SOME RECENT DEVELOPMENTS IN THE CHEMISTRY OF NUCLEIC ACIDS

SOME RECENT DEVELOPMENTS IN THE CHEMISTRY OF NUCLEIC ACIDSAuthor(s): ALEXANDER TODDSource: Journal of the Royal Society of Arts, Vol. 103, No. 4961 (30TH SEPTEMBER, 1955), pp.769-781Published by: Royal Society for the Encouragement of Arts, Manufactures and CommerceStable URL: http://www.jstor.org/stable/41364759 .

Accessed: 28/06/2014 16:20

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp

.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].

.

Royal Society for the Encouragement of Arts, Manufactures and Commerce is collaborating with JSTOR todigitize, preserve and extend access to Journal of the Royal Society of Arts.

http://www.jstor.org

This content downloaded from 91.223.28.130 on Sat, 28 Jun 2014 16:20:34 PMAll use subject to JSTOR Terms and Conditions

http://www.jstor.org/action/showPublisher?publisherCode=thersa

http://www.jstor.org/stable/41364759?origin=JSTOR-pdf

http://www.jstor.org/page/info/about/policies/terms.jsp


SOME RECENT DEVELOPMENTS

IN THE

CHEMISTRY OF NUCLEIC ACIDS

The Pope Memorial Lecture by

SIR ALEXANDER TODD , M.A.y D.Sc.y F.R.S. ,

Professor of Organic Chemistry , University of Cambridge , delivered to the Society on Wednesday , i8th May , J955, with Sir Robert Robinson, О. M., D. Sc., LL.D F.R.S. , Waynflete Professor of Chemistry , Oxford University, in the Chair

the chairman : We are gathered here to-day to hear the seventh of the memorial lectures to the late Sir William Pope, which were instituted by subscriptions from his friends and admirers to be given under the auspices of the Royal Society of Arts.

Of course, Sir William Pope was a man of many parts. It is very interesting to look at the titles and the authors of the previous lectures, because they illustrate in a remarkable way his characteristic versatility. The very first of them was given by Charles Gibson and was largely on the biographical, personal side. Then Dr. Lampitt, in 1947, spoke of a matter in connexion with which Sir William Pope will always be remembered gratefully, namely, the international relations of scientists, and his work for the International Union of Chemistry. This left its mark and has certainly tended to the cementing of good international relations in science. Dr. Mann spoke of a scientific subject for which Sir William Pope was most widely known and recognized, that is, his researches in stereochemistry. Professor Read spoke of still another side of his work and character, namely, his concern with the question of cultural relations in science. This was allied to Dr. Lampitťs theme, but a different aspect of the subject. Professor Norrish spoke of the work of Lyon Playfair. The history of chemistry was of great interest to Sir William Pope and he was a great admirer of Lyon Playfair. That was certainly a very appropriate subject and, to continue this review of Sir William Pope's manifold activities, Sir Owen Wansbrough- Jones spoke of the help he gave to scientific organization and research in the services. You will remember that it was in Sir William Pope's laboratory that many of the important discoveries of chemical warfare agents were made, and he played a very great part in the organization of chemical defence.

Now to-day we have, to continue this survey, Sir Alexander Todd, Professor of Chemistry in the University of Cambridge, who followed Sir William Pope after a short interval of a few years. Sir Alexander will speak of one of the chief research topics which have been prosecuted in the Cambridge laboratories. That is a very appropriate subject for a memorial lecture, for, although its development is due entirely to the initiative of Sir Alexander Todd, it has nevertheless been facilitated by work carried out in the Chemical School at Cambridge where Sir William Pope was so justly famous.

I need not tell you that Sir Alexander Todd is a man on whom many honours have already been showered. He is the Chairman of the Advisory Committee on Scientific Research which directly advises the Cabinet on so many matters affecting scientific policy and personnel. He has received many academic honours, and I think he will value as much as any of them his selection to deliver a lecture in memory of his distinguished predecessor. If I were to tell you all I know to the advantage of

769



JOURNAL OF THE ROYAL SOCIETY OF ARTS 3°TH SEPTEMBER 1 95 5 Sir Alexander Todd, I might perhaps be in danger of provoking the remark which was made by one Welshman to another. The first was enthusiastically praising David Lloyd George, and after a little time the other one said to him: 'Certainly he is a great man, but he is not the Lord God Almighty'. Somewhat taken aback, the first Welshman replied: 'No, but, look you, he is a young man yet'. I think one of the most remarkable things about Sir Alexander Todd is the amount that he has been able to achieve in such a comparatively short period of years.

