sexta-feira, 26 de abril de 2013

Há 60 anos, a dupla hélice começava a girar em nossas mentes!

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James Watson and Francis Crick in 1959. Photograph: Bettmann/Corbis

Hoje, 25 de março de 2013, é o Dia do ADN, por que há exatos 60 anos atrás, saiu o paper da Nature de Watson e Crick (baixe seu original daqui), que desvendou a estrutura exata da dupla hélice da molécula do ADN, base de nosso código genético. Neste mesmo número, saíram outros dois artigos complementares, pois alguns dos experimentos foram utilizados em comum (Wilkins, Stoke & Wilson e Franklin & Gosling).

Foi com as seguintes palavras que a dupla hélice entrou para o rol do conhecimento científico humano:
A structure for nucleic acid has already been proposed by Pauling (4) and Corey1. They kindly made their manuscript available to us in advance of publication. Their model consists of three intertwined chains, with the phosphates near the fibre axis, and the bases on the outside. In our opinion, this structure is unsatisfactory for two reasons:

(1) We believe that the material which gives the X-ray diagrams is the salt, not the free acid. Without the acidic hydrogen atoms it is not clear what forces would hold the structure together, especially as the negatively charged phosphates near the axis will repel each other.

(2) Some of the van der Waals distances appear to be too small.

Another three-chain structure has also been suggested by Fraser (in the press). In his model the phosphates are on the outside and the bases on the inside, linked together by hydrogen bonds. This structure as described is rather ill-defined, and for this reason we shall not comment on it.

We wish to put forward a radically different structure for the salt of deoxyribose nucleic acid (5). This structure has two helical chains each coiled round the same axis (see diagram). We have made the usual chemical assumptions, namely, that each chain consists of phosphate diester groups joining beta-D-deoxyribofuranose residues with 3',5' linkages. The two chains (but not their bases) are related by a dyad perpendicular to the fibre axis. Both chains follow right-handed helices, but owing to the dyad the sequences of the atoms in the two chains run in opposite directions (6) . 

Particularmente triste foi o "esquecimento" do reconhecimento do trabalho de uma das outras autoras, Rosalind Franklin, que infelizmente morreu de câncer em 1958, não podendo sequer concorrer ao Nobel quando Watson e Crick o ganharam (em 1962), se bem que nenhum desses outros autores levou (nem ela levaria, considerando-se que não interpretava seus dados da forma que acabou sendo reconhecida como a correta). Sobre a controvérsia da autoria de todas as idéias reunidas neste achado, cujas feridas recém começam a sarar, a Wikipedia é bastante esclarecedora:
Watson and Crick based their molecular model of the DNA double helix on data that had been collected by researchers in several other laboratories. Watson and Crick were the first to put together all of the scattered fragments of information that were required to produce a successful molecular model of DNA.
Much of the data that were used by Crick and Watson came from unpublished work by Maurice Wilkins, Rosalind Franklin, A.R. Stokes, and H.R. Wilson at King's College London in the University of London. Key data from Wilkins, Stokes, and Wilson, and, separately, by Franklin and Gosling, were published in two separate additional articles in the same issue of Nature with the article by Watson and Crick.[6][7] The article by Watson and Crick did acknowledge that they had been "stimulated" by experimental results from the King's College researchers, and a similar acknowledgment was published by M. H. F. Wilkins, A.R. Stokes, and H. R. Wilson in the following three-page article.
In 1968, Watson published a highly controversial[8] autobiographical account of the discovery of the double-helical, molecular structure of DNA called The Double Helix, and which was not accepted—at least publicly—either by Francis Crick or by M.H.F. Wilkins. Furthermore, Erwin Chargaff also printed a rather "unsympathetic review" of James D. Watson's booklet in the March 29, 1968 issue of Science. In his "autobiographical" booklet, Watson stated among other things that he and Crick had access to some of Franklin's data from a source that she was not aware of, and also that he had seen—without her permission—the B-DNA X-ray diffraction pattern obtained by Franklin and Gosling in May 1952 at King's in London. In particular, in late 1952, Franklin had submitted a progress report to the Medical Research Council, which was reviewed by Dr. Max Perutz, then at the Cavendish Laboratory of the University of Cambridge, UK. Watson and Crick also worked in the MRC-supported Cavendish Laboratory in Cambridge whereas Drs. Wilkins and Franklin were in the MRC supported laboratory at King's in London. Such MRC reports were not usually widely circulated, but Crick read a copy of Dr. Franklin's research summary in early 1953.[9][8]
Max Perutz's justification for passing this information to both Crick and Watson was that the report contained information which Watson has previously heard in November 1951 when Dr. Franklin talked about her unpublished results with Raymond Gosling during a meeting arranged by Dr. M.H.F. Wilkins at King's College, following a request from Crick and Watson; this justification does not hold however for Crick who was not present at this November 1951 meeting, but who also was given access by Max Perutz to Franklin's MRC report data which prompted Crick and Watson to seek permission from Sir Lawrence Bragg--who was at the time the head of the Cavendish Laboratory in Cambridge—to publish in Nature their double-helix molecular model of DNA based on Franklin's and also Wilkins' data. Moreover, in November 1951 Watson had acquired—by his own admission—little training in X-ray crystallography, and therefore had not fully understood (again, according to his own admission, in "The Double Helix") what Dr. Franklin was saying about the structural symmetry of the DNA molecule. Crick, however, knowing the Fourier transforms of Bessel functions that represent the X-ray diffraction patterns of helical structures of atoms, correctly interpreted further one of Dr. Franklin's experimental findings as indicating that DNA was most likely to be a double helix with the two polynucleotide chains running in opposite directions. Crick was thus in a unique position to make this interpretation because he had previously worked on the X-ray diffraction data for other large molecules that had helical symmetry similar to that of DNA. Dr. Franklin, on the other hand, rejected at first the molecular model building approach proposed by Crick and Watson because their first DNA model presented by Watson to her and Dr. M.H.F. Wilkins in 1952 in London had an obviously incorrect structure with hydrated charged groups on the inside of the model, rather than on the outside, as explicitly admitted by James D. Watson in his "Double Helix" booklet.[9] It is therefore questioned by some whether Crick's colleague, Dr. Max Perutz, acted unethically[10] by allowing Crick access to Dr. Franklin's MRC report about the crystallographic unit of the B-DNA and A-DNA structures. Dr. Perutz claimed, however, that he believed he had not because this report was not confidential, and had been designed as part of an effort to promote contact between different MRC research groups.[11]

