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Prospects for Giotto and Halley Origin of Biomolecular Chirality

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Prospects for Giotto and Halley Origin of Biomolecular Chirality 400------------------NEWSANDVIEWS-----------N_A_ru_RE_v_oL_._31_4_4 AP_..;.;Rl;.;;.L..;;;1985"" Halley's comet spacecraft as a whole is hotter than predicted, requiring modifications to radiating surfaces. Assembly and final Prospects for Giotto and Halley testing of the television camera is running late, so installation and checking pro­ from M.K. Wallis cedures have been reorganized around it. One advantageous change to the original AT the final pre-launch preview*, scientists mission options for two days on the out­ schedule gives scientists the option of inte­ involved in the Giotto mission to Halley's ward flight through the extensive gas coma. grating fully-calibrated versions of their comet concluded that the risk to the The meeting heard that the Pathfinder experiments just before delivery of the spacecraft from dust particle impacts (see concept agreed with the Soviet Space Agen­ spacecraft. Nature 303, 659; 1983 and 305, 756; 1983; cy is going ahead; data from the Soviet The planned launch campaign is due to ESA J. 7, 15; 1983; Earth, Moon Planets VEGA spacecraft will be used to cut down commence at the beginning of May 1985 30, 3 I; 1984) is considerably less than had the error in targeting. If the two VEGAs with an expected launch date of 2 July 1985 been feared. are tracked by very long baseline radio­ on an Ariane 1 vehicle from the European Initial pessimism had centred on the interferometry, the error should be under launch site at Kourou, French Guiana. The likelihood of impacts that would cause a 100 km. Supplementary ground-based optimistic hopes of securing data on the in­ deviation of Giotto's spin axis in excess of astrometry is also to be investigated. ner coma and pictures of the nucleus from O I , when radio contact would be lost. The reserve of one month in the original a few hundred kilometres seem likely to be Although milligramme and smaller-sized schedule has now been used up, with final achieved on the basis of this review. D grains impacting the protective shield will instrument acceptance checks due in late contribute to the deviation, it is now agreed March. No significant problems have arisen M.K. Wallis is in the Department of Applied that the original pessimism was based on on subsystems or experiments during the Mathematics and Astronomy, University Col­ an over-weighting of the rather improbable system test programme, although the lege, Cardiff, PO Box 78, Cardiff CF/ IXL, UK. grains exceeding 0.1 g, which rnight anyway smash through the rear shield and destroy Chemical evolution instrumentation. Predictions of survival are based on predictions of the density and size-distri­ Origin of biomolecular chirality bution of dust particles. The probability of survival against 1° perturbations is now from Stephen Mason given as around 90 per cent on the nominal dust model for a 500 km flyby distance, THEORIES accounting for the ubiquity of of enantiomers. The discovery, in 1956, of although calculations need extending to the L-amino acids and the o-sugars in the the non-conservation of mirror-image sym­ give a secure guide for future targeting biochemistry of living organisms centre on metry in the weak nuclear interaction sup­ decisions. mechanisms for the selection of enant­ ported Pasteur's surmise, and the subse­ Much scientific effort has gone into iomers of one hand from the heterochiral quent unification with electromagnetism elaborating models of the comet's dust mixture of a racemic substrate, and for the has allowed the characterization of both the coma: rather than assuming instantaneous propagation of optically pure homochiral sign and the magnitude of the universal formation, it is now agreed that dust grains products. Surprised to learn, in 1953, that chiral electroweak interaction. The sign is can reside in the coma for weeks or even the origin of biomolecular handedness still in accord with the observed products of months, forming into anisotropic structures appeared to be problematic, Frank pro­ natural selection. The L-amino acids and purely because of the comet's motion posed a homogeneous chemical kinetic the L-polypeptides are more stable than around the Sun. Model calculations at the mechanism for an open flow-reactor their o-enantiomers. But the advantage 1 NASA-Jet Propulsion Laboratory (JPL) system • The mechanism contained no in­ ratio of the electroweak enantiomeric have been extended to encompass long­ ternal bias, and the dominance of the L­ energy difference (AE.w) to the thermal lived dust grains and are now in agreement amino acids and the o-sugars in the energy (kT) at ambient temperature is only with an alternative analysis by ESOC scien­ natural products, rather than their respec­ about 10- 17 (ref.3), too small according to tists (Fertig, J. & Schwehm, G.H. ESA tive enantiomers, seemed to be wholly a one school of thought5 to affect the out­ Bull. 38, 36; 1984). matter of chance. By extending the come of biochemical evolution. There has been concern about much mechanism and incorporating a recently Frank envisaged an open reactor system higher dust densities in jet-like regions. JPL evaluated internal bias, Kondepudi and with a continuous input of an achiral scientists (Sekanina, Z. & Larson, S.M., Nelson2 now define the particular condi­ substrate from which both enantiomers, L Astr. J. 89, 1408; 1984) have applied digital tions under which a small electroweak ad­ and o, are formed (see figure). Each of the image-enhancement techniques to vantage factor for the L-series of amino optical isomers autocatalyses its own pro­ photographs of comet Halley's 1910 ap­ acids and polypeptides3 leads to a deter­ duction and competitively inhibits propa­ parition, and infer enhanced densities, minate choice in the evolution of gation of its enantiomer. In general, the some 10-100 times the mean. It is now homochiral biomolecules from an achiral dynamic generation of a racemic product accepted, however, that these would be of or racemic substrate. is metastable and subject to a bifurcation submicron size (like cigarette smoke) and Frank's model avoided the direct catastrophe. Chance fluctuations sooner or present a desirable target rather than a intervention of external agencies, such as later trigger a kinetic switch to one of the threat to the spacecraft's integrity. Study a chiral field or panspermatic celestial homochiral reaction channels, producing of the jet phenomenon might, alternative­ seeding, and the various heterogeneous either the L-isomer or the D-enantiomer. ly, indicate where to expect the dangerous, mechanisms advocated in the grand debate Which is produced, in the absence of an slowly-moving, millimetre-sized grains. on the subject in the correspondence internal bias or an external perturbation, Originally, the mission was scheduled to columns of Nature during 1898 following depends on the sign of the chance 1 terminate one hour after closest approach, F.R. Japp's paper on "Stereochemistry fluctuation • supposing that the spacecraft is unlikely to and Vitalism"4• Ironically, that corre­ The general solution to the kinetic rela­ survive. But there was pressure from ex­ spondence ignored the 1884 conjecture of tionships of the extended mechanism is perimenters at the meeting to retain trans- Pasteur that a dissymetric force pervades shown to be a cubic equation in the enan­ tiomeric excess of the handed products, *European Space Research and Technology Centre, Holland, the physical world, extending even to the 19-20 Feburary, 1985. crystallization dish used for the separation (a/{3 = ([L) - [ol)/([L] + [ol), where [L} © 1985 Nature Publishing Group.
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