more recent but just as is with a deep feeling of satisfac- I have reached the firm conclu- examined, in the main, e pomt of view only-not the ht of view, but one which, un- ves a vista insufficient to reveal tbe greatest development will I believe that the next Molecular Architecture and Biological Reactions LINUS PAULING Chairman, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, Calif. Answers to many basic problems of biology-nature of growth, mechanism of duplication of viruses and genes, action of enzymes, mechanism of physiological activity of drugs, hormones, and vita- mins, structure and action of nerve and brain tissue-may lie in knowledge of molecular structure and intermolecular reactions ****rf***** Eddington has said that the study of the physical world is a search for structure rather than a search for substance. If we ignore the philosophical implications of the words, we may say that the chemist and biologist in their study of living organisms must carry on both a search for structure and a search for substance, and that the second of these must precede the fiist. Investigators have had great success in isolating chemical substances from living organisms, and in determining the chemi- cal composition of the simpler of these sub- stances. The chemical composition is aBo' known of many substances of external origin which exert physiological activity on living organisms. We may consider this work of isolation and identification of active chemical substances as the search for substance in biology. The Search for Structure The search for `structure has also made great progress. From the one side biolo- gists have, by visual observation with the microscope, made thorough studies of the apparent structure of aggregates of cells, of cells themselves, and of certain const,itu- ents of cells, such as chromosomes. This visual observation has provided informa- tion about structures in size extending down to lo-' cm., 10,000 A;. Forty years ago the dark forest of the dimensional un- known stretched from this limit of the visible microscope back indefinitely into the region of smaller dimensions. In re- cent years the region from IO-' down to lo-`* cm., containing atoms and simple molecules, has been thoroughly explored by an expedit,ion outfitted with x-rays and similar tools, and the physicists are strongly pushing back into the region of the structure of atomic nuclei, bclon- 1O-12 cm. Another detailed exploration is being carried out, with the electron microscope. This has pushed the nearer boundary of the unknown back from 10-d to IO-* cm., although the major portion of this region has been only sketchily explored during the few years since the development of the electron microscope, and a very great amount of work still remains to be done. The answers to many of the basic prob- lems of biology-the nature of the process of growth, the mechanism of duplication of viruses, genes, and cells, the basis for the highly specific interactions of these structural constituents, the mode of action of enzymes, the mechanism of physiologi- cal activity of drugs, hormones, vitamins, and other chemical substances, the struc- ture and action of nerve and brain tissue- the answers to all these problems are hiding in the remaining unknown region of the dimensional forest, mostly in the strip between 10 and 100 A., lo-' and 10-e cm, ; and it is only by penetrating into this region that we can track them down. There are many ways of investigating this region-by x-rays, ultracentrifuges, light-scattering techniques, the study of chemical equilibria, t.he techniques of degradation, isolation, identification, and synthesis used by the organic chemist, serological methods, chemical genetics, the use of both radioactive and nonradioactive tracers, the use of electron microscopes of improved resolving power-but no one method is good enough to solve the prob- lem, and all these methods must be applied as effectively as possible if the problem is to be solved. At the present time n-e know in com- plete detail the atomic structure of many simple molecules, including a few amino acids; but we do not know in detail how the amino acids are combined to form pio- teins. We do not know, except very roughly, even the shapes of such important molecules as serum proteins, enzymes, genes, the substances which make up protoplasm-and if we are to obtain a thorough understanding of the structure of living organisms detailed information 1375 aoout the atomic arrangement of these substances must be obtained. Let us imagine ourselves increased in size by the linear factor 250,000,000-the commonly used factor in molecular models, which makes 1 A., lo-* cm., become ap- proximately 1 inch, atoms on this scale being 2 or 3 inches in diameter. With this magnification we would become about equal in height to the distance from the earth to the moon. Let us consider our- selves examining the earth, which would appear to us to be about the size of a bil- liard