Linus Pauling, cientista americano

Linus Pauling, na íntegra Linus Carl Pauling (nascido em 28 de fevereiro de 1901, Portland, Oregon, EUA - morreu em 19 de agosto de 1994, Big Sur, Califórnia), físico teórico americano que se tornou a única pessoa a ganhar dois prêmios Nobel não compartilhados. Seu primeiro prêmio (1954) foi concedido por pesquisas sobre a natureza da ligação química e seu uso na elucidação da estrutura molecular; o segundo (1962) reconheceu seus esforços para proibir o teste de armas nucleares.

Infância e educação

Pauling foi o primeiro de três filhos e o único filho de Herman Pauling, farmacêutico, e Lucy Isabelle (Darling) Pauling, filha de farmacêutico. Após sua educação inicial em Condon e Portland, Oregon, ele frequentou a Oregon Agricultural College (atual Oregon State University), onde conheceu Ava Helen Miller, que mais tarde se tornaria sua esposa, e onde obteve seu diploma de bacharel em ciências em engenharia química. cum laude em 1922. Ele freqüentou o California Institute of Technology (Caltech), onde Roscoe G. Dickinson lhe mostrou como determinar as estruturas dos cristais usando raios-X. Ele recebeu seu Ph.D. em 1925 para uma dissertação derivada de seus documentos sobre estrutura cristalina. Após um breve período como bolsista nacional de pesquisa, ele recebeu uma bolsa Guggenheim para estudar mecânica quântica na Europa.Ele passou a maior parte dos 18 meses no Instituto de Física Teórica de Arnold Sommerfeld, em Munique, Alemanha.

Elucidação de estruturas moleculares

Após concluir os estudos de pós-doutorado, Pauling retornou à Caltech em 1927. Lá, ele iniciou uma longa carreira de ensino e pesquisa. Analisar a estrutura química tornou-se o tema central de seu trabalho científico. Usando a técnica de difração de raios-X, ele determinou o arranjo tridimensional de átomos em vários minerais importantes de silicato e sulfeto. Em 1930, durante uma viagem à Alemanha, Pauling aprendeu sobre a difração de elétrons e, ao retornar à Califórnia, usou essa técnica de espalhar elétrons dos núcleos de moléculas para determinar as estruturas de algumas substâncias importantes. Esse conhecimento estrutural o ajudou a desenvolver uma escala de eletronegatividade na qual ele atribuiu um número representando o poder de um átomo em particular de atrair elétrons em uma ligação covalente.

To complement the experimental tool that X-ray analysis provided for exploring molecular structure, Pauling turned to quantum mechanics as a theoretical tool. For example, he used quantum mechanics to determine the equivalent strength in each of the four bonds surrounding the carbon atom. He developed a valence bond theory in which he proposed that a molecule could be described by an intermediate structure that was a resonance combination (or hybrid) of other structures. His book The Nature of the Chemical Bond, and the Structure of Molecules and Crystals (1939) provided a unified summary of his vision of structural chemistry.

The arrival of the geneticist Thomas Hunt Morgan at Caltech in the late 1920s stimulated Pauling’s interest in biological molecules, and by the mid-1930s he was performing successful magnetic studies on the protein hemoglobin. He developed further interests in protein and, together with biochemist Alfred Mirsky, Pauling published a paper in 1936 on general protein structure. In this work the authors explained that protein molecules naturally coiled into specific configurations but became “denatured” (uncoiled) and assumed some random form once certain weak bonds were broken.

On one of his trips to visit Mirsky in New York, Pauling met Karl Landsteiner, the discoverer of blood types, who became his guide into the field of immunochemistry. Pauling was fascinated by the specificity of antibody-antigen reactions, and he later developed a theory that accounted for this specificity through a unique folding of the antibody’s polypeptide chain. World War II interrupted this theoretical work, and Pauling’s focus shifted to more practical problems, including the preparation of an artificial substitute for blood serum useful to wounded soldiers and an oxygen detector useful in submarines and airplanes. J. Robert Oppenheimer asked Pauling to head the chemistry section of the Manhattan Project, but his suffering from glomerulonephritis (inflammation of the glomerular region of the kidney) prevented him from accepting this offer. For his outstanding services during the war, Pauling was later awarded the Presidential Medal for Merit.

While collaborating on a report about postwar American science, Pauling became interested in the study of sickle-cell anemia. He perceived that the sickling of cells noted in this disease might be caused by a genetic mutation in the globin portion of the blood cell’s hemoglobin. In 1949 he and his coworkers published a paper identifying the particular defect in hemoglobin’s structure that was responsible for sickle-cell anemia, which thereby made this disorder the first “molecular disease” to be discovered. At that time, Pauling’s article on the periodic law appeared in the 14th edition of Encyclopædia.

While serving as a visiting professor at the University of Oxford in 1948, Pauling returned to a problem that had intrigued him in the late 1930s—the three-dimensional structure of proteins. By folding a paper on which he had drawn a chain of linked amino acids, he discovered a cylindrical coil-like configuration, later called the alpha helix. The most significant aspect of Pauling’s structure was its determination of the number of amino acids per turn of the helix. During this same period he became interested in deoxyribonucleic acid (DNA), and early in 1953 he and protein crystallographer Robert Corey published their version of DNA’s structure, three strands twisted around each other in ropelike fashion. Shortly thereafter James Watson and Francis Crick published DNA’s correct structure, a double helix. Pauling’s efforts to modify his postulated structure had been hampered by poor X-ray photographs of DNA and by his lack of understanding of this molecule’s wet and dry forms. In 1952 he failed to visit Rosalind Franklin, working in Maurice Wilkins’s laboratory at King’s College, London, and consequently did not see her X-ray pictures of DNA. Frankin’s pictures proved to be the linchpin in allowing Watson and Crick to elucidate the actual structure. Nevertheless, Pauling was awarded the 1954 Nobel Prize for Chemistry “for his research into the nature of the chemical bond and its application to the elucidation of the structure of complex substances.”