M. Hargittai, I. Hargittai - Symmetry through the Eyes of a Chemist (793765), страница 12
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Alldissymmetric figures are also chiral if dissymmetry means the absenceof usually a plane of reflection. In this sense, dissymmetry is synonymous with chirality.Pasteur was aware of the possible implications of chirality. In hiswords,Is it not necessary and sufficient to admit that atthe moment of the elaboration of the primary principles in the vegetable organism, [a dissymmetric]force is present? . .
. Do these [dissymmetric]actions, possibly placed under cosmic influences,reside in light, electricity, in magnetism, or in heat?Can they be related to the motion of the earth, orto the electric currents by which physicists explainthe terrestrial magnetic poles?Pasteur’s discovery and subsequent work on chirality was a richstarting point for many branches that grew from a common ground.682 Simple and Combined SymmetriesIt is only recently that minute differences have turned up in extremelyaccurate computational works in the energies of chiral moleculesdistinguishing them.An assembly of molecules may be achiral for one of two reasons.Either all the molecules present are achiral, or the two kinds ofenantiomorphs are present in equal amounts. Chemical reactionsbetween achiral molecules lead to achiral products.
Either all productmolecules will be achiral or the two kinds of chiral molecules willbe produced in equal amounts. Chiral crystals may sometimes beobtained from achiral solutions. When this happens, the two enantiomorphs will be obtained in (roughly) equal numbers, as wasobserved by Pasteur. Quartz crystals are an inorganic example ofchirality (Figure 2-36b). Roughly equal numbers of left-handed andright-handed crystals are obtained from the achiral silica melt.Incidentally, Pierre Curie’s teachings on symmetry are probably notso widely known as they should be, considering their fundamentaland general importance. The fact that his works on symmetry werecharacterized by extreme brevity may have contributed to this.
MarieCurie [53] and Aleksei V. Shubnikov [54] have considerably facilitated the dissemination of Curie’s teachings. Our discussion also relieson their works. A critical and fascinating discussion of Pierre Curie’ssymmetry teachings can be found in the literature [55].Pierre Curie’s above quoted statement concerning the role ofdissymmetry in “creating” a phenomenon is part of a broader formulation. It says that in every phenomenon there may be elements ofsymmetry compatible with, though not required by, its existence.What is necessary is that certain elements of symmetry shall not exist.In other words, it is the absence of certain symmetry elements whichis a necessary condition for the phenomenon to exist.Another important statement of Pierre Curie’s is that when severalphenomena are superposed in the same system, the dissymmetries areadded together. As a result, only those symmetry elements that werecommon to each phenomenon will be characteristic of the system.Finally, concerning the symmetry relationships of causes andeffects, Marie Curie formulated the following principles from PierreCurie’s teachings (Figure 2-42) [56].
(1) “When certain causesproduce certain effects, the elements of symmetry in the causes oughtto reappear in the effects produced.” (2) “When certain effects reveala certain dissymmetry, this dissymmetry should be apparent in the2.7. Chirality69Figure 2-42. Bust of Marie and Pierre Curie in Paris (photograph by the authors).causes which have given them birth.” However, (3) “The converse ofthese two statements does not hold . . . [and] the effects produced canbe more symmetrical than their causes.”2.7.2.
Vital ImportanceLiving organisms contain a large number of chiral constituents, butonly L-amino acids are present in proteins and only D-nucleotidesin nucleic acids. This happens in spite of the long-held view thatthe energy of both enantiomorphs is equal and their formation hasequal probability in an achiral environment.
However, only one of thetwo occurs in nature, and the particular enantiomorphs involved inlife processes are the same in humans, animals, plants, and microorganisms. The origin of this phenomenon is a great puzzle which,according to Prelog [57], may be regarded as a problem of “molecular theology.” Lately, very accurate computational work for relatively small systems has revealed minute differences in energy, whichthrough cooperative effects could have contributed to the dominanceof one of the two forms.
The last word has not been said about thedifferences of chiral pairs, and further development may reveal interesting new knowledge.The problem of preference has long fascinated those interested inthe molecular basis of the origin of life [58]. There are, in fact, twoquestions. One is why do all the amino acids in a protein have thesame L-configuration or why do all the components of a nucleic acid,702 Simple and Combined Symmetriesthat is, all its nucleotides, have the same D-configuration? The otherquestion, the more intriguing one, is why that particular configuration happens to be the L for the amino acids and why it happens tobe the D for nucleotides in all living organisms? This second question seems to be impossible to answer satisfactorily at the presenttime.According to Prelog [59], a possible explanation is that the creationof living matter was an extremely improbable event, which occurredonly once.