ball; aud let us concentrate our at- tention on a small organism on the surface of the earth-Sew 1-ork City-which would appear as a spot about 0.01 inch in diameter, barely visible to the naked eye, and showing itself to be living by slow changes in shape and size. To obtain a better view of this organism we could use a microscope, the resolving power of which would be about 1,000 feet; we could distinguish Cent,ral Park, the rivers, and such aggregates of skyscrapers as Rockefeller Center, but the individual skyscrapers xvould not be clearly defined. By "chemical" methods we would know that, running through the veins and arter- ies of this organism, there were substances such as street cars, busses, automobiles, ships, and people; and we might, by the use of membranes of known pore size or by some similar method, obtain the molec- ular weight of these. In addition, we would have obtained, through the applica- tion of a strange method of experimental investigation, the diffraction of x-rays and electron waves complete information about the structhre of objects smaller than about 1 foot in diameter, such as a storage battery, a small electric mot,or, a piece of cable, a small gear wheel, a bolt or rivet. The use of the electron microscope, with resolving power about 10 feet, would give us very much additional information. We would know exactly-that is, to within 10 feet-the shape of the Empire State Build- ing, though we might not be sure about the separate smaller rooms into which it is divided, and we could not obtain by the electron microscope information about the elevators and the machinery for operating them, t,he steel girders of which the build- ing is constructed, and other structural features of similar size. We would be able to see, with the electron microscope, an automobile only as a particle, barely dis- cernible, and roughly spherical in shape, and the human beings in the city would not be visible. We could get complete in- formation about a storage battery, a ring gear, a brake pedal-but not about the automobile built up of these and many other parts; and it is clear that to obtain an understanding of the structure of this city we would still need to find a method of exploring objects in the range 1 to 10 feet. Our hope for achieving precise knowl- edge about biological structures and reac- tions is based largely on the electron micro- scope and on diffraction methods. The dif- 1376 fraction studies of simple molecules have been carried out in sufficient number to permit the formulation of generalizations about atomic radii, bond angles, and other features of molecular configuration; it is still very important that the exact struc- ture be determined of vitamins, bacteria static agents, and other physiologically active substances-the complete crystal structure determination of the rubidium salt of penicillin so ably made by Dorothy Crowfoot and Barbara Rogers-Low (6) has provided not only decisive information about the chemical formula of the sub- st,ance but also the structural basis for later consideration of the detailed mecha- nism of its bacteriostatic activity. Structure of Protein The most important of all structural problems is the problem of the structure of proteins: until this problem is solved all discussions of the exact molecular basis of biological reactions remain in some de- gree speculative. The polypeptide-chain structure of proteins proposed by Fischer is now generally accepted, and there is little doubt t.hat the picture of folded chains held by hydrogen bonds, van der Waals forces, and related weak interac- tions in more or less well-defined configura- tions, as discussed eleven years ago by Mirsky and me (9) is essentially correct. But this whole picture remains very vague-for only a few proteins (such as &lactoglobulin (9)) do we have nearly complete knowledge of the numbers of residues of the different amino acids in the molecule, and for no protein does there exist more than fragmentary information either about the sequence of the different residues in the polypeptide chain or about the way in which the chain is folded. Only for fibrous proteins in the completely ex- tended state do we have knowledge (still very rough) of the configuration and rela- tive orientation of the polypeptide chains (as originally determined by .+&bury), and this knowledge applies only to the back- bone of the chains and not to the side groups. There is urgent need for complete and accurate structure determinations of proteins and related substances. So far these determinations have been' reported for only four such substances (5)-two amino acids and two simple polypeptides- all made in our Pasadena laboratories; and it is my hope that, now that the war is over, precise information will rapidly accrue, including ultimately detailed struc- tures of fibrous proteins, respiratory pig- ments, antibodies, enzymes, reticular proteins of protoplasm, and others. Im>ortance of Shupe Despite the lack of detailed knowledge of the structure of proteins, there is now very strong evidence that the specificity of the physiological activity of substances is de- CHEMICAL termined by t cules, rather than p chemical properties, an shape find expression 1 extent to which certair deten two molecules (at lea: usually a protein) cal n 1 juxtaposition-that is, these regions of the complementary in stru nation of specificity in Lerrn key" complementariness is Ehrlich, who expressed it ofte, such a.