I will now call on Sir Alexander to deliver his lecture on 'Some Recent Develop- ments in the Chemistry of Nucleic Acids'. This subject is of the very greatest possible importance. It lies at the basis of life in the nucleus of the cell and Sir Alexander Todd will tell us of the latest scientific developments in it.

The following lecture , which was illustrated with lantern slides, was then delivered :

THE LECTURE

It is now 84 years since Miescher published his discovery of nuclein in the nuclei of pus cells. In the years that followed, this and similar substances known collectively as nucleic acids were found as characteristic components of all cells, and it was in due course recognized that they rank with carbohydrates and proteins as the three great groups of natural macromolecular substances involved in vital processes. Despite their obvious importance, it is remarkable that until recently the nucleic acids received but little attention from the structural chemist, and our knowledge of them lagged far behind our knowledge of proteins and carbohydrates. Now, however, all this has changed and a remarkable trans- formation has occurred during the past few years; as a result, the chemistry of the nucleic acids can fairly be said to rest now on a firm foundation and their essential structural features are established. Much remains to be done before we can hope to understand their function and their specificity, but a stage has been reached at which it is possible to go forward with some prospect of a successful outcome.

The known nucleic acids are of two types - the ribonucleic acids which yield the sugar D-ribose upon hydrolysis, and the deoxyribonucleic acids which yield 2-deoxy-D-ribose. Graded hydrolysis of nucleic acids gives first the nucleotides, then the nucleosides, and finally a mixture of purine and pyrimidine derivatives together with one or other of the sugars already mentioned. Schematically the hydrolytic breakdown of the nucleic acids can be represented thus:

Nucleic acids

V Nucleotides

I v

Nucleosides -f- phosphoric acid i v

Purines and pyrimidines + D-ribose or 2-deoxy-D-ribose

770



ЗОТН SEPTEMBER 1955 CHEMISTRY OF NUCLEIC ACIDS From ribonucleic acids four nucleosides - adenosine, guanosine, uridine

and cytidine - are obtained, and from the deoxyribonucleic acids, six -

deoxyadenosine, deoxyguanosine, deoxycytidine, thymidine, 5 -methyl- deoxy- cytidine, and 5 -hydroxymethyl-deoxy cytidine - of which the two last-named are of limited occurrence. The exceptional occurrence of other nucleosides cannot, of course, be excluded. Ever since it was realized that the nucleotides are phosphate esters of the nucleosides and that the nucleic acids can be regarded as polynucleotides, attempts were made to formulate structures for the latter» The titrimetric work of Le vene and Simms(l) led to a general concept of nucleic acid structure, in which the individual nucleoside residues were joined together by phosphodiester linkages and eliminated from consideration either ether or pyrophosphate linkages. This simple formulation has become generally accepted and, although other types of linkage have been suggested from time to time, such suggestions lack corroborative chemical evidence. Of these suggestions, the phosphotriester linkage, which might represent a branching point in a polynucleotide chain, has been most widely canvassed. As will be mentioned later, recent evidence suggests that its occurrence is most unlikely. Essentially, the nucleic acids are to be regarded as polydiesters of phosphoric acid. It is the purpose of this lecture to outline briefly the present state of our knowledge of their detailed structure, and to put before you some of the recent findings which bear upon it.

It is clear that an understanding of the details of nucleic acid structure must rest on a knowledge of the precise structure of the individual nucleosides and nucleotides and of the position as well as the nature of the internucleotidic linkage. The study of these problems has been pursued by my colleagues and myself in Cambridge during the past ten years or so, using the methods of organic chemistry, and I believe that a consideration of the development of these studies and of related biochemical work affords the simplest exposition of nucleic acid structure as we know it to-day. Our studies were simplified, at least in their earlier phases, by the pioneer work of earlier investigators, and particularly of Levene, who established the general features of nucleoside structure in a series of outstanding investigations about twenty years ago, and of Gulland, who first provided evidence for the probable location of the sugar residues in nucleosides. In the initial phase of our work we were able to extend this earlier work, and to establish in every detail the structure and stereochemical configuration of the natural ribonucleosides and to confirm our findings by total synthesis. They are in every case ß-D-ribofuranosides, the sugar being attached at N9 in the purine nucleosides and at N3 in the pyrimidine nucleosides* 2) ; typical examples are adenosine (1 ; R = OH) and cytidine (11 ; R = OH). The total synthesis of the natural deoxyribonucleosides is still outstanding but their structure as ß-2-deoxy-D-ribofuranosides with the sugar at N9 in the purine and N3' in the pyrimidine nucleosides has been rigidly established in the cases of deoxyadenosine (1; R = H) deoxycytidine (11; R = H) and thymidine(s) ; the ß -configuration of deoxy-guanosine has been inferred, though not rigidly proven.

