality of modern biomolecules. The experiment did not find any evidence to support the theory, with equal amounts of each enantiomeric crystal being isolated.26... 

First measurements for identifying optical activity in Raman scattering were undertaken soon after the Raman effect itself was discovered but proved unsuccessful [1,2]. The fact that the measurement of solutions of biomolecules is at present the most important application of ROA is remarkable in the face of the experimental difficulties which haunted these early attempts to observe it, even for pure, chiral liquids, the most favorable experimental situation. 

The increased stability and sensitivity of modern instrumentation for the measurement of CPL has opened the door for many future applications of this technique to probe local chiral structure. The improved stability of the instrumentation allows for much longer measurement times, and, consequently, weaker emitters may be studied. Additionally, the ability to work at very low concentrations of sample is an obviously important characteristic in studies of biomolecules that are only available in small quantities. With the availability of high quality strain-free objective lenses, it should even be possible to detect circular... 

In traditional analytical chemistry the determination of enantiomeric purity is sometimes carried out by capillary electrophoresis (CE) in which the electrolyte contains chiral selectors such as cyclodextrin (CD) derivatives [54], Unfortunately the conventional form of this analytical technique allows only a few dozen ee determinations per day. However, as a consequence of the analytical demands arising from the Human Genome Project, CE has been revolutionized in recent years so that efficient techniques for instrumental miniaturization are now available, making ultra-high-through-put analysis of biomolecules possible for the first time [55]. Two different approaches have emerged, namely capillary array electrophoresis (CAE) [55a - e] and CE on microchips (also called CAE on chips) [55f - m[. Both techniques can be used to carry out... 

Chemical Ecology an Overview Chiral Separations of Biomolecules... 

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