~ "only such subs anchored at a particular pa: ism which fit into the molecr . . ent combmation as a piec, In recent years the concept of compL mentariness of surface structure cf Bn, and antibody was empha.sjzed h * yw B and Haurowitz (4), 1\hdd (lo), and .k ander (I), and then was strongly ,. by me (11) in the course of ,,yf2b understand and interpret 5erolo~ca, pk nomena in terms of molecular stmetrr, and molecular interactions. S ;;viyll;r~Ty$a; me lg~ (Dan H. ch,pba , Carol Ike&, L, k Pence, G. G. Wright, S. 11. &Tingle, D,& Brown, J. H. Bryden, .I. L. &a% L. A. R. Hall, Jfiyoshi Ikswa, FrYI Lanni, J. T. llaynard, and A. B. parer) and I have gathered a great amcuntti experimental evidence about mcie, antibody interaction (IL), which ant oo$ supports the general thesis that seml& specificity is the consequence of stmcm complementariness, but provides info- tion about extent of complementarin~ It has been verified that the cl- of fit of an antibody molecule to its ho& ogous haptenic group is to within he& than 1 .&.-that a methyl group (van b Waals radius 2.0 A.) can replace a colon, atom (radius 1.8 A.) in a haptenic m with little interference with its combiplr tion with antibody (as was first shown t Landsteiner), but that interfemna i caused by replacing a hydrogen rta (radius 1.2 h.) by a methyl grouP. `& complementariness in structure X&I m spect to proton-donating and P* accepting hydrogen bond-forming m has been found to be very impomti determining the strength of attraeu~d antibody and haptenic group; and g complementary electrical charge in * body homologous to the p-azoPhW'* methylammonium group has been w I to be within about 2 k of the possible distance from the charg site sign in the haptenic group. amount of quantitative data W been gathered for scores of diflei'sn tens and antigens and successf& terms of molecular str AND ENGINEERING NE atoms in the complex. The one emical phenomenon with high is closely analogous in both its its structural basis to biological y: this phenomenon is crystal- There can be grown from a solu- taining molecules of hundreds of species, crystals of one substance re essentially pure. The reason rest specificity of the phenomenon allization is that a crystal from oe molecule has been removed, is ;ely complementary in structure to ecule, and molecules of other kinds 1 general fit into the cavity in the or are attracted to the cavity less than a molecule of the subst.ance My if the foreign molecule is similar in size and shape and the and nature of active (hydrogen ming) groups to the molecule it is ctural features (such as replace- fa chlorine atom by a methyl group) tendency to serological cross reac- ks of Biokagical Specificity Y isolated examples of biological `ity and biological similarity deter- by molecular size and shape and the d nature of intermolecular forces be mentioned, such as the similarity Ysiological (ant,ipyretic-antineural- :tivity of 4isopropylantipyrine and !thYlaminoantipyrene (pyramidon), is clearly the result of the similarity and shape of the isopropyl group le dimethylamino group. I shall, er, discuss in detail only the speci- )f enzymatic reactions. n the standpoint of molecular Ire and the quantum mechanical 0f chemical reaction, the only rea- e Picture of the catalytic act,iviLy of w is that which involves an active 0f the surface of the enzyme which !`Y complementary in structure not ! substrate molecule itself, in its ' configuration, but rather to the ltf! molecule in a strained configura- cOrresponding to the "activat.ed !X" for the reaction catalyzed by "Yme: the substrate molecule is a to the enzyme, and caused by ">@s of attraction to assume the rte which favors the chemical mt is, the activation energy of but2 txuy uti ld" ;he reaction to proceed at an appreciably greater rate than it would in the absence of the enzyme. This is, I believe, the picture of enzyme activity which is usually accepted. Experimental data have not been gath- ered which permit the induction of so pre- cise a representation of the structure and configuration of the active region of any enzyme as for the antibodies disCussed above, but there do exist some data which support the general concept. If the en- zyme were completely complementary in structure t.o the substrate, then no other molecule would be expected to compete successfully with the substrate in combin- ing with t,he enzyme, which in this respect would be similar in behavior to antibodies; but an enzyme complementary to a strained