771



JOURNAL OF THE ROYAL SOCIETY OF ARTS 30TH SEPTEMBER 1955 OH R OH R

HOHjC HOHjC

^"Y'

nh2 Ùh2 I II

Space does not permit a detailed description of the very extensive researches involved in this phase of the work but mention may be made of an interesting group of nucleoside derivatives, which were discovered during its course and which were used to determine the stereochemical configuration at the glycosidic linkage. These are the гу^/onucleosides which were first discovered during work on the 5'-/>-toluenesulphonyl derivatives of adenosine and cytidine(4) ; on heating, these ̂-toluenesulphonyl derivatives pass readily by intramolecular alkylation, into N3: 5'-cycloadenosine (hi) and O2: 5 '-ryc/ocytidine (iv) salts whose formation is only possible if the original nucleosides are ß-glycosides.

ш 9м ОН он он

кгя

ш

с;н2 ° 0

О nh2 Инг I Ш

More recent work has shown that ryc/onucleoside derivatives are readily obtained from all the pyrimidine nucleosides and that not only O2: 5'- but also O2: 2'- and O2: 3'- cyclonucleosides can be obtained, inversion of configuration of the sugar hydroxyl occurring in the two latter cases. The ease with which ryc/onucleosides are formed is most striking and prompts the question whether they may not have some role in biological processes; no experimental evidence bearing on this question has, however, been reported to date.

Following the clarification of nucleoside structure, the next phase of our studies on nucleic acid structure was clearly the synthesis of the various mono- nucleotides derivable from the nucleosides by introducing a phosphate group

772



30TH SEPTEMBER 1955 CHEMISTRY OF NUCLEIC ACIDS into the carbohydrate portion of the molecule. With this end in view a com- prehensive study of possible phosphorylation procedures was made, and a number of new and flexible methods were devised. Of these, perhaps the most successful employed dibenzyl phosphorochloridate (СвНбСН20)2 POCl(e) as phosphorylating agent. Using these new methods with appropriately protected intermediates, unambiguous syntheses were effected for all the ribonucleoside-5' phosphates*7* and for the 3 and 5'- phosphates of the four main deoxyribonucleosides(e). Unambiguous synthesis of the 2'- and 3'- phosphates of the ribonucleosides proved a difficult undertaking, partly because of the difficulty of obtaining ribonucleosides blocked simultaneously in the 3'- and 5'- positions, and partly because of phosphoryl migration which is discussed below. In their early studies Brown and Todd(e) were able to prepare, for example, adenosine-2' and -3' phosphates by phosphorylating 5'-trityladenosine, but they were unable at the time to say which was the 2'- and which the 3'- derivative; the same was, of course, true of other ribonucleoside 2'- and 3'- phosphates prepared in similar fashion from the appropriate 5'- trityl derivatives.

Until 1949 it had been believed that alkaline hydrolysis of ribonucleic acids yielded only four nucleotides which were believed, on evidence now known to be of doubtful validity, to be the 3'- phosphates of the four ribonucleosides. In that year, however, application of ion-exchange chromatography to alkaline hydrolysates of ribonucleic acid by Carter and Cohn(lo) showed that they contained not four, but eight nucleotides made up of four pairs of isomers - an a and а b nucleotide corresponding to each of the four ribonucleosides. Since it is now known, on the basis of many kinds of evidence, that the four a nucleotides all have the phosphate residue in the same position and the b nucleotides likewise it will simplify matters if, for the moment, we discuss the problem as it concerned one specific case - that of the adenylic acids a and b - bearing in mind that what is said here about this pair of isomers applies equally to each of the other pairs. Adenylic acids a and b were shown to be identical with the synthetic 2'- and 3'- phosphates of adenosine although, as already indicated, it was not at that time known which was the 2'- and which was the 3'- isomer. Brown and Todd*9* observed that, although quite stable in alkaline solution, acid solutions of either isomer undergo phosphoryl migration giving an equilibrium mixture of both. This interconversion can be written formally as follows:

