A NEW DEFINITION OF LIFE 35 such a channel would again be 0.A heterochiral pool of aomped meric substrate Fig.4.The Soai reaction but not homoc were (D).In that would not hay itrepes bydrogen raher tha hydroge ac Thi of THE SIMPLEST FORM OF CARBON-BASED UFE hat t可 ns say that chira his information?The answer is that nents ct.then before an en ceem he chicken-egg dilemr na is olved by assuming that thre struct onal shape age or par r iral building bloc 2 has to 3,etc epicted as the alkoxide) chiral building wha tively.Blackmond hasi the rele mean rela to the in ks n the cas min ma町y be tetra ing two mo s of homo chir zinc (hete chiral speci act more slowly or nota ner er uc The te equiv of the ained in the amind amin han the omaton itse .In an automato In the cas e of the Soai reaction.the but uD in suts in the two ho be another CH group,alike in every way.but with the uct of the Fur Mi the a type to or llular life a portant than the CONCLUSIONS Chirality DOI 10.1002/chisuch a channel would again be 0. A heterochiral pool of enantiomers would not consist of one enzyme that inter￾prets the spatial information of a particular diastereomer and not others, but of multiple analogous enzymes which would interact in different ways with a particular diastereo￾meric substrate. A more pertinent objection can be raised by asking ‘‘What if there were a pool of chiral building blocks, each enantiomerically pure, but not homochiral?’’ For example, suppose of the 19 chiral amino acids, 10 were (L) and 9 were (D). In that case we would not have multiple diaster￾eomeric forms for a given chain of amino acids, but one dia￾stereomer only for each chain. This question goes to the root of the issue of what it means to say that chiral mole￾cules ‘‘interact’’ or exchange information. What precisely is this information? The answer is that a given chiral building block defines the coordinate system. It determines ‘‘left’’/ ‘‘right,’’ ‘‘up’’/‘‘down,’’ and ‘‘in’’/‘‘out’’, or the positive and negative directions of any three-dimensional coordinate system that is preferred. For example, suppose we wish to construct an arbitrary three-dimensional shape as shown in Figure 3 (the solid lines are in the plane of the page or par￾allel to it, the wedges have their usual meaning): In the coordinate system of the drawing, molecule 2 has been attached ‘‘right’’ from molecule 1, and 3 is attached to the ‘‘bottom’’ of molecule 2, 4 ‘‘out’’ relative to 3, etc. But how is the ‘‘constructor’’ (presumably an enzyme made of chiral building blocks) to define precisely what those directions mean relative to the initial building block? It does so by reference to its own set of analogously sub￾stituted, homochiral building blocks. In the case of amino acids, the relative positions of the side chain, hydrogen, carboxyl group and amino group define in a consistent manner where is ‘‘where,’’ A heterochiral mix of such amino acids, either of low % ee or 100% ee but of mixed handedness, would create equivocation of the information concerning location. It is essential therefore to separate the spatial information contained in the amino acids from the information provided by the functional groups of the side chains. As far as spatial information is concerned, all of the amino acids of the homochiral set are identical enti￾ties. The implication of all of this is that homochirality can be thought of as the 21st ‘‘functional group’’ of the amino acid building blocks. It may be difficult at first to imagine what the equivalent of heterochirality would be for func￾tional groups, but consider a functional group such as the

CH2OH group of serine. Its ‘‘enantiomer’’ would have to be another CH2OH group, alike in every way, but with the property that it repels hydrogen rather than hydrogen bonds to it! THE SIMPLEST FORM OF CARBON-BASED LIFE If the foregoing arguments are correct, then before any efficient (in terms of molecular size) life forms may arise, a pool of homochiral building blocks must be available. The chicken-egg dilemma is resolved by assuming that the origin of homochirality and the origin of life were the same event. Further, before a homochiral set of building blocks arose, one homochiral building block was made available. A reaction that could serve as a source of a homochiral building block is exemplified by the Soai reac￾tion.12 This reaction produces, with asymmetric amplifica￾tion, pyrimidyl alcohols as in Figure 4 (the acidic workup is left out to save space so 5 is depicted as the alkoxide). The alcohol 5 is therefore replicating itself stereoselec￾tively. Blackmond has discussed the relevance of the Soai reaction to the origin of biological homochirality.13 Kinetic studies14 indicate that the catalytic entity in the reaction may be tetrameric, involving two molecules of homochiral reduced substrate and two molecules of mono-isopropyl zinc (heterochiral species react more slowly or not at all, and this is the source of the asymmetric amplification). The inclusion of two homochiral alkoxide molecules in the catalyst thus fulfills one of the requirements for self-repli￾cating automata pointed out by Von Neumann15: ‘‘When an automaton performs certain operations, they must be expected to be of a lower degree of complication than the automaton itself. In particular, if an automaton has the ability to construct another one, there must be a decrease in complication as we go from the parent to the construct.’’ In the case of the Soai reaction, the extra ‘‘complication’’ is, in part, the information content of the two homochiral ligands, which results in the production of one molecule of product of the same enantiomer. Further, the reaction kinetics appears to be analogous to the Michaelis-Menton kinetics of enzymes.16 This is only a type example, because the zinc reagent is unstable in water, but analo￾gies to or contradictions with the physical construction of cellular life are less important than the more general anal￾ogy represented by the transmission of three-dimensional information from one molecule to another. CONCLUSIONS Numerous definitions of life have been proposed in the past,17 Fig. 3. An arbitrary three-dimensional object. but the conclusions of this article are clear. First, a Fig. 4. The Soai reaction. A NEW DEFINITION OF LIFE 357 Chirality DOI 10.1002/chir