substrate molecule would attract more strongly to itself a molecule resem- bling the strained substrate molecule than it would the substrate molecule. Ex- amples of this behavior have been found: the hydrolysis of benzoyl-Gtyrosylglycine amide by either chymotrypsin or papain was found by Bergmann and Fruton (2) to be practically completely inhibited by an equal amount of benzoyld-tyrosylglycine amide. This suggests that the strained configuration of the l-isomer during the enzymatic hydrolysis is somewhat similar to the normal configuration of the d-isomer. More extensive quantitative studies of inhibition of enzyme activity might well provide very interesting information about the configuration of the enzyme molecules. Carl Kernann and I have studies of this kind under way. It is highly probable that many chemo- therapeutic agents exercise their activity by acting as inhibitors to an enzymatic re- action through competition with an essen- tial metabolite of similar structure. It was shown by Woods (16) in 1940 that t.he bacteriostatic action of sulfanilamide re- sults from an inhibitory competition with p-aminobeneoic acid, and can be overcome by increasing the concentration of the lat- ter substance. The metabolite and its in- hibitor are closely related in molecular shape, differing in the replacement of a carboxyl group by a sulfonamide group. Other pairs in which a carboxyl group is replaced by a sulfonic acid or sulfonamide group are nicotinic acid and pyridine-3- sulfonic acid or its amide (7), pantothenic acid and pantoyltaurine (14) and the a- aminocarboxylic acids and the correspond- ing a-aminosulfonic acids (8). An interesting case of inhibition is that of thiamine by pyrithiamine (IS), `the corresponding substance with the 6mem- bered pyridine ring in place of the 5-mem- bered thiazole ring. The effective competi- tion of pyrithiamine with thiamine for combination with the enzyme or other macromolecule involved might well have been predicted from the known cross reactivity of aromatic &membered rings containing sulfur and 6membered rings not containing sulfur, as is strikingly shown by the formation of solid solutions Y, UME 94, NO. 10 . . . s MAY 25, 1946 by thiophene and benzene. An analogous situation has been reported (16) by D. 6. Tarbell of the University of Roehester. He has found that any substitution in the benzenoid ring of Zmethylnaphthoquinone destroys its vitamin II activity, but that the substance with a sulfur atom in place of -CH=CH- in the benzenoid ring retains this activity. These facts indicate that in the process of exerting vitamin Ii activity the benzenoid end of the molecule must fit into a pocket carefully tailored to it; that the other end is not so surrounded is shown by the ret.ention of activity on changing the alkyl group in the 2-position. On the other hand, the failure of pyrithi- amine to replace thiamine &s a metabolite indicat.es that the sulfur atom of the thiazole ring in thiamine not only is ef- fective in binding the molecule into its seat of action but also takes part in some way in the subsequent chemical reactions in- volved in the metabolic process. Many Sciences Cooperate The complete inderstanding of physio- logical activity will require consideration not only of molecular structure and weak intermolecular forces, but also of the chemical reactivity of the substances and of such other properties as solubility in different, phases and degree of ionization, as well a.5 of those properties of living organisms which may long defy simplifica- tion to chemical description; the impor- tance of the problem for practical medicine as well as for fundamental biology is SO great as to justify the attention and effort of many workers, in various fields of sci- ence, through whose cooperative effort the solution will some day be found. Literature Cited (1) Alexander. J., Proloplasma. 14, 296 (1931). (2) Bergmann, M., and Fruton. J. S., J. Bid. Chem.. 138, 124, 321 (1941). (3) Brand, E., Said& L. J.. Goldwater. W. H., Ksssell. B.. and Ryan. F. H.. J. Am. Chem. Sm.. 67. 1524 (1945). (4) Breinl, F., and Haurowitz. F., 2. physiol. Chem.. 192, 45 (1930). (5) Corey, R. B., Chem. Reo.. 26,227 (1940). (6) Crowfoot, D., and Rogers-Low B., mentioned in Science, 102, 627 (1946). (7) McIlwain, H., &it. J. EzplZ. Path.. 21, 136 (1940). (8) McIlwain. H., J. Chem. Sot., 1941, 75; Ed. J. Ezpfl. Path.. 22, 148 (1941). (9) Mirsky, A. E.. and Pauling. L.. PVJC. Nat. Acad. Sci.. 22, 439 (1936). (10) Mudd, Stuart, J. Imtn~cnol., 23, 423 (1932). (11) Paulinn. L.. J. Am. Chem. SOC.. 62, 2643 (1940). (12) Pauling, L., and collaborators. J. Am. Chem. Sot., 68, 230 (1946 ), and earlier papM%. (13) Robbins, IV, J., Proc. .%-at. Acad. Sci, 27, 419 (19-11 j ; Wooller. D. W., and White, A. G. C.. J. I?.&. Med., 78, 489 (1943). (14) Snell, E. E., J. Biol. Chem.. 139, 975. 141,121 (19X). (15) Tarbell, D. S., private communication to author. (16) Woods, D. D.. &it. J. Ezptl. Path.. 21. 74 (1940). 1377