°' * °. S> -[-ОН О OH

'/ JC *

'/ - I/

о l'

OH ~~

/' -

/' он о OH О OH о OH

Even more interesting was the behaviour of simple monoesters of the a and b nucleotides. Diesters of phosphoric acid are normally very resistant toward

773



JOURNAL OF THE ROYAL SOCIETY OF ARTS 30TH SEPTEMBER I955 alkaline hydrolysis. Monoesters of the a and b nucleotides are, however, labile to alkali, undergoing ready hydrolysis with loss of the ester group and formation in every case, of a mixture of a and b nucleotides*^. This remarkable ease of hydrolysis is also shown by esters of the glycerophosphoric acids, and it is clear that the structural feature causing alkali-lability in all these cases is the presence of a as-hydroxyl group on a carbon atom adjacent to that bearing the phosphate residue.

The postulated mechanism of hydrolysis given below has received confirmation from kinetic studies using isotopes(ll). The formation of the cyclic phosphate as an intermediate has been demonstrated and the nucleoside^' : 3' cyclic phosphates have been synthesized and shown to have the expected properties(l2).

*>н . он о он о О О I/ 'y X / P - P - . P +BOH-XMÍ ,

/ ' / / ' ^

о OR О OR О OH

It should be noted that the expulsion of the esterifying group R is an inevitable consequence of the cyclization step - one of the esterifying groups clearly must be expelled if the phosphorus atom is to retain its normal valency, and mere rupture of either of the linkages to the nucleoside residue would give no degradation of the molecule ; a neutral cyclic triester of phosphoric acid is not an intermediate in the process (although in formal schematic representations of the hydrolytic process in complex molecules it may at times be convenient to write the intermediate as if it were). The production of a mixture of a and b nucleotides on further hydrolysis of the cyclic phosphate is to be expected in view of the near equivalence of the two ester linkages in it.

The structural theory of the nucleic acids was, in fact, developed during the period when the terms a and b were used for the isomeric nucleotides, but since a resolution of the 2'- and 3'- phosphate problem has since been achieved it will be convenient to report it at this stage. Brown, Fasman, Magrath and Todd(l3) were able to prepare a homogeneous x : 5'- diacetyladenosine and to phosphorylate it. Subsequent removal of the protecting groups yielded exclusively adenylic acid a . This showed clearly that no phosphoryl migration had occurred during any of the operations, and hence that the x : 5'-diacetyl adenosine was the b : 5'diacetyl derivative. The diacetyl compound was then tosylated and the position of the stable tosyl group was established by degradative methods as C2'. It followed rigidly that the diacetyladenosine used was 3' : 5'- diacetyladenosine and hence that adenylic acid a is adenosine-2' phosphate. It was also shown by X-ray crystallographic analysis that adenylic acid b is adenosine -3

' phosphate.

The same conclusion was reached by Khym and Cohn(l4) by an ingenious

774



30TH SEPTEMBER 1955 CHEMISTRY OF NUCLEIC ACIDS

application of differential hydrolysis of the nucleotides on ion-exchange resins. In addition to physical evidence, chemical proof that uridylic acid a is uridine-2' phosphate and hence that cytidylic acid a is cytidine-2' phosphate has been provided by recent work in Cambridge by Dť. Varadarajan(l6). It follows the same general pattern as the work on adenylic acid 0, but the location of the tosyl group in the л-tosyl-ô : 5 '-diacetyluridine was determined by a neat application of the ryc/onucleosides. The tosyldiacetyl derivative was readily converted to a ryc/onucleoside which was then reconverted to a diacetylnucleoside by opening the ry^/o-structure at O2. The nucleoside so obtained was the ß-D-arabofuranoside of uracil (vi) showing conclusively that the tosyl group in the original compound was at C2', that is that it was 2'-tosyl-3' : 5'- diacetyluridine (v).

