In biochemical reactions, sugar molecules such as glucose are synthesized and used as D-sugars (D-glucose). They are chemically related to the structure of D-(+)-glyceraldehyde (see Chap. 2). Sugar molecules are optically active and rotate the plane of linearly polarized light to the right. Some exceptions are known,
7 The name a-helix is not based on the use of a-amino acids as molecular building blocks of proteins. The name a-helix is derived from the classification of X-ray diffraction diagrams of organic fibers. Linus Pauling and Robert B. Corey, biochemists at the California Institute of Technology, were the first to discover and name this helical structure.
as for example the inversion of (+)-saccharose, which decomposes by catalysis to form (+)-glucose and (-)-fructose. (+)-Saccharose exhibits a specific rotation of [a]D = +66,5°, (+)-glucose shows [a]D = +52,7°, and (-)-fructose gives [a]D = -92,4°. The chemical kinetics of this reaction can easily be followed by a polarimeter, since the optical rotation is dominated by the synthesized (-)-fructose and decreases during the ongoing reaction. This reaction is ideal for practical students training; it requires simply cane sugar and hydrochloric acid solution as well as a polarimeter (Forsterling and Kuhn 1985).
Sugars in D-configuration are also used in the nucleic acids DNA and RNA in the form of D-ribofuranose. Ribose has 80 electrons and quantum mechanical calculations have been performed in order to determine sign and value of parity non-conserving energy differences. For the two ribose confirmations that mainly occur in nature, a reduction of the D-enantiomer's energy of 10-20 a.u. was calculated, whereas the L-enantiomer was affected with a higher energy. Consequently, the D-ribose enantiomer used in ribonucleic acid was calculated to be energetically favoured compared to non-biological L-ribose (Rein 1992). In the case of this particular sugar, however, values found in the literature are less coherent. MacDermott and Tranter (1989) calculated that D-deoxyribose is indeed stabilized due to parity non-conserving energy differences by AEPNC (deoxyribose) = -1.2 • 10-20 a.u., whereas D-ribose in its preferentially adopted C3-endo solution confirmation shows a AEPNC (ribose) = + 5.8 • 10-20 a.u. value favouring non-biological L-ribose (see also MacDermott 1993). Nevertheless, the authors argued that the ultimate reason for the evolutionary selection of D-ribofuranose in life's genetic material is not yet clear. Prebiotic synthesis of ribose via the unselective formose reactions leads e.g. to many different stereoisomers and biological D-ribose might have been selected later during evolution by its parent compound, i.e., D-(+)-glyceraldehyde. Subsequent ab initio calculations were therefore performed with glyceraldehyde, the most simple sugar molecule. Glyceraldehyde was discussed to be the precursor molecule of more complex sugar structures in prebiological evolution and is thus a particularly interesting case to study. d-(+)- and L-(-)-glyceraldehyde are used as standards for the d/l-nomenclature. The energetic preference of D-(+)-glyceraldehyde by 10-20 a.u. compared to its mirror-image enantiomer was indeed confirmed by numerical calculations (Rein 1992).
Based on the weak nuclear current, D-(+)-glyceraldehyde shows an intrinsic energetic advantage such as D-deoxyribose, L-alanine, L-valine, L-serine, and the L-aspartate anion are preferred energetically as well. What makes the Yamagata-Rein hypothesis interesting for us is that these mirror image isomers are exactly the specific enantiomers used in biomolecular structures such as proteins and nucleic acids. We have just to keep in mind that their energetic preference is extremely small8 and we will have to discuss whether this determinate intrinsic bias can be amplified by suitable mechanisms.
8 In order to obtain larger parity non-conserving energy differences one may implement more heavy atoms such as phosphor (cf. the sugar-phosphate backbone of DNA), silicon, and sulphur (MacDermott 1993) in highly chiral environments. To cite some examples, larger values for parity non-conserving energy differences were calculated for the (Og)2- helix (AEPNC = + 55.4 • 10-20 a.u.) and a similar (S6)2- helix (AEPNC = + 748 • 10-20 a.u.).
Did "electroweak bioenantioselection" (MacDermott and Tranter 1989) contribute to the asymmetric origins of life? Our main problem to accept this captivating theory is that experimental evidence remains difficult to obtain. The value of the parity non-conserving energy difference of a typical chiral molecule is about 13 orders of magnitude lower, than measurable energetic differences as for example in the fine structure of atomic spectra. This is equivalent to raise the budget of national finances by 1 cent. A Minister of Finance would never consider this raise as an evolutionary advantage in economy and envisage a tax reduction (Rein 1992).
An experimental verification of AEPNC values, however, is of great importance, since competing research teams have difficulties to reproduce the calculation of the parity non-conserving energy differences for the amino acids glycine, alanine, serine, and cysteine (Cintas 2001 and references therein; Wesendrup et al. 2003). Promising experimental trials for an ultimate AEPNC-approval were undertaken and will thus be presented in the next paragraph.
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