Controvérsias à parte, este foi, sim, um grande achado científico. Sempre é interessante conectar esse feito com outros grandes cientistas desse incrível período da história da ciência: a busca da estrutura do ADN recebeu um impulso intelectual especial de ninguém menos de Erwin Schödinger, que 10 anos antes, em suas Trinity Lectures, em Dublin, antecipara certos elementos da estrutura da molécula que continha os genes como sendo um cristal aperiódico de grandes dimensões para, assim, driblar as "complicações" do nível subatômico. Estas idéias foram  inspiradas, por sua vez, pelos experimentos de seu amigo Max Delbrück (ele mesmo Nobel em 1969) com o fago, e aparecem relatadas no notável livro What is Life? (1944 - íntegra do texto aqui). Tão saboroso é este episódio que faço uma metacitação a seguir, isto é, cito Dyson que cita Schrödinger:
Schrödinger’s book is less than a hundred pages long. It was widely read and was influential in guiding the thoughts of the young people who created the new science of molecular biology in the  following decade. It is clearly and simply written, with only five references to the technical literature and less than ten equations from beginning to end. It is, incidentally, a fine piece of English prose. Although Schr ¨ odinger was exiled from his native Austria to Ireland when he was over fifty, he wrote English far more beautifully than most of his English and American contemporaries. He reveals his cosmopolitan background only in the epigraphs that introduce his chapters: three are from Goethe, in German; three are from Descartes and Spinoza, in Latin; and one is from Unamuno, in Spanish. As a sample of his style I quote the opening sentences of his preface:
A scientist is supposed to have a complete and thorough knowledge, at first hand, of some subjects, and therefore he is usually expected not to write on any topic of which he is not a master. This is regarded  as a matter of noblesse oblige. For the present purpose I beg to renounce the noblesse, if any, and to be freed of the ensuing obligation. My excuse is as follows. We have inherited from our forefathers the keen longing for unified, all-embracing knowledge. The very name given to the highest institutions of learning reminds us that from antiquity and throughout many centuries the universal aspect has been the only one to be given full credit. But the spread, both in width and depth, of the multifarious branches of knowledge during the last hundred odd years has confronted us with a queer dilemma. We feel clearly that we are only now beginning to acquire reliable material for welding together the sum-total of what is known into a whole; but, on the other hand, it has become next to impossible for a single mind fully to command more than a small specialized portion of it. I can see no other escape from this dilemma (lest our true aim be lost for ever) than that some of us should venture to embark on a synthesis of facts and theories, albeit with second-hand and incomplete  knowledge of some of them, and at the risk of making fools of themselves. So much for my apology.
Fonte:  Dyson, F. Origins of Life, Cambridge University Press, 1999.

Um dos equívocos comuns nesse tema é achar que Watson e Crick "descobriram" o ADN, o que não é verdade. A molécula em si fora identificada primeiro pelo químico suíço Friedrich Miescher na década de 1860, e uma compreensão razoável emergiu do trabalho do químico russo Phoebus Levene (1919), sem falar nas decisivas contribuições de Avery, Delbrück e Chargaff.

Foi Oswald Avery quem provou, pela primeira vez (em 1944) que o ADN é, de fato o sustentáculo material da hereditariedade, e não as proteínas, como até então se pensava (baixe também esse paper histórico daqui)

Rosalind Franklin.jpg

Por fim, não deixem de ler o verbete sobre Rosalind Franklin neste dia tão simbólico, a quem igualmente homenageamos. Ela, de fato, não "adivinhou" a estrutura correta e, inclusive, caçoou da possibilidade de ser uma hélice num cartão assinado com seu assistente Goslin (abaixo) cerca de um ano antes da publicação.

Bem, ciência funciona assim mesmo, entre (raros) acertos e (mais frequentes) erros...

Saudações ADNéticas!


Não deixe de ver o especial da Nature sobre os 60 anos do ADN
(todos os artigos clássicos citados estão disponíveis aqui

Belo cartaz da empresa Life Technologies

PS: Este é mais um dos artigos que comecei na data certa (25/4) mas só foi ao ar depois da mesma, provando que o dia sempre é mais curto do que gostaríamos...

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