QAc OT00 OH H

kFy

QAc OT00

iai

OH H

AcOHjC HOCHj

- » CYCLONUCLEOSIDE - »

он OH S M

It has long been known that ribonucleic acids hydrolyse readily with alkali giving simple nucleotides; no larger fragments have ever been obtained. Deoxyribonucleic acids, on the other hand, are not readily degraded by alkali and do not by this means yield simple nucleotides. Based on their observations on the alkali-lability of the esters of the a (2') and b (3') nucleotides, Brown and Todd(ie) advanced a simple explanation of the hydrolytic behaviour of the nucleic acids and thence to develop a general theory of their structure. The scheme below (in which Base C2/ C3/ C6, is used as an abbreviated expression for a nucleoside residue) represents in a formal way the alkaline breakdown of a ribonucleic acid; it is based on the strict analogy which exists between the monoester of a nucleotide and a polynucleotide. The scheme predicts what is, in fact, observed, namely complete hydrolysis to a mixture of the 2'- and 3'- phosphates of the respective nucleosides. It may be noted, too, that the intermediate cyclic phosphates have more recently been isolated by very mild alkaline treatment, further supporting the general thesis. The reason for the comparative stability of the deoxyribonucleic acids is apparent; having no hydroxyl on Cy, they cannot undergo a cyclization process (transesterification) and so show the normal stability of diesters of phosphoric acid. The 3' 15'- internucleotidic linkage shown in the scheme was postulated for all nucleic acids by Brown and Todd*ie), although its complete justification depends on other evidence to be mentioned later.

775



JOURNAL OF THE ROYAL SOCIETY OF ARTS 30TH SEPTEMBER 1955 Deoxyribonucleic acids : These acids

are formulated as linear polynucleotides with a recurring 3' : 5' phosphodiester

Bos« Bo, e Bo*. linkage(le).

С2тОН C2-OH C¿rOH

I /° I /° I _ /° p f/Ч но о f°7' но о f°/' но

_

о ' ' но о но о но о ' '¿

но X ' Base C3' cy

i Bose Bose ' x Bose Bose Bose x

III , 4 , c2ro^ ̂ o CfO^ ̂ o c¿ro^ ̂ o

Base C3' , C5' ,

I /р' ô CfO^

I /' ô

I /' 44 C3-0 NOH C3-0 ОН C3-0 OH P

НО I но I HO I ' V V Ч- Base C3' cy

I ' Bose Bose

c¿topo3h2 c2. он This formulation is in agreement with I + | their stability towards alkali and with c3. он c3, opo3h2 £act tjât tjê deoxyribonucieotides I I formed by enzymic hydrolysis have c5. он c5< oh been shown to be nucleoside-5' phos-

phates by enzymic methods*1 7 > and by synthesis*8*. The production of pyrimidine nucleoside 3' : 5 '-diphosphates on

acid hydrolysis*18*, also accords with this structure, showing as it does the participation of both C3, and C5/ in the internucleotidic linkage. Furthermore, the recent synthesis by Michelson and Todd*lö) of a dithymidine dinucleotide containing the 3' : 5 '-internucleotidic linkage and the demonstration that its behaviour towards enzymes is precisely that of the dinucleotidic fragments obtained in solution from enzymic digests of deoxyribonucleic acids, is a further support for the above formulation.

The alkali-stability of the isolated deoxyribonucleic acids would seem to preclude the presence in them of branched chain structures since the only type of branching which could occur would be on phosphorus by way of alkali- labile triester groupings. X-ray evidence, too, favours a linear unbranched structure.

As yet no method is available for determining the sequence of residues in deoxyribonucleic acids.

Ribonucleic acids: Brown and Todd(ie) postulated a recurring 3' : 5 '-linked polynucleotide structure for the main chain in ribonucleic acids

776



30TH SEPTEMBER 1955 CHEMISTRY OF NUCLEIC ACIDS

' P '

Base C2' C3' С/ '

P '

Base C2' C3' C5' '

P '

Base СУ C3' C5' *

The results of alkaline hydrolysis already mentioned show that C2' or C3' is one point of attachment of the internucleotidic linkage. Since treatment of ribonucleic acids with snake venom diesterase* 2o) or with ribonuclease followed by intestinal phosphatase*21* gives large amounts of nucleoside^7 phosphates it is clear that (У is also involved.

A variety of products can be obtained by the action of crystalline pancreatic ribonuclease on ribonucleric acids. Short action gives, in addition to larger oligonucleotides, the cyclic 2' : 3'- phosphates of cytidine and uridine. Further action yields the 3'- phosphates of cytidine and uridine together with a variety of small polynucleotides in which the terminal residue bearing a phosphate group at (У is always a pyrimidine nucleoside residue*22*. The inferred specificity of the enzyme for linkages attached to pyrimidine residues has been confirmed and defined by studying its action on synthetic nucleotide esters*28* and nucleoside-2' : 3' cyclic phosphates*24*. Ribonuclease attacks the cyclic 2' : 3'- phosphates of uridine and cytidine yielding exclusively the corresponding 3' -phosphates; it is without action on the cyclic phosphates of adenosine and guanosine. Again, it has no action on simple esters of purine nucleotides whether the phosphate group be in the 2', 3', or 5' position nor on esters of pyrimidine nucleotides bearing phosphate at C2' or C6', but it hydrolyses esters of pyrimidine nucleoside-3

