Linus Pauling – From Quantum Chemistry to Molecular Biology

Linus Pauling (1901 – 1994)

Linus Pauling (19011994)

On February 28, 1901, American chemist, biochemist, peace activist, author, educator Linus Pauling was born. Pauling was one of the founders of the fields of quantum chemistry and molecular biology. His contributions to the theory of the chemical bond include the concept of orbital hybridisation and the first accurate scale of electronegativities of the elements. New Scientist called him one of the 20 greatest scientists of all time, and as of 2000, he was rated the 16th most important scientist in history.

“Science cannot be stopped. Man will gather knowledge no matter what the consequences – and we cannot predict what they will be. Science will go on — whether we are pessimistic, or are optimistic, as I am. I know that great, interesting, and valuable discoveries can be made and will be made… But I know also that still more interesting discoveries will be made that I have not the imagination to describe — and I am awaiting them, full of curiosity and enthusiasm.”
– Linus Pauling (1947), Lecture at Yale University, “Chemical Achievement and Hope for the Future.”

Linus Pauling Background and Early Studies

Pauling first experimented with chemistry kits during his time in high school. He enrolled at Oregon Agricultural College (now Oregon State University) in 1917 studying mathematics, physics and chemistry. Towards the end of his studies, he became acquainted with the work of Gilbert N. Lewis and Irving Langmuir on the electron structure of atoms and the chemical bonds that enable them to form molecules. This highly influenced Pauling and he decided to concentrate his studies on how the physical and chemical properties of substances are related to their atomic structure. In doing that, Pauling co-founded the science of quantum chemistry. After completing his bachelor’s degree, Pauling became a postgraduate at Caltech in Pasadena, California. There, he managed to publish seven papers on the crystal structures of minerals. Pauling received his doctorate in 1925.

During the 1920s, Linus Pauling traveled to Europe with a Guggenheim scholarship and worked together with Arnold Sommerfeld in Munich, Niels Bohr in Copenhagen and Erwin Schrödinger in Zurich.[5,6] In 1927 and after two years in Europe, he became assistant professor for Theoretical Chemistry at Caltech. He was appointed associate professor in 1929 and became full professor one year later. In 1932, Pauling introduced the concept of electronegativity. Using the numerous properties of molecules such as the energy required to break chemical bonds or the dipole moments of molecules, he determined numerical values for most elements. He arranged these values on a scale, the Pauling scale for electronegativity, which can be used to determine the nature of bonds between atoms and molecules.

Chemical Bonds and the Nobel Prize in Chemistry

In the 1930s Pauling began publishing essays on the nature of chemical bonds, which were published in 1939 in his famous book The Nature of the Chemical Bond. In 1954 he was awarded the Nobel Prize for Chemistry “for his research on the nature of chemical bonds and their application to the elucidation of the structures of complex substances“. Another area of his research was the relationship between an ionic bond, in which electrons are transferred between atoms, and a covalent bond, in which electrons are evenly distributed between atoms. Pauling was able to show that both bond types are only extremes between which most common bonds lie. Pauling’s concept of electronegativity was particularly useful here, because the difference in electronegativity between two atoms is the safest indicator of the degree of ionization of the bond.

Benzenes

Another topic Pauling worked on in the field of the chemical bond, was the enumeration of the structures of aromatics, in particular the prototype benzene. The German chemist Friedrich August Kekulé had described benzene in the most precise way to date by today’s standards.[7] He regarded the compound as a constant change between two structures, both with alternating single and double bonds, each with the double bond at the point where the single bond of the other structure is located. Pauling showed with a suitable description based on quantum mechanics that it is an intermediate structure containing aspects of both. The structure was a superposition of both structures rather than a rapid change between them. The term resonance or mesomerism was given to this phenomenon only later. In a certain way, the phenomenon is similar to that of hybridization, which has been described earlier, since it also involves more than one electron structure to obtain an intermediate result.

Biological Molecules

Throughout his career, Pauling focused his interests more and more towards biology and started working together with Thomas Hunt Morgan,[10Theodosius Dobzhansky,[9Calvin Bridges and Alfred H. Sturtevant.[8] Pauling started researching the structure of hemoglobin and demonstrated that the hemoglobin molecule changes its structure when it binds or releases an oxygen atom. This led him to study the structures of proteins in general and he revisited his method of X-ray diffraction. However, protein molecules are much less suitable for this technique than crystalline minerals. The best X-ray photographs of proteins were taken in the 1930s by the British crystallographer William Astbury. But when Pauling tried to participate in Astbury’s investigations in 1937, he failed.

The Decipherment of DNA

Several years later, Pauling managed to figure out that Astburys images were taken in such a way that they were tilted to their expected positions. Pauling formulated a model of the structure of hemoglobin, in which the atoms are arranged in a helix, and transferred this idea to proteins in general. He published the basic investigations on the secondary structure of proteins with his colleague Robert B. Corey and with the visiting scientist at Caltech Herman Branson around 1951. The double helix postulated by James Watson and Francis Crick for deoxyribonucleic acid (DNA) can also be traced back to this helix structure.[4] Pauling also came very close to this structure. Although his assumed DNA structure was not entirely correct, many who are familiar with his work believe that Pauling would soon have come to the same conclusion as Watson and Crick if Rosalind Franklin,[11] whose work was the basis for Watson and Crick’s publication, had not preceded him.

Later Years

Pauling also looked at enzyme reactions and showed that sickle cell anaemia is due to the alteration of a single amino acid in haemoglobin. As a result of this work, he became interested in the structure of antibodies and was involved in the development of the first synthetic antibodies in 1942. Pauling’s scientific discoveries led to decisive contributions in a diverse array of areas including around 350 publications in the fields of quantum mechanics, inorganic chemistry, organic chemistry, protein structure, molecular biology, and medicine. He is regarded as one of the founders of modern quantum chemistry and the father of molecular biology.

Linus Pauling died on August 19, 1994 in Big Sur, California.

Linus Pauling Lecture: Valence and Molecular Structure.

References and Further Reading:

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