We may then suppose that if there are living formssimilar to ours on a distant planet, their molecular structures maybe the mirror image of the corresponding molecular structures onthe earth. We know of no structural reason at the molecular levelfor living organisms to prefer one type of chirality over another, andthe minute energy differences referred to above have not yet foundany reasonable interpretation. (There may be reasons at the atomicnuclear level. The violation of parity at the nuclear level has alreadybeen referred to in the Introduction). Of course, once the selection ismade, the consequences of this selection must be examined in relation to the first question.
The fact remains, however, that chirality isintimately associated with life. This means that at least dissymmetryand possibly asymmetry are basic characteristics of living matter.Stephen F. Mason has compiled a meticulous and critical review ofthis question in the concluding chapter titled “Biomolecular handedness” in his comprehensive treatise, Chemical Evolution throughthe 1980s [60]. Carefully measured optical activities of crystallinematerials dissolved in water showed appreciable influence of parityviolating energy differences in the crystallization process in somecases [61].In 1960, John B.
S. Haldane published a note in Nature [62] inwhich he returned to Pasteur’s ideas [63] in the wake of the discoveryof parity violation. Haldane is quoting Pasteur in French, but what wequote here we communicate in English translation.‡ Haldane beginswith mentioning the discovery of parity violation that has led to thenotion of the asymmetrical universe.
This was first enunciated byPasteur: “It is inescapable that dissymmetric forces must be operative during the synthesis of the first dissymmetric natural products.”‡We are grateful to Professor Alan L. Mackay (London) for the English translation.2.7. Chirality71Then Haldane continues quoting Pasteur: “What might these forcesbe? I, for my part, think that they are cosmological. The universe isdissymmetric and I am persuaded that life, as it is known to us, is adirect result of the dissymmetry of the universe or of its indirect consequences. The universe is dissymmetric.” Although Pasteur believedthat there is a sharp gap between vital and nonliving processes, heattributed the dissymmetry of living matter to the dissymmetry of thestructure of the universe and not to a vital force.Concerning the first question, Orgel [64] suggests that we comparethe DNA structure to a spiral staircase.
The regular DNA right-handeddouble helix is composed of D-nucleotides. On the other hand, if aDNA double helix were synthesized from L-nucleotides, it would beleft-handed. These two helices can be visualized as right-handed andleft-handed spiral staircases, respectively. Both structures can performuseful functions. A DNA double-helix containing both D- andL-nucleotides, however, could not form a truly helical structure at allsince its handedness would be changing. Orgel suggested consideringthe analogous spiral staircase as depicted in Figure 2-43.If each component of a complex system is replaced by its mirrorimage, the mirror image of the original system is obtained.
However,if only some components of the complex system are replaced bytheir mirror images, a chaotic system emerges. Chemical systems thatare perfect mirror images of each other behave identically, whereassystems in which only some but not all components had been replacedby their mirror images have quite different chemical properties. If,for example, a naturally occurring enzyme made up of L-amino acidssynthesizes a D-nucleotide, then the corresponding artificial enzymeobtained from D-amino acids would synthesize the L-nucleotide. Onthe other hand, a corresponding polypeptide containing both D- andL-amino acids would probably lack the enzymatic activity.
A mostcharming example is given by Lewis Carroll, through his heroine,Alice, when she wonders, “Perhaps Looking-glass milk is not goodto drink. . .” [66].It has been known for some time that the two enantiomers ofdrugs and pesticides may have vastly different responses in a livingorganism. Natural products extracted from plants and animals arepure—in that they contain only one of the two possible enantiomers—722 Simple and Combined SymmetriesFigure 2-43.
Helical staircase with changing handedness from Orgel (reproducedwith permission from Leslie L. Orgel, Ja Jolla, California) [65].while the synthesized ones are obtained in a 1:1 ratio of the twoversions. In some cases, the other member of the twin is harmless,in addition to the one exerting the beneficial action. In other cases,however, the drug molecule has an “evil twin.”A tragic example was the thalidomide case. It was known asContergan in Europe, and it is N-phthaloyl-␣-aminoglutarimide (itsmodel is shown in Figure 2-44). It was originally marketed as a sedative in the late-1950s.
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