' phosphates smoothly to the nucleoside-3

' phosphates, the cyclic

2f : 3- phosphates being intermediates. It follows from these observations that the pyrimidine nucleoside residues in ribonucleic acids must have the internucleotidic linkage at C8' and that ribonuclease acts in the same way as alkali except in so far as it produces only the C8'- nucleotides instead of a mixture of the C2' and C3' isomers. Similar studies of the action of spleen nuclease (which attacks both purine and pyrimidine sites in ribonucleic acids) on simple nucleotide esters show that purine nucleoside residues in the chain are also linked at С3'*2б). It seems therefore that the postulated 3' : internucleotidic linkage may be taken as established, and all other evidence appears to confirm it. There is no real evidence for any other type of internucleotidic linkage.

The possibility that branched-chain structures may occur in ribonucleic acids has been frequently discussed. Bearing in mind the essential requirement

777



JOURNAL OF THE ROYAL SOCIETY OF ARTS 30TH SEPTEMBER I955 of alkali-lability and the mechanism of alkaline hydrolysis, two types of branching can be considered. The first involves branching on phosphorus by way of triester linkages, and this has been frequently canvassed in the past. Recent studies in our laboratory have shown that diesters of ribonucleoside-3

' phosphates (which

are analogous in structure to triester branch points) are extremely unstable even in approximately neutral pH ranges ; it seems, therefore, very unlikely that ribonucleic acids can exhibit branching of this type since the postulated structures could hardly survive the normal isolation procedures. The other possible type of branching is not so easily disposed of; in it C2' in one residue of the main chain, is joined by a phosphodiester linkage to C3' in the first residue of the branching chain(le).

' p '

Base С/ С/ C5' '

P '

Base СУ С/ C5' '

P Base ' I Base C2' C3' C5' C2' ' I P P C,'^^ ' I Base C2 ' C3' C5'

XC5'

In this type of branching, the first residue in the branch cannot be linked to the main chain through C5', for in such a structure neither residue would contain a free hydroxyl adjacent to the phosphate linkage which would permit the cyclization step required in alkaline or ribonucleasé hydrolysis; as a result alkaline hydrolysis of the polynucleotide would give fragments larger than simple nucleotides. No decisive evidence has yet been presented which allows us to decide whether ribonucleic acids are branched or unbranched. All that can be said is that if branching occurs it must be of the type discussed here. Further experimental evidence from synthesis or from physical measurements is necessary before the branching problem can be settled.

Apart from branching, the difference between individual ribonucleic acids presumably lies in the sequence of nucleoside residues in the polynucleotide chain. A method suitable for sequence determination in ribonucleic acids has been proposed(2e). It depends on the oxidation of a terminal nucleoside residue containing a free г' : з'-а -glycol system with periodate followed by expulsion of the oxidized residue under very weakly alkaline conditions by means of an

778



30TH SEPTEMBER 1955 CHEMISTRY OF NUCLEIC ACIDS elimination reaction leaving the rest of the molecule intact. The validity of this stepwise degradation procedure has been confirmed in a number of oligonucleotides^ 7), but it cannot be applied to the nucleic acids themselves until a homogeneous nucleic acid is available for study; unfortunately nucleic acids as ordinarily obtained are not heterogeneous mixtures.

It may be observed that according to whether a nucleic acid bears its terminal phosphoric acid group at C3' or C5' in a nucleoside residue, it may be regarded as a polyester in which the monomeric units are nucleoside^' or nucleoside-5' phosphates. To which of these types the natural nucleic acids belong is at present unknown. True, examples have been reported conforming to both, but since any partial degradation of a polynucleotide of the second type would give products with terminal 3 '- phosphate groups, the significance of these reports is still undertain and their assessment may have to await closer knowledge of the mechanism by which polynucleotides are synthesized in the living organism.

Since the postulation of general structures for the nucleic acids and the avail- ability of precise knowledge about the individual nucleotides, much attention has been devoted to their macromolecular configuration or conformation. The conformation of the large nucleic acid molecules is obviously important in connection with their biological function and a good deal of information is now available regarding the deoxyribonucleic acids whose sodium salts can be obtained in crystalline form as fibres and can be studied by X-ray methods. The position is less satisfactory with regard to the ribonucleic acids which are very difficult to get in fibre form and give generally unsatisfactory X-ray diagrams.

Following earlier suggestions by Pauling and others, Watson and Crick(28) advanced a structure of the macromolecule of deoxyribonucleic acids which appears to be sufficiently in harmony with chemical and X-ray data to be widely accepted, at any rate, in all essential features. According to this view the deoxyribonucleic acid molecule is pictured as a double helix in which two helical chains are coiled round the same axis. Both chains follow right-handed helices but the sequence of residues in the two chains run in opposite directions. The pyrimidine and purine bases are on the inside of the helix and the phosphate groups on the outside. The nucleotides occur at intervals of in the direction of the long axis and the helix repeats itself every ten nucleotides (that is, 34A). In this structural picture the two helical chains are held together by the purine and pyrimidine bases which lie in a plane perpendicular to the long axis of the molecule and are joined together by hydrogen bonds. Assuming the most likely tautomeric forms of the bases, the formation of these hydrogen bonds can be seen on models to be highly specific and only certain pairs of bases can be at once bonded and fitted into the helical structure. The only pairings which seem reasonable are adenine-thymine and guanine-cytosine, so that the sequence of nucleoside residues in one chain in the double helix will, in fact, determine the sequence in the other. This specific pairing of bases is strongly supported by the analytical finding that in the deoxyribonucleric acids the molar ratios adenine/ thymine and guanine/cytosine are close to unity, whereas the ratio between, say, adenine and guanine varies considerably in different acids. We need not

779



JOURNAL OF THE ROYAL SOCIETY OF ARTS 30TH SEPTEMBER 1955 here elaborate the arguments and evidence in detail, but in sum, the Watson- Crick formulation is strongly favoured and is in agreement with the physical and chemical facts.

This picture of the deoxyribonucleic acid molecule has many attractive features, and physicists, as well as chemists and biologists, have not been slow to speculate about its possible implications. One of the most obvious is the way in which it can give a picture of the replication of a specifically con- stituted deoxyribonucleic acid, and hence of the transmission of the hereditary characteristics thought by many workers to be one of the main functions of deoxyribonucleic acids in the cell nucleus. For, if we imagine first of all the separation of the two strands of the double helix and, further, imagine new deoxyribonucleic acid chains being built up alongside these separated strands, it is clear that the specific hydrogen- bonding mentioned above will lead to the duplication of the original double helix in each case. The deoxyribonucleic acid chain in a cell would thus be a kind of permanent template which would be handed on in the reproductive process. Much argument has, of course, developed on the mechanics of such a process of unwinding as I have imagined above, but there can be no doubt that we have here a most attractive idea, and one which may well lead to important developments in biology.

REFERENCES 1. P. A. Levene and H. S. Simms, J. Biol. Chem. 1925, 65, 519 ; 1927, 70, 32 7. 2. For review cf Kenner, Fortschr. Chem. org. Naturstoffe 195 1, 8, 96. 3. D. M. Brown and B. Lythgoe, J. Chem. Soc. 1950, 1990 ; W. Andersen, D. H. Hayes, A. M. Michelson and A. R. Todd, ibid 1954, 1882 ; A. M. Michelson and A. R. Todd, ibid. 1955, 816. 4. V. M. Clark, A. R. Todd and J. Zussman, J. Chem. Soc. 1951, 2952. 5. A. M. Michelson and A. R. Todd, J. Chem. Soc. 1955, 816. 6. F. R. Atherton, H. T. Openshaw and A. R. Todd, J. Chem. Soc. 1945, 382. 7- J. Baddiley and A. R. Todd, J. Chem. Soc. 1947, 648 ; A. M. Michelson and A. R. Todd, ibid. 1949, 2476. 8. A. M. Michelson and A. R. Todd, J. Chem. Soc. 1953, 951 ; 1954, 34 ; D- H. Hayes, A. M. Michelson and A. R. Todd, ibid. 1955, 808. 9. D. M. Brown and A. R. Todd, J. Chem. Soc. 1952, 44. 10. C. E. Carter and W. E. Cohn, Federation Proc. 1949, 8, 190. п. D. Lipkin, P. T. Talbert and M. Cohn, J. Amer. Chem. Soc. 1954, 76, 2871. 12. D. M. Brown, D. I. Magrath and A. R. Todd, J. Chem. Soc. 1952, 2708. 13. D. M. Brown, G. D. Fasman, D. I. Magrath and A. R. Todd, J. Chem. Soc. 1954, 1448. Nature, 1953, 172, 1184. 14. J. X. Khym and W. E. Cohn, J. Amer. Chem. Soc. 1954, 76, 1818. 15. Unpublished results. 16. D. M. Brown and A. R. Todd, J. Chem. Soc. 1952, 52. 17. С. E. Carter,/. Amer. Chem. Soc. 1951, 73, 1537. 18. С. A. Dekker, A. M. Michelson and A. R. Todd, J. Chem. Soc. 1953, 947. 19. A. M. Michelson and A. R. Todd, J. Chem. Soc. 1955, in press. 20. W. E. Cohn and E. Volkin, Arch. Biochem. Biophys. 1952, 35, 465. 21. W. E. Cohn and E. Volkin, Nature, 1951, 167, 483. 22. R. Markham and J. D. Smith, Biochem J. 1952, 52, 558. 23. D. M. Brown and A. R. Todd, J. Chem. Soc. 1953, 2040. 24. D. M. Brown, С. A. Dekker and A. R. Todd, J. Chem. Soc. 1952, 2715. 25. D. M. Brown, L. A. Heppel and R. J. Hilmoe, J. Chem. Soc. 1954, 40. 26. D. M. Brown, M. Fried and A. R. Todd, Chem. Ind. 1953, 352. 27. P. R. Whitfield and R. Markham, Nature, 1953, 171, 346. 28. J. D. Watson and F. H. С. Crick, Nature, 1953, 171, 73 7.

DISCUSSION

the chairman: Sir Alexander Todd has thanked us for the patience with which we have listened to him. I think if instead of that he said impatience he might be more correct, because he has so whetted our appetites that we are anxious to know much more about the subject and to listen to him further. I am sorry that this lecture could not have been twice as long as it was. In introducing Sir Alexander Todd, I did not dwell on his manifold scientific accomplishments. They range from early work with Borsche and other collaborators who did very distinguished work on

780



30TH SEPTEMBER 1955 CHEMISTRY OF NUCLEIC ACIDS vitamin-B2, and now to work on insect pigments and some of the mould metabolites and many other problems. In addition to the outstanding developments of which we have heard this afternoon, he is elucidating the chemistry of В12. I think that we must recognize that all this work which he and his school of collaborators have been able to carry out on the nucleic acids is of quite fundamental importance. I was very impressed with the way in which he stated in a short sentence that the structures of many of these compounds were confirmed by synthesis. There is a great deal of labour implied in that one remark, and if the fascinating vistas which he has opened up to us this afternoon have made it possible for us to take a glimpse into these mysteries, it is only because of the fundamental preparatory studies which Sir Alexander Todd has undertaken, because of his success in laying the foundations, and then perhaps the larger bricks and sections of the actual structures which are found in the nucleic acids.

I think your applause has shown how much you have appreciated the lecture this afternoon and I will now call on Sir John Simonsen to propose a vote of thanks to the lecturer.

sir JOHN simonsen, F.R.s. : We are fortunate to-day in having on the platform two of the most distinguished British chemists, and I stress the word 'British' because Sir Alexander Todd comes from across the border. In the chair this afternoon we have the Past President of the Royal Society and, which is of particular interest to this Society, we have one of our Albert Medallists. I think Sir William Pope, if he could have been here to-day, would have been very gratified to listen to this memorial lecture. It would have appealed to him that the School of Chemistry at Cambridge was such a hive of activity. In his later years he took a great interest in the biological applications of chemistry and for this reason also this lecture would have appealed to him. In Sir Alexander's lecture to-day we see, as in all his work, that he only attempts problems of real difficulty and of fundamental importance. I think there could be few subjects in organic chemistry and biochemistry of greater importance than that of the nucleic acids. He has fascinated us, not only by the originality of thought, by the skill in experimental work, but also by the remarkable manner in which he has presented the facts to us, so that those who are not masters in this field could at any rate quite readily understand the progress which he has made. I should therefore like to propose a vote of thanks both to our chairman, Sir Robert Robinson, and our lecturer, Sir Alexander Todd.

MR. E. MUNRO RUNTZ (Chairman of Council of the Society): I second Sir John Simonsen's vote of thanks with very great pleasure.

The vote of thanks having been carried with acclamation the meeting then ended.

781



Documents

SOME RECENT DEVELOPMENTS IN THE CHEMISTRY OF NUCLEIC ACIDS